The hydrogenation of α-methylstyrene was used as a model reaction for three-phase reaction processes, and the effects of various operational parameters on the enhancement of the reaction rate in a Taylor flow reactor were investigated. The reaction rate was found to be inversely proportional to the thickness of the liquid film and directly proportional to the gas hold-up. A comparison of the predicted reaction rates with the experimental results revealed that, as the reaction rate increased, the discrepancy from the model also grew. It was determined that the primary source of this discrepancy was the prediction of mass transfer within the liquid film. By incorporating the average depth of the roughness features into the diffusion distance, accurate predictions of the reaction rate were achieved with approximately ±20% precision.

This review introduces numerical models based on direct numerical simulation and the phase-field method for dispersion systems in microfiltration, along with their application to membrane performance evaluation. The simulations employ focused ion beam scanning electron microscope (FIB-SEM) image analysis to accurately reconstruct the three-dimensional microstructure of actual microfiltration membranes. To implement the complex geometries of microstructures in microfiltration membranes with a simple algorithm, a signed distance function (SDF) was used for granular flow simulation, and a free-energy-based wetting model was employed for emulsion simulation. These methods enable detailed analysis of colloidal suspension and emulsion dynamics in the membrane microstructures, revealing key correlations with membrane performance.

Alkaline water electrolysis (AWE) has attracted significant attention for producing hydrogen. One of the problems with AWE is the increase in overpotential, which is attributed to bubbles covering the electrode surface at high current densities. Therefore, understanding the relationship between the bubble formation behavior and reactant transfer mechanism is important for reducing the overpotential. In this study, an operando observation system for the oxygen and hydrogen bubble formation behavior on nickel wire electrodes was developed using a microscope-type high-speed video camera. The relationship between the current density-electrode potential curve and bubble generation behavior was then investigated. The results showed that the generation of oxygen/hydrogen bubbles was activated as current density increased. At a high current density, bubbles covered the electrode surface, and the overpotential increased. Electrochemical impedance spectroscopy (EIS) measurements were performed to analyze the resistance components of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). The EIS results showed that the diffusion rate of hydroxide ions through the bubble-covering layer on the anode can be modeled by the Warburg impedance with a finite diffusion boundary. The migration rate of ions driven by the potential gradient in OER and HER was modeled by a tortuosity model with void fraction and bubble-covering layer thickness. A method for promoting bubble detachment via the addition of tert-butyl alcohol and external magnetic fields was introduced. Finally, a computational fluid dynamics (CFD) simulation of the gas-liquid two-phase flow and a deep-learning analysis method for the size distribution of crowded bubbles in an electrolysis cell were introduced.

This paper reviews the authors’ studies on the optical measurement of mesoscale wetting dynamics using phase-shifting ellipsometry, highlighting its capability to capture nanometer-thick liquid film behavior near the contact line. The method enables precise analysis of film thickness profiles, dynamic contact angles, and precursor film lengths, providing valuable insights into the fundamental mechanisms of wetting. The authors’ work also investigates superspreading wetting, where certain nanofluid droplets exhibit larger spreading. While this phenomenon has primarily been associated with aqueous surfactant solutions in previous studies, the authors analyzed it using the phase-shifting ellipsometer and identified it as a phenomenon that cannot be explained by the previously proposed mechanisms. These experimental data and findings offer a novel perspective on droplet behavior on solid substrates.

The phase-change, e.g. condensation, phenomena have been a hot issue in the field of gas-liquid two-phase flow researches. To predict the occurrence of phase-change phenomena accurately in gas-liquid two-phase flows, not only the advanced measurement techniques, but the credible CFD methods should be established. In this study, the authors propose a modified condensation simulation procedure based on an interface-tracking method, in consideration of interface advancement thorough a cell-boundary. The proposed simulation method is verified by solving the well-known Stefan problem to show the superiority to conventional methods. Then, the proposed method is employed to the simulation of a simple experiment of direct contact condensation phenomena to show the applicability to practical complicated condensation flows.

To improve capacitance-based void fraction measurement method in horizontal pipes, the correlation between the two-phase flow pattern in the pipe and the measured capacitance value was investigated by simultaneous measurement with X-ray imaging. The rectangular electrode pairs were placed above and below the horizontal pipe, and the capacitance between the electrodes was measured. X-ray transmission images were also taken at the same measurement position, and the void fraction was estimated. By synchronizing each measurement, the fluctuation of void fraction and capacitance was compared. As a result, the knowledge of the relationship between the measured capacitance and the flow structure was obtained.

