Published papers

The growth rate of vegetables in plant factories can be regulated by environmental factors including light, temperature, and chemicals, which might give rise to mutation in leaf health. Here, we aim to devise a new way that allows for controlling the growth rate of plants in hydroponics as well as maintaining the product quality; we apply underwater ultrasound and dissolved oxygen supersaturation as external stimuli to plants. As an example, we examine the growth of leaf lettuce in hydroponics with exposure to 28-kHz ultrasound and dissolved oxygen supersaturation up to 36 mg/L at 20 °C. Our results show that exposure to the ultrasound of peak-to-peak pressure at 20 kPa or larger works as the growth inhibitor of the leaves and the roots, while the oxygen supersaturation as the growth promoter, without any degradation of chlorophyll in the leaves. This suggests that these external stimuli can be used in the growth control system of plant factories.

A number of acoustic and fluid-dynamic phenomena appear in ultrasonic cleaning baths and contribute to physical cleaning of immersed surfaces. Propagation and repeated reflection of ultrasound within cleaning baths build standing-wave-like acoustic fields; when an ultrasound intensity gradient appears in the acoustic fields, it can in principle induce steady streaming flow. When the ultrasound intensity is sufficiently large, cavitation occurs and oscillating cavitation bubbles are either trapped in the acoustic fields or advected in the flow. These phenomena are believed to produce mechanical action to remove contaminant particles attached at material surfaces. Recent studies suggest that the mechanical action of cavitation bubbles is the dominant factor of particle removal in ultrasonic cleaning, but the bubble collapse resulting from high-intensity ultrasound may be violent enough to give rise to surface erosion. In this paper, we aim to carefully examine the role of cavitation bubbles from ultrasonic cleaning tests with varying dissolved gas concentration in water. In our cleaning tests using 28-kHz ultrasound, oxygen-supersaturated water is produced by oxygen-microbubble aeration and used as a cleaning solution, and glass slides spin-coated with silica particles of micron/submicron sizes are used to define cleaning efficiency. High-speed camera recordings and Particle Image Velocimetry analysis with a pressure oscillation amplitude of 1.4 atm at the pressure antinode show that the population of cavitation bubbles increases and streaming flow inside the bath is promoted, as the dissolved oxygen supersaturation increases. The particle removal is found to be achieved mainly by the action of cavitation bubbles, but there exists optimal gas supersaturation to maximize the removal efficiency. Our finding suggests that low-intensity ultrasound irradiation under the optimal gas supersaturation in cleaning solutions allows for having mild bubble dynamics without violent collapse and thus cleaning surfaces without cavitation erosion. Finally, observations of individual bubble dynamics and the resulting particle removal are reported to further support the role of cavitation bubbles as cleaning agents.

We theoretically and experimentally study the quasistatic growth of bubbles in a gelatin gel under dissolved-gas supersaturation in order to examine the role of the gel elasticity in the mass-diffusion-driven process. First, we model the diffusion-driven bubble growth with the classical Epstein-Plesset approach for quasistatic bubble growth, accounting for elasticity of the medium surrounding the bubbles. Next, we devise an experimental technique to visualize the bubble growth in an air-supersaturated gel of different gelatin concentrations and to obtain the growth rate of the bubble. We show, from comparisons between the theory and experiments, that the bubble growth is hindered by the gel elasticity.

Visualization experiments were performed to study the relation between free-surface motion and bubble translation in a 1-MHz ultrasonic cleaning bath. From the visualization with a video camera, the characteristic frequencies of the free-surface oscillation (under the acoustic radiation force) and the translational velocity of cavitation bubbles (trapped via the primary Bjerknes force) were extracted, showing that there is a strong correlation between the free-surface oscillation and bubble translation. From the context of megasonic cleaning, such free-surface oscillation is expected to contribute to uniform cleaning performance with cavitation bubbles.

In order to study the role of growing cavitation bubbles in the context of ultrasonic cleaning, we perform two-dimensional, axisymmetric Navier-Stokes simulation for compressible, multicomponent flow and examine the so-called Rayleigh growth of an air bubble (with initial radius 33 µm and pressure 10 MPa) near a rigid wall. The simulation suggests that strong shear stress, which is important in physical cleaning such as particle removal, appears as a result of the bubble-growth-induced shock passage. The parametric study with varying a standoff distance of the bubble to the wall shows that the wall shear stress linearly decreases against the standoff distance.

Visualization experiments were performed to examine whether acoustic bubbles play a role in ultrasonic water flow cleaning, as in convention cleaning with ultrasonic baths. Schlieren visualization confirmed the standing-wave-like acoustic field in ultrasonic water flow that collides with a glass surface. Backlight visualization showed that cavitation bubbles appear in the water flow spreading over the glass surface. These bubbles are found to oscillate in volume and move inside film flow and thus expected to play a role as active cleaning agents.

Visualization experiments are performed to examine the role of acoustic cavitation bubbles that appear in 0.43-MHz ultrasonic water flow spreading over glass surfaces in the context of physical cleaning. The cleaning performance is evaluated using glass samples on which small silica particles are spin-coated. The visualization suggests that acoustic cavitation bubbles play a major role in particle removal as in the case of conventional cleaning with ultrasonic cleaning baths.

