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

Simple, and highly efficient edge-effect surface acoustic wave atomizer

Conventional surface acoustic wave (SAW) atomizers require a direct water supply on the surface, which can be complex and cumbersome. This paper presents a novel SAW atomizer that uses lateral acoustic wetting to achieve atomization without a direct water supply. The device works by simply pressing a piece of wetted paper strip against the bottom of an excited piezoelectric transducer. The liquid then flows along the side to the unmodified surface edge, where it is atomized into a well-converging mist in a stable and sustainable manner. We identified this phenomenon as the edge effect, using numerical simulation results of surface displacement mode. The feasibility of the prototype design was demonstrated by observing and investigating the integrated process of liquid extraction, transport, and atomization. We further explored the hydrodynamic principles of the change and breakup in liquid film geometry under different input powers. Experiments demonstrate that our atomizer is capable of generating high-quality fine liquid particles stably and rapidly even at very high input power. Compared to conventional SAW atomizer, the dispersion of mist width can be scaled down by 70%, while the atomization rate can be increased by 37.5%. Combined with the advantages of easy installation and robustness, the edge effect-based atomizer offers an attractive alternative to current counterparts for applications requiring high efficiency and miniaturization, such as simultaneous synthesis and encapsulation of nanoparticles, pulmonary drug delivery and portable inhalation therapy.

Optimizing coupling layer and superstrate thickness in attachable acoustofluidic devices

Superstrate-based acoustofluidic devices, where the fluidic elements are reversibly coupled to a transducer rather than bonded to it, offer advantages for cost, interchangeability and preventing contamination between samples. A variety of coupling materials can be used to transmit acoustic energies into attachable superstrates, though the dimensions and material composition of the system elements are not typically optimized. This work analyzes these coupling layers for bulk wavefront transmission, including water, ultrasound gel and polydimethylsiloxane (PDMS), as well as the material makeup and thickness of the superstrate component, which is commonly comprised of glass, quartz or silicon. Our results highlight the importance of coupling layer and superstrate dimensions, identifying frequencies and component thicknesses that maximize transmission efficiency. Our results indicate that superstrate thicknesses 0.55 times the acoustic wavelength result in maximal acoustic coupling. While various coupling layers and superstrate materials are capable of similar acoustic energy transmission, the inherent dimensional stability of the PDMS coupling layers, somewhat less common in superstrate work compared to liquid-based agents, presents advantages for practically maximizing acoustic efficiency.

Acoustic resonance effects and cavitation in SAW aerosol generation

The interaction of surface acoustic waves (SAWs) with liquids enables the production of aerosols with adjustable droplet sizes in the micrometer range expelled from a very compact source. Understanding the nonlinear acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is essential for acousto-microfluidics platforms, particularly aerosol generators. In this study, we demonstrate the presence of micro-cavitation in a MHz-frequency SAW aerosol generation platform, which is touted as a leap in aerosol technology with versatile application fields including biomolecule inhalation therapy, micro-chromatography and spectroscopy, olfactory displays, and material deposition. Using analysis methods with high temporal and spatial resolution, we demonstrate that SAWs stabilize spatially arranged liquid micro-domes atop the generator's surface. Our experiments show that these liquid domes become acoustic resonators with highly fluctuating pressure amplitudes that can even nucleate cavitation bubbles, as supported by analytical modeling. The observed fragmentation of liquid domes indicates the participation of three droplet generation mechanisms, including cavitation and capillary-wave instabilities. During aerosol generation, the cavitation bubbles contribute to the ejection of droplets from the liquid domes and also explain observed microstructural damage patterns on the chip surface eventually caused by cavitation-based erosion.

Acoustic Atomization-Induced Pumping Based on a Vibrating Sharp-Tip Capillary

Pumping is an essential component in many microfluidic applications. Developing simple, small-footprint, and flexible pumping methods is of great importance to achieve truly lab-on-a-chip systems. Here, we report a novel acoustic pump based on the atomization effect induced by a vibrating sharp-tip capillary. As the liquid is atomized by the vibrating capillary, negative pressure is generated to drive the movement of fluid without the need to fabricate special microstructures or use special channel materials. We studied the influence of the frequency, input power, internal diameter (ID) of the capillary tip, and liquid viscosity on the pumping flow rate. By adjusting the ID of the capillary from 30 µm to 80 µm and the power input from 1 V_pp to 5 V_pp, a flow rate range of 3 to 520 µL/min can be achieved. We also demonstrated the simultaneous operation of two pumps to generate parallel flow with a tunable flow rate ratio. Finally, the capability of performing complex pumping sequences was demonstrated by performing a bead-based ELISA in a 3D-printed microdevice.

Integrating an aerosolized drug delivery device with conventional static cultures and a dynamic airway barrier microphysiological system

Organ on a chip or microphysiological systems (MPSs) aim to resolve current challenges surrounding drug discovery and development resulting from an unrepresentative static cell culture or animal models that are traditionally used by generating a more physiologically relevant environment. Many different airway MPSs have been developed that mimic alveolar or bronchial interfaces, but few methods for aerosol drug delivery at the air-liquid interface exist. This work demonstrates a compact Surface Acoustic Wave (SAW) drug delivery device that generates an aerosol of respirable size for delivery of compounds directly onto polarized or differentiated epithelial cell cultures within an airway barrier MPS and conventional static inserts. As proof of principle, the SAW drug delivery device was used to nebulize viral dsRNA analog poly I:C and steroids fluticasone and dexamethasone without disrupting their biological function.

