Research Activity

List of recent publications

* A. Vasan, R. Weekes, W. Connacher, J. Sieker, M. Stambaugh, P. Suresh, D. E. Lee, W. Mazzei, E. Schlaepfer, T. Vallejos, J. Petersen, S. Merritt, L. Petersen, and J. Friend. **MADVent: A low-cost ventilator for patients with COVID-19**. Medical Devices and Sensors, Accepted 26 May 2020.
* L. G. Petersen, J. Friend, and S. Merritt. **Single ventilator for multiple patients during COVID19 surge: matching and balancing patients**. Critical Care, Accepted 26 May 2020.
* A. M. Girgis, M. N. Aziz, T. C. Gopesh, J. Friend, A. M. Grant, J. A. Sandubrae, and D. A. Banks. [**Novel Coronavirus Disease 2019 (COVID-19) Aerosolization Box: Design Modifications for Patient Safety**]( Journal of Cardiothoracic and Vascular Anesthesia, 2020.
*  N. Zhang, J. Mei, T. Gopesh, and J. Friend. [**Optimized, omnidirectional surface acoustic wave source: 152 degree Y-rotated cut of lithium niobate for acoustofluidics**]( IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Accepted 7 May 2020.
* A. Huang, H. Liu, O. Manor, P. Liu, and J. Friend. [**Enabling Rapid Charging Lithium Metal Batteries via Surface Acoustic Wave‐Driven Electrolyte Flow**]( Advanced Materials, 1907516, 2020.
* S. Zhang, A. Huang, A. Bar-Zion, J. Wang, O. V. Mena, M. G. Shapiro, and J. Friend. [**The Vibration Behavior of Sub-Micrometer Gas Vesicles in Response to Acoustic Excitation Determined via Laser Doppler Vibrometry**]( Advanced Functional Materials, 30(13):2000239, 2020.
* N. Zhang, Y. Wen, and J. Friend. [**MHz-Order Surface Acoustic Wave Thruster for Underwater Silent Propulsion**]( Micromachines, 11(4), 2020.
* J. Mei, N. Zhang, and J. Friend. [**Fabrication of Surface Acoustic Wave Devices on Lithium Niobate**]( Journal of Visualized Experiments, Accepted 16 February 2020.
* N. Zhang and J. Friend. [**Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics**]( Journal of Visualized Experiments, e60648, 2020.
* A. Vasan, W. Connacher, and J. Friend. **Fabrication and Methods of Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics**. Journal of Visualized Experiments, Accepted 12 February 2020.
* J. Mei and J. Friend. [**A review: controlling the propagation of surface acoustic waves via waveguides for potential use in acoustofluidics**]( Bulletin of the JSME, 7(1):1-25, 2020.
* G. Tilvawala, A. Camp, M. Unanian, J. Friend, and R. Weinreb. [**Rapid and accurate intraocular pressure sensing device for patients with artificial corneas**](). Translational Vision Science and Technology, 9(28), 1-9, 2019.
* A. Vasan and J. Friend. [**Medical Devices for low and middle income countries: A review and directions for development**]( ASME Journal of Medical Devices, 14(1):010803, 2019.
* M. Miansari, M. D. Mehta, J. M. Schilling, Y. Kurashina, H. H. Patel, and J. Friend. [**Inducing Mild Traumatic Brain Injury in C. elegans via Cavitation-Free Surface Acoustic Wave-Driven Ultrasonic Irradiation**]( Scientific Reports, 9(12775)2019.
* Y. Kurashina, C. Imashiro, M. Hirano, T. Kuribara, K. Totani, K. Ohnuma, J. Friend, and K. Takemura. [**Enzyme-free release of adhered cells from standard culture dishes using intermittent ultrasonic traveling waves**]( Nature Communications Biology, 2(1):393, 2019.
* W. Connacher, N. Zhang, A. Huang, J. Mei, S. Zhang, T. Gopesh, and J. Friend. [**Micro/nano acoustofluidics: materials, phenomena, design, devices, and applications**]( Lab Chip, 18:1952-1996, 2018.
* S. Collignon, O. Manor, and J. Friend. **Improving and Predicting Fluid Atomization via Hysteresis-Free Thickness Vibration of Lithium Niobate**. Advanced Functional Materials, 28(1704359)2018.
* A. Huang, M. Miansari, and J. Friend. **Driving morphological changes in magnetic nanoparticle structures through the application of acoustic waves and magnetic fields**. Applied Physics Letters, 113(3):034103, 2018.
* M. Kumaraswamy, S. Collignon, C. Do, J. Kim, V. Nizet, and J. Friend. [**Decontaminating surfaces with atomized disinfectants generated by a novel thickness-mode lithium niobate device**]( Applied Microbiology and Biotechnology, 15:6459-6467, 2018.
* Y. Kurashina, K. Takemura, J. Friend, S. Miyata, and J. Komotori. [**Efficient Subculture Process for Adherent Cells by Selective Collection Using Cultivation Substrate Vibration**]( IEEE Transactions on Biomedical Engineering, 64(3):580-587, 2017.
* Y. Kurashina, K. Takemura, and J. Friend. **Cell agglomeration in the wells of a 24-well plate using acoustic streaming**. Lab on a Chip, 17(5):876--886, 2017.
* Y. Ma, Q. Huang, T. Li, J. Villanueva, N. H. Nguyen, J. Friend, and D. J. Sirbuly. **A Local Nanofiber-Optic Ear**. ACS Photonics, 3(10):1762--1767, 2016.
* K. M. Ang, L. Y. Yeo, J. R. Friend, Y. M. Hung, and M. K. Tan. [**Acoustically enhanced heat transport**]( Review of Scientific Instruments, 87(1)2016.
* M. Miansari and J. R. Friend. [**Acoustic Nanofluidics via Room-Temperature Lithium Niobate Bonding: A Platform for Actuation and Manipulation of Nanoconfined Fluids and Particles**]( Advanced Functional Materials, 26(43):7861--7872, 2016.
* M. Miansari, A. Qi, L. Y. Yeo, and J. R. Friend. **Vibration-Induced Deagglomeration and Shear-Induced Alignment of Carbon Nanotubes in Air**. Advanced Functional Materials, 25(7):1014-1023, 2015.
* C. Cortez-Jugo, A. Qi, A. Rajapaksa, J. R. Friend, and L. Y. Yeo. [**Pulmonary monoclonal antibody delivery via a portable microfluidic nebulization platform**]( Biomicrofluidics, 9(5), 2015.
* Y. Wang, J. Z. Ou, A. F. Chrimes, B. J. Carey, T. Daeneke, M. M. Y. A. Alsaif, M. Mortazavi, S. Zhuiykov, N. Medhekar, M. Bhaskaran, J. R. Friend, M. S. Strano, and K. Kalantar-Zadeh. [**Plasmon Resonances of Highly Doped Two-Dimensional MoS2**]( Nano Letters, 15(2):883-890, 2015.
* S. Collignon, J. Friend, and L. Yeo. [**Planar microfluidic drop splitting and merging**]( Lab Chip, , 2015.
* K. M. Ang, L. Y. Yeo, J. R. Friend, Y. M. Hung, and M. K. Tan. **Nozzleless spray cooling using surface acoustic waves**. Journal of Aerosol Science, 79:48-60, 2015.
* J. E. Sader and J. R. Friend. [**Note: Improved calibration of atomic force microscope cantilevers using multiple reference cantilevers**]( Review of Scientific Instruments, 86(5), 2015.
* A. G. McDonnell, T. C. Gopesh, J. Lo, M. O'Bryan, L. Y. Yeo, J. R. Friend, and R. Prabhakar. [**Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows**]( Soft Matter, 11:4658-4668, 2015.
* X. Zhang, J. Wang, L. Bao, E. Dietrich, R. C. A. van der Veen, S. Peng, J. Friend, H. J. W. Zandvliet, L. Yeo, and D. Lohse. [**Mixed mode of dissolving immersed nanodroplets at a solid-water interface**]( Soft Matter, , 2015.