Large-scale cell suspension culture technology opens up opportunities for numerous medical and bioengineering applications. For these purposes, scale-up of the culture system is paramount. For initial small-scale culture, a simple static suspension culture (SSC) is generally employed. However, cell sedimentation due to the lack of agitation limits the culture volume feasible for SSC. Thus, when scaling up, cell suspensions must be manually transferred from the culture flask to another vessel suitable for agitation, which increases the risk of contamination and human error. Ideally, the number of culture transfer steps should be kept to a minimum. The present study describes the fabrication of an ultrasonic suspension culture system that stirs cell suspensions with the use of acoustic streaming generated by ultrasound irradiation at a MHz frequency. This system was applied to 100-mL suspension cultures of Chinese hamster ovary cells—a volume ten-fold larger than that generally used. The cell proliferation rate in this system was 1.88/day when applying an input voltage of 40 V to the ultrasonic transducer, while that of the SSC was 1.14/day. Hence, the proposed method can extend the volume limit of static cell suspension cultures, thereby reducing the number of cell culture transfer steps.

We experimentally observe and theoretically analyse oscillations of a spherical bubble in a gelatin gel under ultrasound irradiation to quantify viscoelastic effects on the nonlinear bubble dynamics. A bubble nucleus is generated by focusing a laser pulse into a 6 wt% gelatin gel supersaturated with dissolved air, which enables us to control the bubble radius at mechanical equilibrium via influx of the gas air into the bubble. Linearized and finite-amplitude oscillations of the bubble are driven by 28 kHz ultrasound and recorded by a high-speed camera; the resonance curves of the oscillation amplitude as a function of the equilibrium radius are constructed for different ultrasound intensities. First, the viscosity and shear modulus of the gel are obtained by fitting the resonance curve (for the lowest ultrasound intensity) to the linearized solution of the Rayleigh–Plesset model that accounts for the gel's nonlinear elasticity of neo-Hookean type and diffusive effects on the bubble dynamics. Next, finite-amplitude oscillations of the bubble are compared with the nonlinear Rayleigh–Plesset calculations. The comparison suggests a need to include the gel's elasticity in the calculations to more accurately reproduce the nonlinear bubble dynamics. Another important finding is that the so-called spring softening feature appears in the experimentally determined resonance curve as the oscillation amplitude increases, which can be predicted by the Rayleigh–Plesset model. Furthermore, our experiment with the highest ultrasound intensity shows non-spherical oscillation of mode 1 that does not appear in the case of water but can be predicted by shape instability theory.

Acoustic cavitation is used for megasonic cleaning in the semiconductor industry, especially of wafers with fragile pattern structures. Control of transient cavitation is necessary to achieve high particle removal efficiency (PRE) and low pattern damage (PD). In this study, the cleaning performance of solutions with different concentrations of dissolved gas (H2) and anionic surfactant (sodium dodecyl sulfate, SDS) in DIW (DI water) on silicon (Si) wafers was evaluated in terms of PRE and PD. When only DIW was used, PRE was low and PD was high. An increase in dissolved H2 gas concentration in DIW increased PRE; however, PD also increased accordingly. Thus, we investigated the megasonic cleaning performance of DIW and H2-DIW solutions with various concentrations of the anionic surfactant, SDS. At 20 ppm SDS in DIW, PRE reached a maximum value and then decreased with increasing concentration of SDS. PRE decreased slightly with increasing concentrations of SDS surfactant when dissolved in H2-DIW. Furthermore, PD decreased significantly with increasing concentrations of SDS surfactant in both DIW and H2-DIW cases. A high-speed camera setup was introduced to analyze bubble dynamics under a 0.96 MHz ultrasonic field. Coalescence, agglomeration, and the population of multi-bubbles affected the PRE and PD of silicon wafers differently in the presence of SDS surfactant. We developed a hypothesis to explain the change in bubble characteristics under different chemical environmental conditions.

Ultrasound facilitates the penetration of macromolecular compounds through the skin and offers a promising non-invasive technique for transdermal delivery. However, technical difficulties in quantifying ultrasound-related parameters have restricted further analysis of the sonophoresis mechanism. In this study, we devise a bolt-clamped Langevin transducer-based sonophoresis device that enables us to measure with a thin lead zirconate titanate (PZT) sensor. One-dimensional acoustic theory accounting for wave interaction at the skin interface indicates that the acoustic pressure and cavitation onset on the skin during sonophoresis are sensitive to the subcutaneous support, meaning that there is a strong need to perform the pressure measurement in an experimental environment replacing the human body. From a series of the experiments with our new device, the transdermal penetration of polystyrene, silica and gold nanoparticles is found to depend on the size and material of the particles, as well as the hardness of the subcutaneous support material. We speculate from the acoustic pressure measurement that the particles' penetration results from the mechanical action of cavitation.

Current amplification plays an essential role in electrochemistry by improving the productivity of the electrochemical production of industrial materials and enhancing the sensitivity of environmental and biomedical sensing. Various approaches have been explored to enhance steady-state current, such as thin-layer reactors, microelectrodes, and rotating-disk electrodes. Thin-layer reactors have several advantages, including the ability to generate larger currents using bulk-sized electrodes and simple fabrication processes. In this study, we developed a thin-layer reactor using boron-doped diamond (BDD) electrodes with an interelectrode distance of several tens of micrometers, which is comparable to the thickness of a diffusion layer. The use of BDD electrodes enabled reversible redox cycling in the thin-layer reactor, resulting in more than 2-fold current amplification compared to conventional thin-layer reactors. This effect was observed only when BDD electrodes were used for both the working and counter electrodes, and the interelectrode distance was 200 μm and below. Based on the experimental results of the present study, we propose a novel concentration profile model and reaction mechanism for BDD thin-layer reactors that cannot be explained by conventional thin-layer reactor models. The model involves a three-step reversible redox cycle: (1) consumption of reduced species and generation of oxidized species at the working electrode, (2) regeneration of reduced species at the counter electrode, and (3) resupply of reduced species to the working electrode. The BDD thin-layer reactor also demonstrated twice the sensitivity, and a detection limit one-tenth those of the conventional bulk-reactor in electrochemical detection based on the proposed model.