Nebulization of siRNA for inhalation therapy based on a microfluidic surface acoustic wave platform

The local delivery of therapeutic small interfering RNA or siRNA to the lungs has the potential to improve the prognosis for patients suffering debilitating lung diseases. Recent advances in materials science have been aimed at addressing delivery challenges including biodistribution, bioavailability and cell internalization, but an equally important challenge to overcome is the development of an inhalation device that can deliver the siRNA effectively to the lung, without degrading the therapeutic itself. Here, we report the nebulization of siRNA, either naked siRNA or complexed with polyethyleneimine (PEI) or a commercial transfection agent, using a miniaturizable acoustomicrofluidic nebulization device. The siRNA solution could be nebulised without significant degradation into an aerosol mist with tunable mean aerodynamic diameters of approximately 3 µm, which is appropriate for deep lung deposition via inhalation. The nebulized siRNA was tested for its stability, as well as its toxicity and gene silencing properties using the mammalian lung carcinoma cell line A549, which demonstrated that the gene silencing capability of siRNA is retained after nebulization. This highlights the potential application of the acoustomicrofluidic device for the delivery of efficacious siRNA via inhalation, either for systemic delivery via the alveolar epithelium or local therapeutic delivery to the lung.

Experimental research on surface acoustic wave microfluidic atomization for drug delivery

This paper demonstrates that surface acoustic wave (SAW) atomization can produce suitable aerosol concentration and size distribution for efficient inhaled lung drug delivery and is a potential atomization device for asthma treatment. Using the SAW device, we present comprehensive experimental results exploring the complexity of the acoustic atomization process and the influence of input power, device frequency, and liquid flow rate on aerosol size distribution. It is hoped that these studies will explain the mechanism of SAW atomization aerosol generation and how they can be controlled. The insights from the high-speed flow visualization studies reveal that it is possible by setting the input power above 4.17 W, thus allowing atomization to occur from a relatively thin film, forming dense, monodisperse aerosols. Moreover, we found that the aerosol droplet size can be effectively changed by adjusting the input power and liquid flow rate to change the film conditions. In this work, we proposed a method to realize drug atomization by a microfluidic channel. A SU-8 flow channel was prepared on the surface of a piezoelectric substrate by photolithography technology. Combined with the silicon dioxide coating process and PDMS process closed microfluidic channel was prepared, and continuous drug atomization was provided to improve the deposition efficiency of drug atomization by microfluidic.

Effect of the area of a lithium niobate transducer on the efficiency of ultrasonic atomization driven by resonance vibration

In recent years, individual control of one's personal environment has been drawing increasing attention due to the growing interest in health care. Wearable devices are especially useful because of their controllability regardless of location. Humidity is one of the inevitable factors in the personal environment as a preventive against infectious diseases. Although atomization devices are commonly used as a method of humidity control, at present, there are no wearable humidity control devices. Vibration of a lithium niobate (LN) device in the thickness mode is a promising piezoelectric method for miniaturization of atomization devices for humidity control. To miniaturize the atomization device, the transducer size needs to be small not so much as to decrease the atomization efficiency. However, the effect of the device area on the atomization efficiency of LN at a size suitable for mounting in wearable devices has not been studied. Here, we conducted an atomization demonstration of LN devices with different sizes to evaluate particle size and atomization efficiency. Furthermore, to reveal the relationship between vibration behavior and atomization efficiency, resonance vibration in the MHz frequency band was evaluated by the finite element method and an impedance analyzer. The results showed that the peak size of water particles atomized by each device was in the range of 3.2 to 4.2 µm, which is smaller than particles produced by typical piezoelectric ceramics. Moreover, the best LN size for efficient atomization was found to be 8 mm × 10 mm among the five LN device sizes used in experiments. From the relationship between vibration behavior and atomization efficiency, the size of the transducer was suggested to affect the vibration mode. The obtained result suggested that the LN device is suitable for small wearable nebulizer devices.

Periodicity in ultrasonic atomization involving beads-fountain oscillations and mist generation: Effects of driving frequency

Ultrasonic atomization induced by high driving frequency, generally on the order of 1 MHz or higher, could involve a liquid fountain in the form of a corrugated jet, or a chain of "beads" of submillimeter diameter in contact. This study concerns dynamics/instability of such beads fountain, observed under lower input power density (≤ 6 W/cm²) of the "flat" ultrasound transducer with a "regulating" nozzle equipped, exhibiting time-varying characteristics with certain periodicity. High-speed, high-resolution images are processed for quantitative elucidation: frequency analysis (fast Fourier transform) and time-frequency analysis (discrete wavelet transform) are employed, respectively, to evaluate dominant frequencies of beads-surface oscillations and to reveal factor(s) triggering mist emergence. The resulting time variation in the measured (or apparent) fountain structure, associated with the recurring-beads size scalable to the ultrasound wavelength, subsumes periodic nature predictable from simple physical modeling as well as principle. It is further found that such dynamics in (time-series data for) the fountain structure at given height(s) along a series of beads would signal "bursting" of liquid droplets emanating out of a highly deformed bead often followed by a cloud of tiny droplets, or mist. In particular, the bursting appears to be not a completely random phenomenon but should concur with the fountain periodicity with a limited extent of probability.

Nanoscale plasma-activated aerosol generation for in situ surface pathogen disinfection

Plasma treatment constitutes an efficient method for chemical-free disinfection. A spray-based system for dispensing plasma-activated aerosols onto surfaces would facilitate disinfection of complex and/or hidden surfaces inaccessible to direct line-of-sight (for example, UV) methods. The complexity and size of current plasma generators (for example, plasma jet and cometary plasma systems)-which prohibit portable operation, together with the short plasma lifetimes, necessitate a miniaturized in situ technique in which a source can be simultaneously activated and administered on-demand onto surfaces. Here, we demonstrate this possibility by combining two nanoscale technologies for plasma and aerosol generation into an integrated device that is sufficiently small and lightweight. Plasma is generated on a carpet of zinc oxide nanorods comprising a nanoneedle ensemble, which when raised to a high electric potential, constitutes a massive point charge array with near-singular electric fields to effect atmospheric breakdown. The plasma is then used to activate water transported through an underlying capillary wick, that is subsequently aerosolized under MHz-order surface acoustic waves. We show that the system, besides being amenable to miniaturization and hence integration into a chipscale device, leads to a considerable improvement in plasma-activation over its macroscale cometary discharge predecessor, with up to 20% and 127% higher hydrogen peroxide and nitrite ion concentrations that are respectively generated in the plasma-activated aerosols. This, in turn, leads to a 67% reduction in the disinfection time to achieve 95% bacterial load reduction, therefore demonstrating the potential of the technology as an efficient portable platform for on-demand field-use surface disinfection.