* J. Behrens, S. Langelier, A. R. Rezk, G. Lindner, L. Y. Yeo, and J. R. Friend. **Microscale anechoic architecture: acoustic diffusers for ultra low power microparticle separation via traveling surface acoustic waves**. Lab on a Chip, 15(1):43-46, 2015.
* A. G. McDonnell, N. N. Jason, L. Yeo, J. R. Friend, W. Cheng, and P. Ranganathan. **Extensional viscosity of copper nanowire suspensions in an aqueous polymer solution**. Soft Matter, Accepted 27 August 2015.
* M. Miansari, J. R. Friend, and L. Y. Yeo. [**Enhanced Ion Current Rectification in 2D Graphene-Based Nanofluidic Devices**]( Advanced Science, 2(6):n/a--n/a, 2015.
* O. Manor, A. R. Rezk, J. R. Friend, and L. Y. Yeo. [**Dynamics of liquid films exposed to high-frequency surface vibration**]( Phys. Rev. E, 91:053015, 2015.
* A. R. Rezk, S. Walia, R. Ramanathan, H. Nili, J. Z. Ou, V. Bansal, J. R. Friend, M. Bhaskaran, L. Y. Yeo, and S. Sriram. [**Acoustic--Excitonic Coupling for Dynamic Photoluminescence Manipulation of Quasi-2D MoS2 Nanoflakes**]( Advanced Optical Materials, 2015.
* X. Zhang, J. Yao, M. Ali, J. Wei, H. Wang, L. Y. Yeo, J. R. Friend, and D. R. MacFarlane. **UV/ozone-assisted low temperature preparation of mesoporous TiO2 with tunable phase composition and enhanced solar light photocatalytic activity**. Journal of Materials Chemistry A, 2(44):18791-18795, 2014.
* D. Yudistira, A. Boes, A. R. Rezk, L. Y. Yeo, J. R. Friend, and A. Mitchell. **UV Direct Write Metal Enhanced Redox (MER) Domain Engineering for Realization of Surface Acoustic Devices on Lithium Niobate**. Advanced Materials Interfaces, 1(1400006):1-7, 2014.
* J. E. Sader, M. Yousefi, and J. Friend. **Uncertainty in Least-Squares Fits to the Thermal Noise Spectra of Nanomechanical Resonators with Applications to the Atomic Force Microscope**. Review of Scientific Instruments, 85(025104):1-5, 2014.
* A. Boes, D. Yudistira, T. Crasto, H. Steigerwald, V. Sivan, T. Limboeck, J. Friend, S. Mailis, E. Soergel, and A. Mitchell. **Ultraviolet laser induced domain inversion on chromium coated lithium niobate crystals**. Optical Materials Express, 4(2):241-254, 2014.
* A. M. Gracioso Martins, N. R. Glass, S. Harrison, A. R. Rezk, N. A. Porter, P. D. Carpenter, J. Du Plessis, J. R. Friend, and L. Y. Yeo. [**Toward Complete Miniaturisation of Flow Injection Analysis Systems: Microfluidic Enhancement of Chemiluminescent Detection**]( Analytical Chemistry, 86(21):10812-10819, 2014.
* L. Y. Yeo and J. R. Friend. **Surface Acoustic Wave Microfluidics**. Annual Review of Fluid Mechanics, 46:379-406, 2014.
* D. Yudistira, A. Boes, A. R. Rezk, L. Y. Yeo, J. R. Friend, and A. Mitchell. **Surface Acoustic Devices: UV Direct Write Metal Enhanced Redox (MER) Domain Engineering for Realization of Surface Acoustic Devices on Lithium Niobate (Adv. Mater. Interfaces 4/2014)**. Advanced Materials Interfaces, 1(1400006):1-7, 2014.
* A. R. Rezk, J. R. Friend, and L. Y. Yeo. [**Simple, low cost MHz-order acoustomicrofluidics using aluminium foil electrodes**]( Lab Chip, 14:1802-1805, 2014.
* X. Zhang, J. Yao, D. Li, X. Chen, H. Wang, L. Y. Yeo, and J. R. Friend. **Self-assembled highly crystalline TiO$_2$ mesostructures for sunlight-driven, pH-responsive photodegradation of dyes**. Materials Research Bulletin, 55:13-18, 2014.
* J. Guo, J. L. W. Li, Y. Chen, L. Y. Yeo, J. R. Friend, and Y. Kang. **RF-activated standing surface acoustic wave for on-chip particle manipulation**. IEEE Transactions on Microwave Theory and Techniques, 62(9):1898-1904, 2014.
* A. R. Rezk, L. Y. Yeo, and J. R. Friend. **Poloidal Flow and Toroidal Particle Ring Formation in a Sessile Drop Driven by Megahertz Order Vibration**. Langmuir, 30(37):11243--11247, 2014.
* J. E. Sader and J. R. Friend. **Note: Calibration of atomic force microscope cantilevers using only their resonant frequency and quality factor**. Review of Scientific Instruments, 85(11):116101, 2014.
* X. Zhang, W. Lu, J. Dai, L. Bourgeois, J. Yao, H. Wang, J. R. Friend, D. Zhao, and D. R. MacFarlane. **Nanofabrication of highly ordered, tunable metallic mesostructures via quasi-hard-templating of lyotropic liquid crystals**. Scientific Reports, 4(7420):1-5, 2014.
* D. Yudistira, A. Boes, B. Djafari-Rouhani, Y. Pennec, L. Y. Yeo, A. Mitchell, and J. R. Friend. [**Monolithic Phononic Crystals with a Surface Acoustic Band Gap from Surface Phonon-Polariton Coupling**]( Phys. Rev. Lett., 113:215503, 2014.
* A. Al-Abboodi, R. Tjeung, P. M. Doran, L. Y. Yeo, J. Friend, Y. Chan, and P. Pui. **In Situ Generation of Tunable Porosity Gradients in Hydrogel-Based Scaffolds for Microfluidic Cell Culture**. Advanced healthcare materials, 3(10):1655-1670, 2014.
* A. Qi, S. P. Hoo, J. Friend, L. Yeo, Z. Yue, and P. P. Chan. **Hydroxypropyl Cellulose Methacrylate as a Photo-Patternable and Biodegradable Hybrid Paper Substrate for Cell Culture and Other Bioapplications**. Advanced healthcare materials, 3(4):543--554, 2014.
* M. Miansari, J. R. Friend, P. Banerjee, M. Majumder, and L. Y. Yeo. **Graphene-Based Planar Nanofluidic Rectifiers**. The Journal of Physical Chemistry C, 118(38):21856-21865, 2014.
* M. B. Dentry, L. Y. Yeo, and J. R. Friend. [**Frequency effects on the scale and behavior of acoustic streaming**]( Phys. Rev. E, 89:013203, 2014.
* A. Rajapaksa, A. Qi, L. Yeo, R. Coppel, and J. R. Friend. [**Enabling Practical Surface Acoustic Wave Nebulizer Drug Delivery via Amplitude Modulation**]( Lab Chip, 14(11):1858-1865, 2014.
* A. E. Rajapaksa, J. J. Ho, A. Qi, R. Bischof, T.-H. Nguyen, M. Tate, D. Piedrafita, M. P. McIntosh, L. Y. Yeo, E. Meeusen, R. L. Coppel, and J. R. Friend. [**Effective pulmonary delivery of an aerosolized plasmid DNA vaccine via surface acoustic wave nebulization**]( Respiratory Research, 15(60):1-12, 2014.
* A. Rezk, O. Manor, L. Y. Yeo, and J. R. Friend. **Double Flow Reversal in Thin Liquid Films Driven by MHz Order Surface Vibration**. Proceedings of the Royal Society A, 470(20130765)2014.
* X. Zhang, Y. Zheng, D. G. McCulloch, L. Y. Yeo, J. R. Friend, and D. R. Macfarlane. **Controlled morphogenesis and self-assembly of bismutite nanocrystals into three-dimensional nanostructures and their applications**. Journal of Materials Chemistry A, 2(7):2275-2282, 2014.
* K. Kulkarni, L. Y. Yeo, J. R. Friend, and P. Perlmutter. **An emerging reactor technology for chemical synthesis: Surface acoustic wave-assisted closed-vessel Suzuki coupling reactions**. Ultrasonics Sonochemistry, 21(4):1305-1309, 2014.

