Acoustially-mediated microfluidic nanofiltration through graphene films

Influence of MHz-order acoustic waves on bacterial suspensions

The development of alternative techniques to efficiently inactivate bacterial suspensions is crucial to prevent transmission of waterborne illness, particularly when commonly used techniques such as heating, filtration, chlorination, or ultraviolet treatment are not practical or feasible. We examine the effect of MHz-order acoustic wave irradiation in the form of surface acoustic waves (SAWs) on Gram-positive (Escherichia coli) and Gram-negative (Brevibacillus borstelensis and Staphylococcus aureus) bacteria suspended in water droplets. A significant increase in the relative bacterial load reduction of colony-forming units (up to 74%) can be achieved by either increasing (1) the excitation power, or, (2) the acoustic treatment duration, which we attributed to the effect of the acoustic radiation force exerted on the bacteria. Consequently, by increasing the maximum pressure amplitude via a hybrid modulation scheme involving a combination of amplitude and pulse-width modulation, we observe that the bacterial inactivation efficiency can be further increased by approximately 14%. By combining this scalable acoustic-based bacterial inactivation platform with plasma-activated water, a 100% reduction in E. coli is observed in less than 10 mins, therefore demonstrating the potential of the synergistic effects of MHz-order acoustic irradiation and plasma-activated water as an efficient strategy for water decontamination.

Revisiting the Electroacoustic Phenomenon in the Presence of Surface Acoustic Waves

Recently, one has been observing abundant studies on the application of surface acoustic waves (SAWs) in solid substrates for manipulating liquids and particulates in micron-to-nanometer thick films and channels and in porous media. At these length scales, contributions of SAWs to the electrical double layer (EDL) of ions and of the latter to particulates and flow may become appreciable. However, the nature of the interplay between SAWs and EDLs is unknown. We demonstrate the contribution of a SAW to the near-equilibrium electrical and ion-concentration fields in an EDL near inert and piezoelectric substrates. In particular, we concentrate on the leakage of transient and steady components of electrical potential through the excited EDL. Far from the solid, the leakage may be interpreted by different models of the EDL to give information about the EDL characteristic relaxation time, ζ-potential, and the Stern layer therein. In addition, the analysis given here may explain observed SAW-induced electrochemical effects on piezoelectric substrates.

Microfluidic Isolation and Enrichment of Nanoparticles

Over the past decades, nanoparticles have increased in implementation to a variety of applications ranging from high-efficiency electronics to targeted drug delivery. Recently, microfluidic techniques have become an important tool to isolate and enrich populations of nanoparticles with uniform properties (e.g., size, shape, charge) due to their precision, versatility, and scalability. However, due to the large number of microfluidic techniques available, it can be challenging to identify the most suitable approach for isolating or enriching a nanoparticle of interest. In this review article, we survey microfluidic methods for nanoparticle isolation and enrichment based on their underlying mechanisms, including acoustofluidics, dielectrophoresis, filtration, deterministic lateral displacement, inertial microfluidics, optofluidics, electrophoresis, and affinity-based methods. We discuss the principles, applications, advantages, and limitations of each method. We also provide comparisons with bulk methods, perspectives for future developments and commercialization, and next-generation applications in chemistry, biology, and medicine.

Development of Nanoporous AAO Membrane for Nano Filtration Using the Acoustophoresis Method

A concept of a nanoporous anodic aluminum oxide (AAO) membrane as a vibro-active micro/nano-filter in a micro hydro mechanical system for the filtration, separation, and manipulation of bioparticles is reported in this paper. For the fabrication of a nanoporous AAO, a two-step mild anodization (MA) and hard anodization (HA) technique was used. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the surface morphology of nanoporous AAO. A nanoporous structure with a pore diameter in the range of 50–90 nm, an interpore distance of 110 nm, and an oxide layer thickness of 0.12 mm with 60.72% porosity was obtained. Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS) were employed to evaluate AAO chemical properties. The obtained results showed that the AAO structure is of hexagonal symmetry and showed where Al₂O₃ is dominant. The hydrophobic properties of the nanoporous surface were characterized by water contact angle measurement. It was observed that the surface of the nanoporous AAO membrane is hydrophilic. Furthermore, to determine whether a nanomembrane could function as a vibro-active nano filter, a numerical simulation was performed using COMSOL Multiphysics 5.4 (COMSOL Inc, Stockholm, Sweden). Here, a membrane was excited at a frequency range of 0–100 kHz for surface acoustics wave (SAW) distribution on the surface of the nanoporous AAO using a PZT 5H cylinder (Piezo Hannas, Wuhan, China). The SAW, standing acoustic waves, and travelling acoustic waves of different wavelengths were excited to the fabricated AAO membrane and the results were compared with experimental ones, obtained from non-destructive testing method 3D scanning vibrometer (PSV-500-3D-HV, Polytec GmbH, Waldbronn, Germany) and holographic interferometry system (PRISM, Hy-Tech Forming Systems (USA), Phoenix, AZ, USA). Finally, a simulation of a single nanotube was performed to analyze the acoustic pressure distribution and time, needed to center nanoparticles in the nanotube.

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.

Enhancing greywater treatment via MHz-Order surface acoustic waves

There is a pressing need for efficient biological treatment systems for the removal of organic compounds in greywater given the rapid increase in household wastewater produced as a consequence of rapid urbanisation. Moreover, proper treatment of greywater allows its reuse that can significantly reduce the demand for freshwater supplies. Herein, we demonstrate the possibility of enhancing the removal efficiency of solid contaminants from greywater using MHz-order surface acoustic waves (SAWs). A key distinction of the use of these high frequency surface acoustic waves, compared to previous work on its lower frequency (kHz order) bulk ultrasound counterpart for wastewater treatment, is the absence of cavitation, which can inflict considerable damage on bacteria, thus limiting the intensity and duration, and hence the efficiency enhancement, associated with the acoustic exposure. In particular, we show that up to fivefold improvement in the removal efficiency can be obtained, primarily due to the ability of the acoustic pressure field in homogenizing and reducing the size of bacterial clusters in the sample, therefore providing a larger surface area that promotes greater bacteria digestion. Alternatively, the SAW exposure allows the reduction in the treatment duration to achieve a given level of removal efficiency, thus facilitating higher treatment rates and hence processing throughput. Given the low-cost of the miniature chipscale platform, these promising results highlight its possibility for portable greywater treatment for domestic use or for large-scale industrial wastewater processing through massive parallelization.

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.