Spray-Assisted Formation of Micrometer-Sized Emulsions

Emulsion drops with defined sizes are frequently used to conduct chemical reactions on picoliter scales or as templates to form microparticles. Despite tremendous progress that has been achieved in the production of emulsions, the high throughput formation of drops with well-defined diameters of a few micrometers remains challenging. Drops of this size, however, are in high demand, for example, for many pharmaceutical, food, and materials science applications. Here, we introduce a scalable method to produce water-in-oil emulsion drops possessing controlled diameters of just a few micrometers: We fabricate calibrated aerosol drops and transfer them into an oil bath to form stable emulsions at rates up to 480 μL min^-1 of the dispersed phase. We demonstrate that the emulsification is thermodynamically driven such that design principles to successfully form emulsions can easily be deduced. We employ these emulsion drops as templates to form well-defined micrometer-sized hydrogel spheres and capsules.

Prospects of Inhaled Phage Therapy for Combatting Pulmonary Infections

With respiratory infections accounting for significant morbidity and mortality, the issue of antibiotic resistance has added to the gravity of the situation. Treatment of pulmonary infections (bacterial pneumonia, cystic fibrosis-associated bacterial infections, tuberculosis) is more challenging with the involvement of multi-drug resistant bacterial strains, which act as etiological agents. Furthermore, with the dearth of new antibiotics available and old antibiotics losing efficacy, it is prudent to switch to non-antibiotic approaches to fight this battle. Phage therapy represents one such approach that has proven effective against a range of bacterial pathogens including drug resistant strains. Inhaled phage therapy encompasses the use of stable phage preparations given via aerosol delivery. This therapy can be used as an adjunct treatment option in both prophylactic and therapeutic modes. In the present review, we first highlight the role and action of phages against pulmonary pathogens, followed by delineating the different methods of delivery of inhaled phage therapy with evidence of success. The review aims to focus on recent advances and developments in improving the final success and outcome of pulmonary phage therapy. It details the use of electrospray for targeted delivery, advances in nebulization techniques, individualized controlled inhalation with software control, and liposome-encapsulated nebulized phages to take pulmonary phage delivery to the next level. The review expands knowledge on the pulmonary delivery of phages and the advances that have been made for improved outcomes in the treatment of respiratory infections.

Periodic Rayleigh streaming vortices and Eckart flow arising from traveling-wave-based diffractive acoustic fields

Recent studies have demonstrated that periodic time-averaged acoustic fields can be produced from traveling surface acoustic waves (SAWs) in microfluidic devices. This is caused by diffractive effects arising from a spatially limited transducer. This permits the generation of acoustic patterns evocative of those produced from standing waves, but instead with the application of a traveling wave. While acoustic pressure fields in such systems have been investigated, acoustic streaming from diffractive fields has not. In this work we examine this phenomenon and demonstrate the appearance of geometry-dependent acoustic vortices, and demonstrate that periodic, identically rotating Rayleigh streaming vortices result from the imposition of a traveling SAW. This is also characterized by a channel-spanning flow that bridges between adjacent vortices along the channel top and bottom. We find that the channel dimensions determine the types of streaming that develops; while Eckart streaming has been previously presumed to be a distinguishing feature of traveling-wave actuation, we show that Rayleigh streaming vortices also results. This has implications for microfluidic actuation, where traveling acoustic waves have applications in microscale mixing, separation, and patterning.

Acoustic resonance and atomization for gas-liquid systems in microreactors

It is shown that a liquid slug in gas-liquid segmented flow in microchannels can act as an acoustic resonator to disperse large amounts of small liquid droplets, commonly referred to as atomization, into the gas phase. We investigate the principles of acoustic resonance within a liquid slug through experimental analysis and numerical simulation. A mechanism of atomization in the confined channels and a hypothesis based on high-speed image analysis that links acoustic resonance within a liquid slug with the observed atomization is proposed. The observed phenomenon provides a novel source of confined micro sprays and could be an avenue, amongst others, to overcome mass transfer limitations for gas-liquid processes in flow.

Coalescence of Droplets in a Microwell Driven by Surface Acoustic Waves

Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet-droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications.

Ultrafast assembly of swordlike Cu3(1,3,5-benzenetricarboxylate)n metal-organic framework crystals with exposed active metal sites

Owing to their large surface area and high uptake capacity, metal-organic frameworks (MOFs) have attracted considerable attention as potential materials for gas storage, energy conversion, and electrocatalysis. Various strategies have recently been proposed to manipulate the MOF surface chemistry to facilitate exposure of the embedded metal centers at the crystal surface to allow more effective binding of target molecules to these active sites. Nevertheless, such strategies remain complex, often requiring strict control over the synthesis conditions to avoid blocking pore access, reduction in crystal quality, or even collapse of the entire crystal structure. In this work, we exploit the hydrodynamics and capillary resonance associated with acoustically-driven dynamically spreading and nebulizing thin films as a new method for ultrafast synthesis of swordlike Cu₃(1,3,5-benzenetricarboxylate)_n (Cu-BTC) MOFs with unique monoclinic crystal structures (P2₁/n) distinct to that obtained via conventional bulk solvothermal synthesis, with 'swordlike' morphologies whose lengths far exceed their thicknesses. Through pulse modulation and taking advantage of the rapid solvent evaporation associated with the high nebulisation rates, we are also able to control the thicknesses of these large aspect ratio (width and length with respect to the thickness) crystals by arresting their vertical growth, which, in turn, allows exposure of the metal active sites at the crystal surface. An upshot of such active site exposure on the crystal surface is the concomitant enhancement in the conductivity of the MOF, evident from the improvement in its current density by two orders of magnitude.