Research details

Mild traumatic brain injury in C. elegans via cavitation-free surface acoustic wave-driven ultrasonic irradiation 

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Mild traumatic brain injury is an all-too-common outcome from modern warfare and sport, and lacks a reproducible model for assessment of potential treatments and protection against it. Here we consider the use of surface acoustic wave (SAW) irradiation of C. elegans worms—without cavitation— as a potential, ethically reasonable animal-on-a-chip model for inducing traumatic brain injury in an animal, producing significant effects on memory and learning that could prove useful in a model that progress from youth to old age in but a few weeks. We show a significant effect by SAW on the ability of worms to learn post-exposure through associative learning chemotaxis. At higher SAW intensity, we find immediate, thorough, but temporary paralysis of the worms. We further explore the importance of homogeneous exposure of the worms to the SAW-driven ultrasound, an aspect poorly controlled in past efforts, if at all, and demonstrate the absence of cavitation through a change in fluids from a standard media for the worms to the exceedingly viscous polyvinyl alcohol. Likewise, we demonstrate that acoustic streaming, when present, is not directly responsible for paralysis nor learning disabilities induced in the worm, but is beneficial at low amplitudes to ensuring homogeneous ultrasound exposure. Appears in Scientific Reports.
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Micro/nano acoustofluidics: materials, phenomena, design, devices, and applications (review)

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Acoustic actuation of fluids at small scales may finally enable a comprehensive lab-on-a-chip revolution in microfluidics, overcoming long-standing difficulties in fluid and particle manipulation on-chip. In this com- prehensive review, we examine the fundamentals of piezoelectricity, piezoelectric materials, and trans- ducers; revisit the basics of acoustofluidics; and give the reader a detailed look at recent technological ad- vances and current scientific discussions in the discipline. Recent achievements are placed in the context of classic reports for the actuation of fluid and particles via acoustic waves, both within sessile drops and closed channels. Other aspects of micro/nano acoustofluidics are examined: atomization, translation, mixing, jetting, and particle manipulation in the context of sessile drops and fluid mixing and pumping, par- ticle manipulation, and formation of droplets in the context of closed channels, plus the most recent re- sults at the nanoscale. These achievements will enable applications across the disciplines of chemistry, biology, medicine, energy, manufacturing, and we suspect a number of others yet unimagined. Basic de- sign concepts and illustrative applications are highlighted in each section, with an emphasis on lab-on-a- chip applications.
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Improving and predicting fluid atomization via hysteresis-free thickness vibration of lithium niobate

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Acoustically driven atomization from the broad perspective of materials choice, vibration mode, and fluid characteristics is considered to identify a simple method for improving both the understanding of the atomization phenomena and the overall efficiency of atomization. Whether by the definition of a “figure of merit”, its output vibration displacement at a given input power, or the fluid flow rate during atomization, it is found that the combination of single-crystal 127.86° Y-rotated lithium niobate and thickness- mode vibration produces an order of magnitude greater atomization flow rate and efficiency in comparison to all known atomizers, including classic lead zirconate-based devices and newer, Rayleigh wave or Rayleigh/Lamb spurious-mode-based devices alike. By using this improved approach, for the first time, fluids with viscosities up to 48 cP are reported to be atomized, and an atomization Reynolds number ReA is defined which can be used to both predict the atomization flow rate for ReA ≳ 40 and the inability to atomize a given fluid at a particular vibration amplitude when ReA ≲ 40.
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Decontaminating surfaces with atomized disinfectants generated by a novel thickness-mode lithium niobate device

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It is now possible to freely manipulate fluids at the nanoscale using acoustics. We are looking at ways to manipulate attoliter to femtoliter volumes of fluid in droplet form in nanostructures without concern for evaporation, and without the use of surrounding oil. We are also looking at extremely high pressures from new forms of acoustic streaming due to interference between the nanostructures as the acoustic waves propagate through. Applications that are anticipated include acoustophoresis and, molecular separation, deagglomeration, and analysis. Our paper in Advanced Functional Materials reports the initial results.
Link to paper
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Driving morphological changes in magnetic nanoparticle structures through the application of acoustic waves and magnetic fields

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The growing interest in acoustic manipulation of particles in micro to nanofluidics using surface acoustic waves, together with the many applications of magnetic nanoparticles—whether individual or in arrays—underpins our discovery of how these forces can be used to rapidly, easily, and irreversibly form 1D chains and 2D films. These films and chains are difficult to produce by other methods yet offer many advantages over suspensions of individual nanoparticles by making use of the scale of the structures formed, 10􏰀9 to 10􏰀5 m, and by taking a balance of the relevant external and interparticle forces, the underlying mechanisms responsible for the phenomena become apparent. For loosely connected 1D chains, the magnetic field alone is sufficient, though applying an acoustic field drives a topology change to interconnected loops of 􏰁10–100 particles. Increasing the acoustic field intensity drives a transition from these looped structures to dense 2D arrays via interparticle Bjerknes forces. Inter-particle drainage of the surrounding fluid leaves these structures intact after removal of the externally applied forces. The self-evident morphology transitions depend solely upon the relative amplitudes of the Brownian, Bjerknes, and magnetic forces.
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A Local Nanofiber-Optic Ear