In situ generation of plasma-activated aerosols via surface acoustic wave nebulization for portable spray-based surface bacterial inactivation

The presence of reactive species in plasma-activated water is known to induce oxidative stresses in bacterial species, which can result in their inactivation. By integrating a microfludic chipscale nebulizer driven by surface acoustic waves (SAWs) with a low-temperature atmospheric plasma source, we demonstrate an efficient technique for in situ production and application of plasma-activated aerosols for surface disinfection. Unlike bulk conventional systems wherein the water is separately batch-treated within a container, we show in this work the first demonstration of continuous plasma-treatment of water as it is transported through a paper strip from a reservoir onto the chipscale SAW device. The significantly larger surface area to volume ratio of the water within the paper strip leads to a significant reduction in the duration of the plasma-treatment, while maintaining the concentration of the reactive species. The subsequent nebulization of the plasma-activated water by the SAW then allows the generation of plasma-activated aerosols, which can be directly sprayed onto the contaminated surface, therefore eliminating the storage of the plasma-activated water and hence circumventing the typical limitation in conventional systems wherein the concentration of the reactive species diminishes over time during storage, resulting in a reduction in the efficacy of bacterial inactivation. In particular, we show up to 96% reduction in Escherichia coli colonies through direct spraying with the plasma-activated aerosols. This novel, low-cost, portable and energy-efficient hybrid system necessitates only minimal maintenance as it only requires the supply of tap water and battery power for operation, and is thus suitable for decontamination in home environments.

High frequency acoustic nebulization for pulmonary delivery of antibiotic alternatives against Staphylococcus aureus

The increasing prevalence of multidrug resistant bacteria has warranted the search for new antimicrobial agents as existing antibiotics lose their potency. Among these, bacteriophage therapy, as well as the administration of specific bacteriolysis agents, i.e., lytic enzymes, have emerged as attractive alternatives. Nebulizers offer the possibility for delivering these therapeutics directly to the lung, which is particularly advantageous as a non-invasive and direct route to treat bacterial lung infections. Nevertheless, nebulizers can often result in significant degradation of the bacteriophage or protein, both structurally and functionally, due to the large stresses the aerosolization process imposes on these entities. In this work, we assess the capability of a novel low-cost and portable hybrid surface and bulk acoustic wave platform (HYDRA) to nebulize a Myoviridae bacteriophage (phage K) and lytic enzyme (lysostaphin) that specifically targets Staphylococcus aureus. Besides its efficiency in producing phage or protein-laden aerosols within the 1-5 μm respirable range for optimum delivery to the lower respiratory tract where lung infections commonly take place, we observe that the HYDRA platform-owing to the efficiency of driving the aerosolization process at relatively low powers and high frequencies (approximately 10 MHz)-does not result in appreciable denaturation of the phages or proteins, such that the loss of antimicrobial activity following nebulization is minimized. Specifically, a low (0.1 log₁₀ (pfu/ml)) titer loss was obtained with the phages, resulting in a high viable respirable fraction of approximately 90%. Similarly, minimal loss of antimicrobial activity was obtained with lysostaphin upon nebulization wherein its minimum inhibitory concentration (0.5 μg/ml) remained unaltered as compared with the non-nebulized control. These results therefore demonstrate the potential of the HYDRA nebulization platform as a promising strategy for pulmonary administration of alternative antimicrobial agents to antibiotics for the treatment of lung diseases caused by pathogenic bacteria.

Acoustic fields and microfluidic patterning around embedded micro-structures subject to surface acoustic waves

Recent research has shown that interactions between acoustic waves and microfluidic channels can generate microscale interference patterns with the application of a traveling surface acoustic wave (SAW), effectively creating standing wave patterns with a traveling wave. Forces arising from this interference can be utilized for precise manipulation of micron-sized particles and biological cells. The patterns that have been produced with this method, however, have been limited to straight lines and grids from flat channel walls, and where the spacing resulting from this interference has not previously been comprehensively explored. In this work we examine the interaction between both straight and curved channel interfaces with a SAW to derive geometrically deduced analytical models. These models predict the acoustic force-field periodicity near a channel interface as a function of its orientation to an underlying SAW, and are validated with experimental and simulation results. Notably, the spacing is larger for flat walls than for curved ones and is dependent on the ratio of sound speeds in the substrate and fluid. Generating these force-field gradients with only travelling waves has wide applications in acoustofluidic systems, where channel interfaces can potentially support a range of patterning, concentration, focusing and separation activities by creating locally defined acoustic forces.

Influence of Waterproof Films on the Atomization Behavior of Surface Acoustic Waves

One of the reasons why commercial application of surface acoustic wave (SAW) atomization is not possible is due to the condensation of aerosol droplets generated during atomization, which drip on the interdigitated transducer (IDT), thereby causing electrodes to short-circuit. In order to solve this problem, a SU-8-2002 film coating on an IDT is proposed in this paper. The waterproof performance of the film coating was tested on a surface acoustic wave (SAW) device several times. The experimental results reveal that the film coating was robust. The experiment also investigated the effects of the SU-8-2002 film on atomization behavior and heating.

Performance of five ultrasonic transducers modified for efficient atomization

Performance of five ultrasonic transducers modified with different shapes of electrodes is investigated for efficient atomization. A circular silver electrode, which is conventionally used as standard on a transducer, is chemically replaced with five shapes of silver electrodes (circular, toroidal, singlet, doublet, and cross). Each electrode reflects its precise shape on the water surface and statically forms characteristic three-dimensional geometry of a water column. The modified electrode also affects the dynamics of this water column, generating two types of wobbling on the column and inducing three types of atomization depending on the shape of the electrodes. Statistical analysis indicates that the shape of the electrode on an ultrasonic transducer affects the speed of atomization, showing that the singlet electrode exhibits the highest speed of atomization (4 mg/s). The mechanisms of atomization are analyzed from the viewpoint of energy transformation with reference to mass transformation of the oscillating liquid, indicating that the vibration energy of the transducer is transferred to the water film through resonance, consuming this vibration energy with four kinds of energy such as kinetic energy of atomized mists and work function of atomization, which are defined in this study. This analysis clarifies why the speed of atomization increases with a decreasing amount of water on the transducer. Application of the appropriate shape of an electrode will greatly contribute to the fields of engineering, medicine, and biology where various types of atomization are highly desired as the situation demands.