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The development of acoustic sensors that are compact, have simple read-out mechanisms, and have geometries that enable them to be inserted/embedded deep in materials is of great interest for acoustic-based imaging technologies and novel analytical instruments. Fiber-optic-based detectors are the most logical choice to satisfy these demands, but scaling down the size to sub-micrometer dimensions and uncovering transduction mechanisms that can be more robust than interferometric techniques in dynamic environments has been challenging. In this work, we demonstrate a non-interference-based acoustic ear that utilizes the movement of plasmonic nanoparticles embedded in the near field of a nanofiber optic. The modulated optical signal induced by sound waves can be read-out through transmission through the nanofiber or by tracking the scattering of the nanoparticles in the far field. By utilizing a thin, compressible cladding on the nanofibers, acoustic intensities of <10−8 W/m2 can be detected by the devices over an interaction area of <4 μm2, representing a measured acoustic power at the sensor of 10−21 W. With the ability to modify the mechanical properties of the cladding, change the size of the plasmonic nanoparticle, and alter the guided wavelength, the performance of this platform is highly tunable and ideal for compact, deep-body acoustic probes.
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From Acoustofluidics and Acoustic Microfluidics to Acoustic Nanofluidics

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It is now possible to freely manipulate fluids at the nanoscale using acoustics. We are looking at ways to manipulate attoliter to femtoliter volumes of fluid in droplet form in nanostructures without concern for evaporation, and without the use of surrounding oil. We are also looking at extremely high pressures from new forms of acoustic streaming due to interference between the nanostructures as the acoustic waves propagate through. Applications that are anticipated include acoustophoresis and, molecular separation, deagglomeration, and analysis. Our paper in Advanced Functional Materials reports the initial results.
Link to paper

Cell agglomeration in the wells of a 24-well plate using acoustic streaming

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Cell agglomeration is essential both to the success of drug testing and to the development of tissue engi- neering. Here, a MHz-order acoustic wave is used to generate acoustic streaming in the wells of a 24-well plate to drive particle and cell agglomeration. Acoustic streaming is known to manipulate particles in microfluidic devices, and even provide concentration in sessile droplets, but concentration of particles or cells in individual wells has never been shown, principally due to the drag present along the periphery of the fluid in such a well. The agglomeration time for a range of particle sizes suggests that shear-induced migration plays an important role in the agglomeration process. Particles with a diameter of 45 μm ag- glomerated into a suspended pellet under exposure to 2.134 MHz acoustic waves at 1.5 W in 30 s. Addi- tionally, BT-474 cells also agglomerated as adherent masses at the center bottom of the wells of tissue- culture treated 24-well plates. By switching to low cell binding 24-well plates, the BT-474 cells formed suspended agglomerations that appeared to be spheroids, fully fifteen times larger than any cell agglomer- ates without the acoustic streaming. In either case, the viability and proliferation of the cells were maintained despite acoustic irradiation and streaming. Intermittent excitation was effective in avoiding tem- perature excursions, consuming only 75 mW per well on average, presenting a convenient means to form fully three-dimensional cellular masses potentially useful for tissue, cancer, and drug research.
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Flow Reversal using Acoustic Waves

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Fluid films exposed to acoustic waves reverse direction not once but twice: submicron films have now been found to flow with the acoustic wave, while slightly thicker films flow against it, and even thicker films flow with the acoustic wave again. The peculiar behavior is explored in our paper in the Proceedings of the Royal Society A here.
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Vibration-Induced Deagglomeration and Shear-Induced Alignment of Carbon Nanotubes in Air

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Carbon nanotubes (CNTs) are widely known to agglomerate into difficult to separate, 10–100 μm bundles, even after suspension in solution. Here, a dry and rapid (≈10 s) method to deagglomerate bulk, unbound multi-walled CNT bundles due to surface acoustic waves (SAW) in a piezoelectric substrate is reported for the first time in Advanced Functional Materials here.
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Mixed mode of dissolving immersed nanodroplets at a solid-water interface

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Carbon nanotubes (CNTs) are widely known to agglomerate into difficult to separate, 10–100 μm bundles, even after suspension in solution. Here, a dry and rapid (≈10 s) method to deagglomerate bulk, unbound multi-walled CNT bundles due to surface acoustic waves (SAW) in a piezoelectric substrate is reported for the first time in Advanced Functional Materials here.
Link to paper
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Nozzleless spray cooling using surface acoustic waves

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Surface acoustic wave (SAW) atomization is an attractive approach for generating monodispersed microdroplets for a diversity of applications, from drug delivery to mass spectrometry, due to its reliability, miniaturizability, and portability. Here, we demonstrate a nozzleless spray cooling technique based on SAW atomization, with the key advantage of downward scalability: increasing the operating frequency facilitates the fabrication of a chip-sized atomizer to use in compact cooling of electronic devices. Using deionised water, cooling is improved by 15% when the atomization rate is increased by 40%; when the gap separating the SAW device and heat source is halved, the cooling is improved by 20%. By constructing the device such that the atomized droplets are easily deposited upstream of the flow circulation, the performance is improved further. The atomization of CuO nanoparticle suspensions (at 3%) increased the cooling performance by 30%. The paper is published in the Journal of Aerosol Science.
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Monolithic Phononic Crystals with a Surface Acoustic Band Gap

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We theoretically and experimentally demonstrate the existence of complete surface acoustic wave band gaps in surface phonon-polariton phononic crystals, in a completely monolithic structure. The manuscript appears in Physical Review Letters here.
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Calibration of atomic force microscope cantilevers using only their resonant frequency and quality factor

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Using laser Doppler vibrometry, we rigorously assess the accuracy of a new method for calibrating atomic force microscope cantilevers, employing a series of commercially available cantilevers and exploring their performance under non-ideal conditions. This shows that the simplified method is highly accurate and can be easily implemented to perform fast, robust, and non-invasive spring constant calibration. Paper is reported online at the Review of Scientific Instruments.
Link to paper
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Acoustic–Excitonic Coupling for Dynamic Photoluminescence Manipulation of Quasi-2D MoS2 Nanoflakes

Our paper appears in Advanced Optical Materials. It shows that a surface acoustic wave (SAW) can effectively and reversibly modulate the photoluminescence in quasi-2D MoS2. This technique does not result in any structural or compositional change, which suggests the tremendous potential of integrating 2D materials on to SAW platforms for designing controllable, state-of-the-art optoelectronic systems. Thanks to Sharath Sriram’s leadership it made the cover of AOM.

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Microscale anechoic architecture: acoustic diffusers for ultra low power microparticle separation via traveling surface acoustic waves

This paper presents a versatile and very low-power traveling SAW microfluidic sorting device able to displace and separate particles of different diameter in aqueous suspension; the travelling wave propagates through the fluid bulk and diffuses via a Schröder diffuser, adapted from its typical use in concert hall acoustics to be the smallest such diffuser to be suitable for microfluidics. The effective operating power range is two to three orders of magnitude less than current SAW devices, uniquely eliminating the need for amplifiers, and by using traveling waves to impart forces directly upon suspended microparticles, they can be separated by size. The manuscript appears in Lab on a Chip.

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Extensional viscosity of copper nanowire suspensions in an aqueous polymer solution

This paper looks at the behavior of fluids that carry copper nanowires (left above) in suspension as an example of an extremely long and narrow nanoparticle. The purpose is to determine the fluid rheology so that such particles can be deposited on surfaces for a diverse range of applications, from solar cells to conductive electrodes as a replacement for the dwindling supply of indium.

Extensional viscosities from 3 mPa⋅s (1 mPa⋅s shear viscosity) to 37.2 Pa⋅s were observed via changes in the suspension concentration, thus capturing low viscosities that have been historically very challenging to measure (right above). These changes equate to an increase in the relative extensional viscosity of nearly 12,200 times at a volume fraction of just 0.027. Interactions between the wires and the necessary polymer additive strongly affect the rheology, with a reduction in the elasticity of the fluid as the buffer relaxation time falls from 819 to 59 μs above a critical particle concentration.

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Toward Complete Miniaturisation of Flow Injection Analysis Systems: Microfluidic Enhancement of Chemiluminescent Detection

Conventional flow injection systems for aquatic environmental analysis typically comprise large laboratory benchscale equipment, which place considerable constraints for portable field use. Here, we demonstrate the use of an integrated acoustically driven microfluidic mixing scheme to enhance detection of a chemiluminescent species tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate—a common chemiluminescent reagent widely used for the analysis of a wide range of compounds such as illicit drugs, pharmaceuticals, and pesticides.

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Laser technique offers nano breakthrough

Using the new technique, RMIT University researchers have been able to more easily and efficiently produce surface acoustic waves on a piezoelectric chip - a type of crystal that generates these waves when certain electrical voltages are applied to it.

Surface acoustic waves are sound waves used in mobile and optical telecommunications and have ground-breaking potential applications in the fields of ultrasound technology, energy harvesting and sensors for harsh environments.

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Effective pulmonary delivery of an aerosolized plasmid DNA vaccine via surface acoustic wave nebulization

We have demonstrated the ability—for the first time—to effectively transfect a large animal (sheep) with a naked plasmid DNA-based influenza vaccine (H1N1 Solomon Islands) using nebulisation of the pDNA and inhalation delivery. The work is published in Respiratory Research here and a PDF of the work is available here.

Pulmonary-delivered gene therapy promises to mitigate vaccine safety issues and reduce the need for needles and skilled personnel to use them. While plasmid DNA (pDNA) offers a rapid route to vaccine production without side effects or reliance on cold chain storage, its delivery to the lung has proved challenging. Conventional methods, including jet and ultrasonic nebulizers, fail to deliver large biomolecules like pDNA intact due to the shear stresses present during nebulization. In vitro structural analysis followed by in vivo protein expression studies served in assessing the integrity of the pDNA subjected to surface acoustic wave (SAW) nebulisation. In vivo immunization trials were then carried out in rats using SAW nebulized pDNA (influenza A, human hemagglutinin H1N1) condensate delivered via intratracheal instillation. Finally, in vivo pulmonary vaccinations using pDNA for influenza was nebulised and delivered via respirator to sheep.

The SAW nebulizer was effective at generating pDNA aerosols with sizes optimal for deep-lung delivery. Successful gene expression was observed in mouse lung epithelial cells, when SAW-nebulized pDNA was delivered to a male Swiss mouse via intratracheal instillation. Effective systemic and mucosal antibody responses was found in rats via post-nebulized, condensed fluid instillation. Significantly, we demonstrated the suitability of the SAW nebulizer to administer unprotected pDNA encoding an influenza A virus surface glycoprotein to respirated sheep via aerosolized inhalation.

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Self-assembled highly crystalline TiO2 mesostructures for sunlight-driven, pH-responsive photodegradation of dyes

Dyes are a routine contaminant of water, breaking down slowly, if at all, and presenting substantial environmental risks for a rapidly dwindling resource: water. We have now a route to breaking down such dyes using nanostructured titania, that, upon exposure to sunlight, effectively break down the dyes as reported in the Materials Research Bulletin.

The development of new strategies and photocatalytic materials for practical environmental solutions remains a great challenge, particularly due to the large energy demands associated with various remediation processes. In this paper, we report the fabrication of self-assembled ordered mesoporous TiO2 with highly crystalline anatase structures as well as high surface area, and characterize their photocatalytic performance on the degradation of three typical dyes, including anionic methyl orange, cationic methylene blue, and neutral rhodamine B driven merely by sunlight. The results show that the dye photodegradation strongly depends on the charging state of both mesoporous TiO2 surface and dyes, which can be adjusted by the pH value of the solutions. Such charge-dependent photocatalytic functionality of mesoporous TiO2 can thus be exploited for highly efficient and selective dye photodegradation.

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Simple and Low Cost Acoustomicrofluidics Using Aluminium Foil

While some have adopted our SAW microfluidics devices and are researching that, we’ve found an entirely easier way to accomplish the same outcomes: using simple aluminum foil electrodes, published in Lab on a Chip here, and available for download here. We’ve also put a patent out this and related technologies fro commercialisation.

We show that it is possible to circumvent the necessity for costly and complex cleanroom fabrication procedures required for the production of MHz-order acoustically- driven microfluidic platforms through the use of electrode strips simply cut from kitchen aluminium foil. By completely removing the high device production costs and addressing issues associated with the reliability of complicated electrode technology, this exceptionally simple and low-cost acoustofluidic platform, on which we demonstrate rapid and efficient fluid transport and manipulation, microcentrifugation, and, remarkably, even nebulisation, in both sessile drops as well as paper-based substrates, is therefore a significant step closer towards commercially-viable consumer diagnostic devices, especially for use in the developing world.

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Enabling Practical Surface Acoustic Wave Nebulizer Drug Delivery via Amplitude Modulation

A practical, commercially viable microfluidic device relies upon the miniaturization and integration of all its components—including pumps, circuitry, and power supply—onto a chip-based platform. Surface acoustic waves (SAW) have become popular in microfluidic manipulation, in solving the problems of microfluidic manipulation, but practical applications employing SAW still require more power than available via a battery. Introducing amplitude modulation at 0.5–40 kHz in SAW nebulization, which requires the highest energy input levels of all known SAW microfluidic processes, halves the power required to 1.5 W even while including the power in the sidebands, suitable for small lithium ion batteries, and maintains the nebulization rate, size, and size distributions vital to drug inhalation therapeutics. This simple yet effective means to enable an integrated SAW microfluidics device for nebulization exploits the relatively slow hydrodynamics and is furthermore shown to deliver shear-sensitive biomolecules—plasmid DNA and antibodies as exemplars of future pulmonary gene and vaccination therapies—undamaged in the nebulized mist. Altogether, the approach demonstrates a means to offer truly micro-scale microfluidics devices in a handheld, battery powered SAW nebulization device.

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An emerging reactor technology for chemical synthesis: Surface acoustic wave-assisted closed-vessel Suzuki coupling reactions

In this paper we demonstrate the use of an energy-efficient surface acoustic wave (SAW) device for driving closed-vessel SAW-assisted (CVSAW), ligand-free Suzuki couplings in aqueous media. The reactions were carried out on a µmolar scale with low to ultra-low catalyst loadings. The reactions were driven by heating resulting from the penetration of acoustic energy derived from RF Raleigh waves generated by a piezoelectric chip via a renewable fluid coupling layer. The yields were uniformly high and the reactions could be executed without added ligand and in water. In terms of energy density this new technology was determined to be roughly as efficient as microwaves and superior to ultrasound.