Size distributions of droplets produced by ultrasonic nebulizers

In many applications where small, similar-sized droplets are needed, ultrasonic nebulizers are employed. Little is known about the mechanism of nebulization, for example about what determines the median droplet size. Even less understood, is the droplet size distribution, which is often simply fitted with a log-normal distribution or assumed to be very narrow. We perform the first systematic study of droplet size distributions for different nebulizer technologies, showing that these distributions can be very well fitted with distributions found for sprays, where the size distribution is completely determined by the corrugation of ligaments and the distribution of ligament sizes. In our case, breakup is believed to be due to pinch-off of Faraday instabilities. The droplet size distribution is then set by the distribution of wavelengths of the standing capillary waves and the roughness of the pinch-off ligaments. We show that different nebulizer technologies produce different size distributions, which we relate to (variation in) wavelengths of the waves that contribute to the droplet formation. We further show that the median droplet size scales with the capillary wavelength, with a proportionality constant that depends only slightly on the type of nebulizer, despite order-of-magnitude differences in other parameters.

Tailoring surface acoustic wave atomisation for cryo-electron microscopy sample preparation

Surface acoustic wave (SAW) atomisation has been widely explored for use in pharmacological delivery, hence performance is characterised predominately in terms of droplet size and maximum delivery of fluid, to ensure sufficient dosage is delivered to the right location. For the application of cryo electron microscopy grid preparation, however, what is required is the transfer of very little fluid onto the grid in a well-defined manner. To meet this requirement, the analysis of SAW atomisation needs to focus on very different characteristics. Specifically, we examine the aerosol jet geometry, in terms of width, cone angle, and elevation angle, and its stability at low power, and hence low flow rates. The variables used are the width and the location of the channel delivering the fluid to the site of atomization. From the experiments, it is observed that we can reach a flowrate as low as 0.55 μl s-1 with reasonable aerosol jet stability, a jet width of 0.5 mm wide and an elevation angle variation as low as 2°.

Enhancing rate of water absorption in seeds via a miniature surface acoustic wave device

Seeds, which are high in protein and essential nutrients, must go through a hydration process before consumption. The ability to rapidly increase water absorption can significantly reduce the soaking time as well as the amount of energy needed for cooking seeds. Many studies in the literature employ high-power (10² W) low-frequency (10⁴ Hz) ultrasound; although their results are very promising where more than 100% increase in water content can be obtained between the treated and untreated seeds, the high-power and low-frequency ultrasound often causes acoustic cavitation under high intensity, which can severely disrupt the cell walls and damage the seeds. In our study, however, we demonstrate that treating the seeds via a miniature surface acoustic wave device, which operates at low-power (10⁰ W) and high-frequency (10⁷ Hz) range, gives rise to a higher water absorption rate without the acoustic cavitations. By comparing the water content between the treated and untreated seeds, an increase of up to 2600% (for chickpeas) and 6350% (for mung bean) can be obtained after 60 min. A significantly higher water absorption in mung beans can be attributed to the larger pore size when compared with the acoustic wavelength in water, enabling an efficient transmission of acoustic wave inside the pores. Our results also indicate that the germination time can be reduced by half for treated seeds as compared to the untreated seeds.

Versatile platform for performing protocols on a chip utilizing surface acoustic wave (SAW) driven mixing

We present and demonstrate a dextrous microfluidic device which features a reaction chamber with volume flexibility. This feature is critical for developing protocols directly on chip when the exact reaction is not yet defined, enabling bio/chemical reactions on chip to be performed without volumetric restrictions. This is achieved by the integration of single layer valves (for reagent dispensing) and surface acoustic wave excitation (for rapid reagent mixing). We show that a single layer valve can control the delivery of fluid into, an initially air-filled, mixing chamber. This chamber arrangement offers flexibility in the relative volume of reagents used, and so offers the capability to not only conduct, but also develop protocols on a chip. To enable this potential, we have integrated a SAW based mixer into the system, and characterised its mixing time based on frequency and power of excitation. Numerical simulations on the streaming pattern inside the chamber were conducted to probe the underlying physics of the experimental system. To demonstrate the on-chip protocol capability, the system was utilised to perform protein crystallization. Furthermore, the effect of rapid mixing, results in a significant increase in crystal size uniformity.

High frequency acoustic permeabilisation of drugs through tissue for localised mucosal delivery

The majority of infectious diseases enter the body through mucosal membranes that line the ocular, nasal, oral, vaginal and rectal surfaces. As infections can be effectively prevented by instigating a local immune response in the immunocyte-rich regions of the mucosa, an efficacious route of vaccine administration is to directly target their delivery to these surfaces. It is nevertheless challenging to provide sufficient driving force to penetrate both the mucus lining as well as the epithelial barrier of the mucosal surfaces, which are designed to effectively keep foreign entities out, but not excessively such that the therapeutic agent penetrates deeper into the vascularised submucosal regions where they are mostly taken up by the systemic circulation, thus resulting in a far weaker immune response. In this work, we demonstrate the possibility of controllably localising and hence maximising the delivery of both small and large molecule model therapeutic agents in the mucosa of a porcine buccal model using high frequency acoustics. Unlike their low (kHz order) frequency bulk ultrasonic counterpart, these high frequency (>10 MHz) surface waves do not generate cavitation, which leads to large molecular penetration depths beyond the 100 μm order thick mucosal layer, and which has been known to cause considerable cellular/tissue damage and hence scarring. Through system parameters such as the acoustic irradiation frequency, power and exposure duration, we show that it is possible to tune the penetration depth such that over 95% of the delivered drug are localised within the mucosal layer, whilst preserving their structural integrity.