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Uncertainty in least-squares fits to the thermal noise spectra of nanomechanical resonators with applications to the atomic force microscope

Thermal noise spectra of nanomechanical resonators are used widely to characterize their physical properties. These spectra typically exhibit a Lorentzian response, with additional white noise due to extraneous processes. Least-squares fits of these measurements enable extraction of key parameters of the resonator, including its resonant frequency, quality factor, and stiffness. Here, we present general formulas for the uncertainties in these fit parameters due to sampling noise inherent in all thermal noise spectra. Good agreement with Monte Carlo simulation of synthetic data and measurements of an Atomic Force Microscope (AFM) cantilever is demonstrated. These formulas enable robust interpretation of thermal noise spectra measurements commonly performed in the AFM and adaptive control of fitting procedures with specified tolerances.

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Frequency effects on the scale and behavior of acoustic streaming

From Physical Review E, indicating what happens to acoustic streaming as you continue to increase in frequency from low MHz to GHz order.

Acoustic streaming underpins an exciting range of fluid manipulation phenomena of rapidly growing significance in microfluidics, where the streaming often assumes the form of a steady, laminar jet emanating from the device surface, driven by the attenuation of acoustic energy within the beam of sound propagating through the liquid. The frequencies used to drive such phenomena are often chosen ad hoc to accommodate fabrication and material issues. In this work, we seek a better understanding of the effects of sound frequency and power on acoustic streaming. We present and, using surface acoustic waves, experimentally verify a laminar jet model that is based on the turbulent jet model of Lighthill, which is appropriate for acoustic streaming seen at micro- to nanoscales, between 20 and 936 MHz and over a broad range of input power. Our model eliminates the critically problematic acoustic source singularity present in Lighthill’s model, replacing it with a finite emission area and enabling determination of the streaming velocity close to the source, predicting (as shown in figure above) a transition from laminar jetting to turbulent jetting in a small water drop.

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Controlled morphogenesis and self-assembly of bismutite nanocrystals into three-dimensional nanostructures and their applications

From the Journal of Materials Chemistry A, showing the complex micro/nanostructure formed by Na3C6H5O7 at different pH values.

It is now possible to control the morphogenesis and self-assembly of bismutite nanocrystals with fully tunable morphologies from square plates, octagonal sheets, and round disks into three-dimensional hierarchical nanostructures. The results show that the nucleation, growth and self-assembly of bismutite nanocrystals strongly depend on the synergistic effect between hydroxide and citrate ions. The three-dimensional hierarchical nanostructures are formed through an oriented-attachment of bismutite nanocrystals along the 〈001〉 directions. The bismutite hierarchical nanostructures can be utilized for efficient and selective adsorption and separation. A novel surface-enhanced Raman spectroscopy platform based on a bismutite/gold nanoparticles core–shell structure has been developed for ultrasensitive detection of aromatic molecules with a detection limit down to 1 nM. 

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Ultraviolet laser-induced domain inversion on chromium-coated lithium niobate crystals

Direct UV laser writing on chromium coated lithium niobate (LiNbO3) crystals is found to produce spontaneous domain inversion associated with the exposed UV laser tracks. Experimental evidence suggests that this effect is attributed to local out-diffusion of oxygen, reducing the LiNbO3 crystal surface due to the presence of chromium. The thin chromium film becomes hot and reactive after absorbing the UV laser radiation thus acting as an oxygen getter. This very efficient process enables the inversion of domains at lower intensities as compared to other direct laser based poling methods practically eliminating the deleterious surface damage induced by the direct absorption of the UV laser radiation by the crystal. Furthermore, the versatility of this domain fabrication method, is demonstrated by the production of inverted domain structures on Z-, Y- and 128 deg YX-cut substrates.

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Continuous flow actuation between external reservoirs in small-scale devices driven by surface acoustic waves

We have designed and characterized a surface acoustic wave (SAW) fluid actuation platform that significantly improves the transmission of sound energy from the SAW device into the fluid in order to obtain enhanced performance. This is in distinct contrast to previous SAW microfluidic devices where the SAW substrate is simply interfaced with a microchannel without due consideration given to the direction at which the sound energy is transmitted into the fluid, thus resulting in considerable reflective and dissipative losses due to reflection and absorption at the channel walls. For the first time, we therefore demonstrate the ability for continuous fluid transfer between independent reservoirs driven by the SAW in a miniature device, and report the associated pressure–flow rate relationship, in which a maximum flowrate of 100 μl/min and pressure of 15 Pa was obtained. The pumping efficiency is observed to increase with input power, and, at peak performance, offers an order-of-magnitude improvement over that of existing SAW micropumps that have been reported to date.

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Hydroxypropyl Cellulose Methacrylate as a Photo‐Patternable and Biodegradable Hybrid Paper Substrate for Cell Culture and Other Bioapplications

In addition to the choice of appropriate material properties of the tissue construct to be used, such as its biocompatibility, biodegradability, cytocompatibility, and mechanical rigidity, the ability to incorporate microarchitectural patterns in the construct to mimic that found in the cellular microenvironment is an important consideration in tissue engineering and regenerative medicine. Both these issues are addressed by demonstrating a method for preparing biodegradable and photo-patternable constructs, where modified cellulose is cross-linked to form an insoluble structure in an aqueous environment. Specifically, hydroxypropyl cellulose (HPC) is rendered photocrosslinkable by grafting with methylacrylic anhydride, whose linkages also render the cross-linked construct hydrolytically degradable. The HPC is then cross-linked via a photolithography-based fabrication process. The feasibility of functionalizing these HPC structures with biochemical cues is verified post-fabrication, and shown to facilitate the adhesion of mesenchymal progenitor cells. The HPC constructs are shown to be biocompatible and hydrolytically degradable, thus enabling cell proliferation and cell migration, and therefore constituting an ideal candidate for long-term cell culture and implantable tissue scaffold applications. In addition, the potential of the HPC structure is demonstrated as an alternative substrate to paper microfluidic diagnostic devices for protein and cell assays.

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Polariton-based band gap and generation of surface acoustic waves in acoustic superlattice lithium niobate

We report the presence of surface acoustic wave (SAW, a) band gap (b) on acoustic superlattice (ASL) in a single-crystal lithium niobate structure. The calculated band gap appears at a frequency twice the value expected from purely acoustic Bragg scattering. We have identified the band gap as originating from a polariton-based mechanism due to the coupling between the electromagnetic wave and the surface vibrations. We have examined the influence of the band gap on SAW generation with the ASL and have shown that the © calculated frequency resonance of the SAW lies in the vicinity of the upper stop-band edges. This results in the localization of the SAW in the ASL. Experimental confirmation is achieved through direct measurement of the SAW displacement by laser vibrometry on an actual ASL SAW transducer.

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Microscale Capillary Wave Turbulence Excited by High Frequency Vibration

Low frequency (O(10 Hz–10 kHz)) vibration excitation of capillary waves has been extensively studied for nearly two centuries. Such waves appear at the excitation frequency or at rational multiples of the excitation frequency through nonlinear coupling due to the finite displacement of the wave, most often at one-half the excitation frequency in so-called Faraday waves and twice this frequency in superharmonic waves. Less understood, however, are the dynamics of capillary waves driven by high frequency vibration (>O(100 kHz)) and small interface length scales, an arrangement ideal for a broad variety of applications, from nebulisers for pulmonary drug delivery to complex nanoparticle synthesis. In the few studies conducted to date, a marked departure from the predictions of classical Faraday wave theory has been shown, with the appearance of broadband capillary wave generation from 100 Hz to the excitation frequency and beyond, but there has not yet been a clear explanation. We show that weak wave turbulence is the dominant mechanism in the behavior of the system, evident from wave height frequency spectra that closely follows the Rayleigh-Jeans spectral response η ∼ ω^(−17/12) as a consequence of a period-halving, weakly turbulent cascade that appears within a 1 mm water drop whether driven by thickness-mode or surface acoustic Rayleigh wave excitation. However, such a cascade is one-way, from low to high frequencies. The mechanism of exciting the cascade with high frequency acoustic waves is an acoustic streaming-driven turbulent jet in the fluid bulk, driving the fundamental capillary wave resonance through the well-known coupling between bulk flow and surface waves. Unlike capillary waves, turbulent acoustic streaming can exhibit subharmonic cascades from high to low frequencies; here it appears from the excitation frequency all the way to the fundamental modes of the capillary wave some four orders of magnitude in frequency less than the excitation frequency, enabling the capillary weakly turbulent wave cascade to form from the fundamental capillary wave upwards. 