Ultrafast Acoustofluidic Exfoliation of Stratified Crystals

While the remarkable properties of 2D crystalline materials offer tremendous opportunities for their use in optics, electronics, energy systems, biotechnology, and catalysis, their practical implementation largely depends critically on the ability to exfoliate them from a 3D stratified bulk state. This goal nevertheless remains elusive, particularly in terms of a rapid processing method that facilitates high yield and dimension control. An ultrafast multiscale exfoliation method is reported which exploits the piezoelectricity of stratified materials that are noncentrosymmetric in nature to trigger electrically-induced mechanical failure across weak grain boundaries associated with their crystal domain planes. In particular, it is demonstrated that microfluidic nebulization using high frequency acoustic waves exposes bulk 3D piezoelectric crystals such as molybdenum disulphide (MoS₂ ) and tungsten disulphide (WS₂ ) to a combination of extraordinarily large mechanical acceleration (≈10⁸ m s^-2 ) and electric field (≈10⁷ V m^-1 ). This results in the layered bulk material being rapidly cleaved into pristine quasi-2D-nanosheets that predominantly comprise single layers, thus constituting a rapid and high throughput chip-scale method that opens new possibilities for scalable production and spray coating deposition.

Delivery of femtolitre droplets using surface acoustic wave based atomisation for cryo-EM grid preparation

Cryo-Electron Microscopy (cryo-EM) has become an invaluable tool for structural biology. Over the past decade, the advent of direct electron detectors and automated data acquisition has established cryo-EM as a central method in structural biology. However, challenges remain in the reliable and efficient preparation of samples in a manner which is compatible with high time resolution. The delivery of sample onto the grid is recognized as a critical step in the workflow as it is a source of variability and loss of material due to the blotting which is usually required. Here, we present a method for sample delivery and plunge freezing based on the use of Surface Acoustic Waves to deploy 6-8 µm droplets to the EM grid. This method minimises the sample dead volume and ensures vitrification within 52.6 ms from the moment the sample leaves the microfluidics chip. We demonstrate a working protocol to minimize the atomised volume and apply it to plunge freeze three different samples and provide proof that no damage occurs due to the interaction between the sample and the acoustic waves.

Droplet trapping and fast acoustic release in a multi-height device with steady-state flow

We demonstrate a novel multilayer polydimethylsiloxane (PDMS) device for selective storage and release of single emulsion droplets. Drops are captured in a microchannel cavity and can be released on-demand through a triggered surface acoustic wave pulse. The surface acoustic wave (SAW) is excited by a tapered interdigital transducer (TIDT) deposited on a piezoelectric lithium niobate (LiNbO₃) substrate and inverts the pressure difference across the cavity trap to push a drop out of the trap and back into the main flow channel. Droplet capture and release does not require a flow rate change, flow interruption, flow inversion or valve action and can be achieved in as fast as 20 ms. This allows both on-demand droplet capture for analysis and monitoring over arbitrary time scales, and continuous device operation with a high droplet rate of 620 drops per s. We hence decouple long-term droplet interrogation from other operations on the chip. This will ease integration with other microfluidic droplet operations and functional components.

Fluorescence activated cell sorting via a focused traveling surface acoustic beam

Fluorescence activated cell sorting (FACS) has become an essential technique widely exploited in biological studies and clinical applications. However, current FACS systems are quite complex, expensive, bulky, and pose potential sample contamination and biosafety issues due to the generation of aerosols in an open environment. Microfluidic technology capable of precise cell manipulation has great potential to reinvent and miniaturize conventional FACS systems. In this work, we demonstrate a benchtop scale FACS system that makes use of a highly focused traveling surface acoustic wave beam to sort out micron-sized particles and biological cells upon fluorescence interrogation at ∼kHz rates. The highly focused acoustic wave beam has a width of ∼50 μm that enables highly accurate sorting of individual particles and cells. We have applied our acoustic FACS system to isolate fluorescently labeled MCF-7 breast cancer cells from diluted whole blood samples with the purity of sorted MCF-7 cells higher than 86%. The cell viability before and after acoustic sorting is higher than 95%, indicating excellent biocompatibility that should enable a variety of cell sorting applications in biomedical research.

Acoustomicrofluidic application of quasi-shear surface waves

The paper analyzes the possibility of using predominantly boundary polarized surface acoustic waves for actuating fluidic effects in microchannels fabricated inside containers made of PDMS. The aim is to remove a shortcoming peculiar to conventionally utilized predominantly vertically polarized waves. Such waves strongly attenuate while they propagate under container side walls because of the leakage into them. Due to a specific feature of PDMS - extremely small shear elastic modulus - losses of boundary polarized modes should be far smaller. The amplitude of vertical mechanical displacements can be increased right inside the channel owing to the scattering of acoustic fields. As an example, the predominantly vertically polarized surface wave on 128YX LiNbO₃ is compared with the quasi-shear leaky wave on 64YX LiNbO₃. Our computations predict that, given the electric power supplied to the launching transducer, the quasi-shear wave will drive the fluid more efficiently than the surface wave on 128YX LiNbO₃ when the container wall thickness is larger than 25-30 wavelengths, if there are no additional scatterers inside the channel. In the presence of a scatterer, such as a thin gold strip, the quasi-shear wave can be more efficient when the wall thickness exceeds 10-15 wavelengths.

Acoustially-mediated microfluidic nanofiltration through graphene films

We exploit the possibility of enhancing the molecular transport of liquids through graphene films using amplitude modulated surface acoustic waves (SAWs) to demonstrate effective and efficient nanoparticle filtration. The use of the SAW, which is an extremely efficient means for driving microfluidic transport, overcomes the need for the large mechanical pumps required to circumvent the large pressure drops encountered in conventional membranes for nanoparticle filtration. 100% filtration efficiency was obtained for micron-dimension particulates, decreasing to only 95% for the filtration of particles of tens of nanometers in dimension, which is comparable to that achieved with other methods. To circumvent clogging of the film, which is typical with all membrane filters, a backwash operation to flush the nanoparticles is incorporated simply by reversing the SAW-induced flow such that 98% recovery of the initial filtration rate is recovered. Given these efficiencies, together with the low cost and compact size of the chipscale SAW devices, we envisage the possibility of scaling out the process by operating a large number of devices in parallel to achieve typical industrial-scale throughputs with potential benefits in terms of substantially lower capital, operating and maintenance costs.

Continuous micro-vortex-based nanoparticle manipulation via focused surface acoustic waves

Despite increasing demand in the manipulation of nanoscale objects for next generation biological and industrial processes, there is a lack of methods for reliable separation, concentration and purification of nanoscale objects. Acoustic methods have proven their utility in contactless manipulation of microscale objects mainly relying on the acoustic radiation effect, though the influence of acoustic streaming has typically prevented manipulation at smaller length scales. In this work, however, we explicitly take advantage of the strong acoustic streaming in the vicinity of a highly focused, high frequency surface acoustic wave (SAW) beam emanating from a series of focused 6 μm substrate wavelength interdigital transducers patterned on a piezoelectric lithium niobate substrate and actuated with a 633 MHz sinusoidal signal. This streaming field serves to focus fluid streamlines such that incoming particles interact with the acoustic field similarly regardless of their initial starting positions, and results in particle displacements that would not be possible with a travelling acoustic wave force alone. This streaming-induced manipulation of nanoscale particles is maximized with the formation of micro-vortices that extend the width of the microfluidic channel even with the imposition of a lateral flow, occurring when the streaming-induced flow velocities are an order of magnitude larger than the lateral one. We make use of this acoustic streaming to demonstrate the continuous and differential focusing of 100 nm, 300 nm and 500 nm particles.

Frequency and wavelength prediction of ultrasonic induced liquid surface waves

A theoretical investigation of parametric excitation of liquid free surface by a high frequency sound wave is preformed, using potential flow theory. Pressure and velocity distributions, resembling the sound wave, are applied to the free surface of the liquid. It is found that for impinging wave two distinct capillary frequencies will be excited: One of them is the same as the frequency of the sound wave, and the other is equal to the natural frequency corresponding to a wavenumber equal to the horizontal wavenumber of the sound wave. When the wave propagates in vertical direction, mathematical formulation leads to an equation, which has resonance frequency equal to half of the excitation frequency. This can explain an important contradiction between the frequency and the wavelength of capillary waves in the two cases of normal and inclined interaction of the sound wave and the free surface of the liquid.

Graphene-mediated microfluidic transport and nebulization via high frequency Rayleigh wave substrate excitation

The deposition of a thin graphene film atop a chip scale piezoelectric substrate on which surface acoustic waves are excited is observed to enhance its performance for fluid transport and manipulation considerably, which can be exploited to achieve further efficiency gains in these devices. Such gains can then enable complete integration and miniaturization for true portability for a variety of microfluidic applications across drug delivery, biosensing and point-of-care diagnostics, among others, where field-use, point-of-collection or point-of-care functionality is desired. In addition to a first demonstration of vibration-induced molecular transport in graphene films, we show that the coupling of the surface acoustic wave gives rise to antisymmetric Lamb waves in the film which enhance molecular diffusion and hence the flow through the interstitial layers that make up the film. Above a critical input power, the strong substrate vibration displacement can also force the molecules out of the graphene film to form a thin fluid layer, which subsequently destabilizes and breaks up to form a mist of micron dimension aerosol droplets. We provide physical insight into this coupling through a simple numerical model, verified through experiments, and show several-fold improvement in the rate of fluid transport through the film, and up to 55% enhancement in the rate of fluid atomization from the film using this simple method.

Virtual membrane for filtration of particles using surface acoustic waves (SAW)

Surface acoustic wave (SAW) based particle manipulation is contactless, versatile, non-invasive and biocompatible making it useful for biological studies and diagnostic technologies. In this work, we present a sensitive particle sorting system, termed the virtual membrane, in which a periodic acoustic field with a wavelength on the order of particle dimensions permits size-selective filtration. Polystyrene particles that are larger than approximately 0.3 times the acoustic half-wavelength experience a force repelling them from the acoustic field. If the particle size is such that, at a given acoustic power and flow velocity, this repulsive force is dominant over the drag force, these particles will be prohibited from progressing further downstream (i.e. filtered), while smaller particles will be able to pass through the force field along the pressure nodes (akin to a filter's pores). Using this mechanism, we demonstrate high size selectivity using a standing SAW generated by opposing sets of focused interdigital transducers (FIDTs). The use of FIDTs permits the generation of a highly localized standing wave field, here used for filtration in μl min(-1) order flow rates at 10s of mW of applied power. Specifically, we demonstrate the filtration of 8 μm particles from 5 μm particles and 10.36 μm particles from 7.0 μm and 5.0 μm particles, using high frequency SAW at 258 MHz, 192.5 MHz, and 129.5 MHz, respectively.

Computation of the pressure field generated by surface acoustic waves in microchannels

The high-frequency pressure induced by a surface acoustic wave in the fluid filling a microchannel is computed by solving the full scattering problem. The microchannel is fabricated inside a container attached to the top of a piezoelectric substrate where the surface wave propagates. The finite element method is used. The pressure found in this way is compared with the pressure obtained by solving boundary-value problems formulated on the basis of simplifications which have been introduced in earlier papers by other research studies. The considered example shows that the difference between the results can be significant, ranging from several tens of percent up to several times in different points inside the channel.