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Unique fingering instabilities and soliton-like wave propagation in thin acoustowetting films

Acoustic–fluid interactions not only has had a long history but has recently experienced renewed scrutiny because of their vast potential for microscale fluid and particle manipula- tion. Here we unravel a fascinating and anomalous ensemble of dynamic ‘acoustowetting’ phenomena in which a thin film drawn from a sessile drop first spreads in opposition to the acoustic wave propagation direction. The advancing film front then exhibits fingering instabilities akin to classical viscous fingering, but arising through a different and novel mechanism: transverse Fresnel diffraction of the underlying acoustic wave. Peculiar ‘soliton-like’ wave pulses are observed to grow above these fingers, which, on reaching a critical size, translate away along the wave propagation direction. By elucidating the complex hydrodynamics underpinning the spreading, and associated flow reversal and instability phenomena, we offer insight into the possibility of acoustically controlling fast and uniform film spreading, constituting a flexible and powerful alternative for microfluidic transport.

Figure: (a) The setup with oil drop, (b) top view with fluorescence, © exposure to SAW draws a film toward the acoustic source, (d) the leading edge of the film deepens and forms rapidly moving drops away from the source, and (e) this develops into a broadly occurring phenomenon that maintains mass conservation.

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Precise drop dispensation on superhydrophobic surfaces using acoustic nebulization

The adhesion forces of liquid drops on superhydrophobic surfaces are typically in the nano-Newton range which presents problems in their dispensation from pipettes. Furthermore, since the liquid adheres more strongly to the pipette tip, some portion of the liquid will tend to remain on the tip, causing inaccuracy in the volume dispensed. We advance a novel approach here, in which the spray from an acoustic nebulizer is sent to a superhydrophobic receptacle and the volume ascertained precisely using a weighing scale. The volume dispensed was found to vary linearly with the operation time of the nebulizer.

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Fast Surface Acoustic Wave-Matrix-Assisted Laser Desorption Ionization Mass Spectrometry of Cell Response from Islets of Langerhans

A desire for higher speed and performance in molecular profiling analysis at a reduced cost is driving a trend in miniaturization and simplification of procedures. Here we report the use of a surface acoustic wave (SAW) atomizer for fast sample handling in matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) peptide and protein profiling of Islets of Langerhans, for future type 2 diabetes (T2D) studies. Here the SAW atomizer was used for ultrasound (acoustic) extraction of insulin and other peptide hormones released from freshly prepared islets, stimulated directly on a membrane. A high energy propagating SAW atomizes the membrane-bound liquid into approximately 2 μm diameter droplets, rich in cell-released molecules. Besides acting as a sample carrier, the membrane provides a purification step by entrapping cell clusters and other impurities within its fibers. A new SAW-based sample-matrix deposition method for MALDI MS was developed and characterized by a strong insulin signal, and a limit of detection (LOD) lower than 100 amol was achieved. Our results support previous work reporting the SAW atomizer as a fast and inexpensive tool for ultrasound, membrane-based sample extraction. When interfaced with MALDI MS, the SAW atomizer constitutes a valuable tool for rapid cell studies. Other biomedical applications of SAW-MALDI MS are currently being developed, aiming at fast profiling of biofluids.

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Atomization off thin water films generated by high-frequency substrate wave vibrations

Generating aerosol droplets via the atomization of thin aqueous films with high frequency surface acoustic waves (SAWs) offers several advantages over existing nebulization methods, particularly for pulmonary drug delivery, offering droplet sizes in the 1–5 μm range ideal for effective pulmonary therapy. Nevertheless, the physics underlying SAW atomization is not well understood, especially in the context of thin liquid film formation and spreading and how this affects the aerosol production. Here, we demonstrate that the film geometry, governed primarily by the applied power and frequency of the SAW, indeed plays a crucial role in the atomization process and, in particular, the size of the atomized droplets. In contrast to the continuous spreading of low surface energy liquids atop similar platforms, high surface energy liquids such as water, in the present case, are found to undergo transient spreading due to the SAW to form a quasisteady film whose height is determined by self-selection of the energy minimum state associated with the acoustic resonance in the film and whose length arises from a competition between acoustic streaming and capillary effects. This is elucidated from a fundamental model for the thin film spreading behavior under SAW excitation, from which we show good agreement between the experimentally measured and theoretically predicted droplet dimension, both of which consistently indicate a linear relationship between the droplet diameter and the mechanical power coupled into the liquid by the SAW (the latter captured by an acoustic Weber number to the two thirds power, and the reciprocal of the SAW frequency).

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Fluid-structure interaction in deformable microchannels

A polydimethylsiloxane microfluidic device composed of a single microchannel with a thin flexible layer present over a short length along one side of the channel was fabricated and modelled in order to investigate the complex fluid-structure interaction that arises between a flowing fluid and a deformable wall. Experimental measurements of thin layer deformation and pressure drop are compared with predictions of two- and three-dimensional computational models that numerically solve the coupled set of equations governing both the elasticity of the thin layer and the fluid. It is shown that the two-dimensional model, which assumes the flexible thin layer comprises an infinitely wide elastic beam of finite thickness, reasonably approximates a three- dimensional model, and is in excellent agreement with experimental observations of the thin layer profile when the width of the thin layer is beyond a critical value, roughly twice the length of the thin layer.

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Spring constant calibration of atomic force microscope cantilevers of arbitrary shape

Measuring atomic force microscope probes using laser Doppler vibrometry permits measurement of their spring constants without excitation: the thermal excitation from the atomic motion within the cantilever drives the cantilever’s resonances at around 5 pm in amplitude, vibration we can see. 

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The appearance of boundary layers and drift flows due to high-frequency surface waves

The classical Schlichting boundary layer theory is extended to account for the excitation of generalized surface waves in the frequency and velocity amplitude range commonly used in microfluidic applications, including Rayleigh and Sezawa surface waves and Lamb, flexural and surface-skimming bulk waves. These waves possess longitudinal and transverse displacements of similar magnitude along the boundary, often spatiotemporally out of phase, giving rise to a periodic flow shown to consist of a superposition of classical Schlichting streaming and uniaxial flow that have no net influence on the flow over a long period of time. Correcting the velocity field for weak but significant inertial effects results in a non-vanishing steady component, a drift flow, itself sensitive to both the amplitude and phase (prograde or retrograde) of the surface acoustic wave propagating along the boundary. We validate the proposed theory with experimental observations of colloidal pattern assembly in microchannels filled with dilute particle suspensions to show the complexity of the boundary layer, and suggest an asymptotic slip boundary condition for bulk flow in microfluidic applications that are actuated by surface waves.

Image: Spatial comparison between the theoretical variation of the drift velocity along the solid boundary,

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UV epoxy bonding for enhanced SAW transmission and microscale acoustofluidic integration

Surface acoustic waves (SAWs) are appealing as a means to manipulate fluids within lab-on-a-chip systems. However, current acoustofluidic devices almost universally rely on elastomeric materials, especially PDMS, that are inherently ill-suited for conveyance of elastic energy due to their strong attenuation properties. Here, we explore the use of a low-viscosity UV epoxy resin for room temperature bonding of lithium niobate (LiNbO3), the most widely used anisotropic piezoelectric substrate used in the generation of SAWs, to standard micromachined superstrates such as Pyrex1 and silicon. The bonding methodology is straightforward and allows for reliable production of sub- micron bonds that are capable of enduring the high surface strains and accelerations needed for conveyance of SAWs. Devices prepared with this approach display as much as two orders of magnitude, or 20 dB, improvement in SAW transmission compared to those fabricated using the standard PDMS elastomer. This enhancement enables a broad range of applications in acoustofluidics that are consistent with the low power requirements of portable battery-driven circuits and the development of genuinely portable lab-on-a-chip devices. The method is exemplified in the fabrication of a closed-loop bidirectional SAW pumping concept with applications in micro-scale flow control, and represents the first demonstration of closed channel SAW pumping in a bonded glass/LiNbO3 device.

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Multi-degree-of-freedom ultrasonic micromotor for guidewire and catheter navigation: The NeuroGlide actuator

A 240-µm diameter ultrasonic micromotor is presented as a potential solution for an especially difficult task in minimally invasive neurosurgery, navigating a guidewire to an injury in the neurovasculature as the first step of surgery. The peak no-load angular velocity and maximum torque were 600 rad/s and 1.6 nN-m, respectively, and we obtained rotation about all three axes. By using a burst drive scheme, open-loop position and speed control were achieved. The construction method and control scheme proposed in this study remove most of the current limitations in minimally invasive, catheter-based actuation, enabling minimally invasive vascular surgery concepts to be pursued for a broad variety of applications. 

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Miniaturized Lab-on-a-Disc (miniLOAD)

A miniaturized centrifugal microfluidic platform for lab-on-a-chip applications is presented. Unlike its macroscopic Lab-on-a-CD counterpart, the miniature Lab-on-a- Disc (miniLOAD) device does not require moving parts to drive rotation of the disc, is inexpensive, disposable, and significantly smaller, comprising a 10-mm-diameter SU-8 disc fabricated through two-step photolithography. The disc is driven to rotate using surface acoustic wave irradiation incident upon a fluid coupling layer from a pair of offset, opposing single-phase unidirectional transducers patterned on a lithium niobate substrate. The irradiation causes azimuthally oriented acoustic streaming with sufficient intensity to rotate the disc at several thousand revolutions per minute. In this first proof-of-concept, the capability of the miniLOAD platform to drive capillary-based valving and mixing in microfluidic structures on a disc similar to much larger Lab-on-a-CD devices is shown. In addition, the ability to concentrate aqueous particle suspensions at radial positions in a channel in the disc dependent on the particles’ size is demonstrated. To the best of our knowledge, the miniLOAD concept is the first centrifugal microfluidic platform small enough to be self-contained in a handheld device. 

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Uniform mixing in paper-based microfluidic systems using surface acoustic waves

Paper-based microfluidics has recently received considerable interest due to their ease and low cost, making them extremely attractive as point-of-care diagnostic devices. The incorporation of basic fluid actuation and manipulation schemes on paper substrates, however, afford the possibility to extend the functionality of this simple technology to a much wider range of typical lab-on-a-chip operations, given its considerable advantages in terms of cost, size and integrability over conventional microfluidic substrates. We present a convective actuation mechanism in a simple paper-based microfluidic device using surface acoustic waves to drive mixing. Employing a Y-channel structure patterned onto paper, the mixing induced by the 30 MHz acoustic waves is shown to be consistent and rapid, overcoming several limitations associated with its capillary-driven passive mixing counterpart wherein irreproducibilities and nonuniformities are often encountered in the mixing along the channel.

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Template-free Synthesis and Encapsulation Technique for Layer-by-Layer Polymer Nanocarrier Fabrication

The encapsulation of therapeutic molecules within multiple layers of biocompatible and biodegrad- able polymeric excipients allows exquisite design of their release profile, to the extent the drug can be selectively delivered to a specific target location in vivo. Here, we develop a novel technique for the assembly of multilayer polyelectrolyte nanocarriers based on surface acoustic wave atomization as a rapid and efficient alternative to conventional layer-by-layer assembly, which requires the use of a sacrificial colloidal template over which consecutive polyelectrolyte layers are deposited. Polymer nanocarriers are synthesized by atomizing a polymer solution and suspending them within a complementary polymer solution of opposite charge subsequent to their solidification in-flight as the solvent evaporates; reatomizing this suspension produces nanocarriers with a layer of the second polymer deposited over the initial polymer core. Successive atomizationsuspension layering steps can then be repeated to produce as many additional layers as desired. Specifically, we synthesize nanocarriers comprising two and three, and up to eight, alternating layers of chitosan (or polyethyleneimine) and carboxymethyl cellulose within which plasmid DNA is encapsulated and show in vitro DNA release profiles over several days. Evidence that the plasmid’s viability is preserved and hence the potential of the technique for gene delivery is illustrated through efficient in vitro transfection of the encapsulated plasmid in human mesenchymal progenitor and COS-7 cells.

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Surface acoustic wave micromotor with arbitrary axis rotational capability

A surface acoustic wave (SAW) actuated rotary motor is reported here, consisting of a millimeter-sized spherical metal rotor placed on the surface of a lead zirconate titanate piezoelectric substrate upon which the SAW is made to propagate. At the design frequency of 3.2 MHz and with a fixed preload of 41.1 lN, the maximum rotational speed and torque achieved were approximately 1900rpm and 5.37lN-mm, respectively, producing a maximum output power of 1.19lW. The surface vibrations were visualized using laser Doppler vibrometry and indicate that the rotational motion arises due to retrograde elliptical motions of the piezoelectric surface elements. Rotation about orthogonal axes in the plane of the substrate has been obtained by using orthogonally placed interdigital electrodes on the substrate to generate SAW impinging on the rotor, offering a means to generate rotation about an arbitrary axis in the plane of the substrate.

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Principal Investigator: Professor James Friend 
Phone/Zoom: +1-858-26o-95o8 • 
Personal Assistant: Lusia Veksler 
Center for Medical Devices
Department of Mechanical and Aerospace Engineering • Jacobs School of Engineering
Department of Surgery • School of Medicine 
University of California, San Diego
9500 Gilman Drive MC0411
La Jolla, CA 92093-0411

Offices: 344K/345C&F Structural and Materials Engineering Building 
Lab: 320 Structural and Materials Engineering Building 
Maps to parking, laboratories and offices.

Shipping address:
University of California, San Diego
Attn: Prof James Friend, (858) 260-9508
320 SME MC0411
7835 Trade Street
San Diego CA 92121-2460

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