Acoustic tweezers via sub-time-of-flight regime surface acoustic waves

Micrometer-scale acoustic waves are highly useful for refined optomechanical and acoustofluidic manipulation, where these fields are spatially localized along the transducer aperture but not along the acoustic propagation direction. In the case of acoustic tweezers, such a conventional acoustic standing wave results in particle and cell patterning across the entire width of a microfluidic channel, preventing selective trapping. We demonstrate the use of nanosecond-scale pulsed surface acoustic waves (SAWs) with a pulse period that is less than the time of flight between opposing transducers to generate localized time-averaged patterning regions while using conventional electrode structures. These nodal positions can be readily and arbitrarily positioned in two dimensions and within the patterning region itself through the imposition of pulse delays, frequency modulation, and phase shifts. This straightforward concept adds new spatial dimensions to which acoustic fields can be localized in SAW applications in a manner analogous to optical tweezers, including spatially selective acoustic tweezers and optical waveguides.

Stability and efficacy of synthetic cationic antimicrobial peptides nebulized using high frequency acoustic waves

Surface acoustic wave (SAW), a nanometer amplitude electroelastic wave generated and propagated on low-loss piezoelectric substrates (such as LiNbO3), is an extremely efficient solid-fluid energy transfer mechanism. The present study explores the use of SAW nebulization as a solution for effective pulmonary peptide delivery. In vitro deposition characteristics of the nebulized peptides were determined using a Next Generation Cascade Impactor. 70% of the peptide-laden aerosols generated were within a size distribution favorable for deep lung distribution. The integrity of the nebulized peptides was found to be retained, as shown via mass spectrometry. The anti-mycobacterial activity of the nebulized peptides was found to be uncompromised compared with their non-nebulized counterparts, as demonstrated by the minimum inhibition concentration and the colony forming inhibition activity. The peptide concentration and volume recoveries for the SAW nebulizer were significantly higher than 90% and found to be insensitive to variation in the peptide sequences. These results demonstrate the potential of the SAW nebulization platform as an effective delivery system of therapeutic peptides through the respiratory tract to the deep lung.

Dynamics of sessile and pendant drops excited by surface acoustic waves: Gravity effects and correlation between oscillatory and translational motions

When sessile droplets are excited by ultrasonic traveling surface acoustic waves (SAWs), they undergo complex dynamics with both oscillations and translational motion. While the nature of the Rayleigh-Lamb quadrupolar drop oscillations has been identified, their origin and their influence on the drop mobility remains unexplained. Indeed, the physics behind this peculiar dynamics is complex with nonlinearities involved both at the excitation level (acoustic streaming and radiation pressure) and in the droplet response (nonlinear oscillations and contact line dynamics). In this paper, we investigate the dynamics of sessile and pendant drops excited by SAWs. For pendant drops, so-far unreported dynamics are observed close to the drop detachment threshold with the suppression of the translational motion. Away from this threshold, the comparison between pendant and sessile drop dynamics allows us to identify the role played by gravity or, more generally, by an initial or dynamically induced stretching of the drop. In turn, we elucidate the origin of the resonance frequency shift, as well as the origin of the strong correlation between oscillatory and translational motion. We show that for sessile drops, the velocity is mainly determined by the amplitude of oscillation and that the saturation observed is due to the nonlinear dependence of the drop response frequency on the dynamically induced stretching.

HYbriD Resonant Acoustics (HYDRA)

The existence of what is termed here as a surface-reflected bulk wave is unraveled and elucidated, and it is shown, quite counterintuitively, that it is possible to obtain an order-of-magnitude improvement in microfluidic manipulation efficiency, and, in particular, nebulization, through a unique combination of surface and bulk waves without increasing complexity or cost.

Highly focused high-frequency travelling surface acoustic waves (SAW) for rapid single-particle sorting

High-speed sorting is an essential process in a number of clinical and research applications, where single cells, droplets and particles are segregated based on their properties in a continuous flow. With recent developments in the field of microscale actuation, there is increasing interest in replicating the functions available to conventional fluorescence activated cell sorting (FACS) flow cytometry in integrated on-chip systems, which have substantial advantages in cost and portability. Surface acoustic wave (SAW) devices are ideal for many acoustofluidic applications, and have been used to perform such sorting at rates on the order of kHz. Essential to the accuracy of this sorting, however, is the dimensions of the region over which sorting occurs, where a smaller sorting region can largely avoid inaccurate sorting across a range of sample concentrations. Here we demonstrate the use of flow focusing and a highly focused SAW generated by a high-frequency (386 MHz), 10 μm wavelength set of focused interdigital transducers (FIDTs) on a piezoelectric lithium niobate substrate, yielding an effective sorting region only ~25 μm wide, with sub-millisecond pulses generated at up to kHz rates. Furthermore, because of the use of high frequencies, actuation of particles as small as 2 μm can be realized. Such devices represent a substantial step forward in the evolution of highly localized forces for lab-on-a-chip microfluidic applications.

Spatiotemporally controllable acoustothermal heating and its application to disposable thermochromic displays

Disposable thermochromic displays using spatiotemporally controllable acoustothermal heating are reported. A variety of thermochromic displays are presented to prove the applicability of the proposed thermochromic display system.

Two-dimensional single-cell patterning with one cell per well driven by surface acoustic waves

In single-cell analysis, cellular activity and parameters are assayed on an individual, rather than population-average basis. Essential to observing the activity of these cells over time is the ability to trap, pattern and retain them, for which previous single-cell-patterning work has principally made use of mechanical methods. While successful as a long-term cell-patterning strategy, these devices remain essentially single use. Here we introduce a new method for the patterning of multiple spatially separated single particles and cells using high-frequency acoustic fields with one cell per acoustic well. We characterize and demonstrate patterning for both a range of particle sizes and the capture and patterning of cells, including human lymphocytes and red blood cells infected by the malarial parasite Plasmodium falciparum. This ability is made possible by a hitherto unexplored regime where the acoustic wavelength is on the same order as the cell dimensions.