The growth and collapse of a laser-induced bubble inside and outside water-agarose boundaries were observed with a high-speed camera. Agarose gels with Young's modulus E = 2.3 kPa, 7.1 kPa, and 13 kPa were used as tissue-mimicking phantoms. The effects of Young's modulus of agarose and the dimensionless stand-off distance γ (negative γ means that the bubble is produced inside the agarose gel) on the dynamics of laser-induced bubbles (i.e., bubble oscillation time and bubble migration) were investigated. It was shown that the oscillation time of a bubble became longer with an increase in γ. When a bubble was produced in the agarose gel, the oscillation time became shorter with an increase in Young's modulus. It was also shown that the bubble migrated toward the water region and penetrated the water-agarose gel boundary when it was produced in the gel and close to the boundary; the larger Young's modulus of agarose became, the larger the bubble migration became. When a bubble grew and collapsed across the water-agarose gel boundary, the bubble part occupied in the agarose collapsed earlier than in the water because the pressure gradient around the former part differed from that around the latter. This pressure gradient around the bubble resulted in the acceleration of the bubble migration toward the water region.

Bubble visualization using a high-speed video-camera has been used as a measurement technique of bubble diameters and velocities. However, the bubble detection was difficult under the condition of the high void fraction because the overlapping bubbles for the sight direction of the camera increase with the void fraction. Additionally, the visualization for a system with objects, such as rod bundle flow channels, becomes more difficult. In this study, we applied a deep learning-based bubble detection technique with Shifted Window Transformer to bubble images shoot from two directions to identify the bubble size, three-dimensional (3D) positions of bubbles, 3D bubble trajectories in the rod bundle flow channel. Furthermore, we used perfluoroalkoxy alkane tubes with almost the same reflection as water in the channel to visualize the bubbly flow in the whole of the flow channel. We confirmed that the detection technique can segment individual bubbles in overlapping bubbles and bubbles behind the rod. By using the detection results, we estimated the diameter and velocity of each bubble and cross-sectional void fraction.

The bubble-bubble interaction of two spherical bubbles rising along an inclined flat wall was investigated experimentally. The relative position of all pairs of two bubbles was statistically analyzed. It was observed that as the two bubbles rose along the wall and the rise distance increased, the bubble alignment changed from vertical to horizontal. It was also observed that the bubble alignment changed from vertical to horizontal with an increase in the bubble generation frequency. This suggests that both bubble generation frequency and rise distance are key factors in determining the formation of bubble clusters. The comparison of obtained 2D discrete pair distribution functions between different bubble generation frequencies and rise distances showed some similarities between them; the bubble accumulation for low bubble generation frequency at large rise distance was similar to that for high bubble generation frequency at small rise distance. The correlation coefficient between them was found to be more than 0.8 in some cases.

One of the most widely used film-forming technologies in industry and research is the spin-coating method. Spin coating is a technology in which a coating solution containing dispersed functional materials is dropped onto a rotating substrate and the film is formed by centrifugal force generated on the rotating substrate. One of the major challenges in spin coating is the control of undulation film thickness. The authors have proposed a method to control the centrifugal force on the substrate by employing a planetary rotation mechanism that adds orbital rotation to the substrate's spinning rotation. Undulation film thickness is often caused by the centrifugal force in one direction due to the spinning rotation of the substrate, and a planetary rotation makes it possible to achieve homogeneous film formation by varying the centrifugal force on the substrate over time. In this paper, based on the results of film formation experiments using alumina slurry by planetary spin-coating and the observation of coating liquid flow on the substrate using black-and-white ink, the suppression effect of uneven film thickness was investigated, and undulation film thickness was successfully suppressed by planetary spin-coating and the suppression mechanism was discussed through the observation of coating liquid flow.

Jet cleaning is a method used for various applications, such as oil cleaning and oral cavity cleaning, and many studies have been conducted on methods to improve cleaning capability. However, there are few studies focused on the behavior of dirt by jet cleaning, and the removal process has not been clarified. In this study, gas-liquid two-phase pulsating jet is focused on and the deformation behavior of dirt by this jet was investigated with detailed visualization. Silicon oil of various viscosities was used as simulated dirt. It was found that the impact of the jet stream on the dirt deforms the dirt into a crater shape. In gas-liquid two-phase pulsating jets, where there are many bubbles and voids in the jet, fine fluctuations in collision pressure were observed, and it is certain that craters quickly reach the baseplate due to continuous flow inside the sample. Detailed visualization and measurement of this behavior confirmed that cleaning proceeds in two stages: “crater formation process by jet intrusion” before the crater reaches the baseplate, and “crater expansion process by diffusion flow” after the crater reaches the baseplate.

The roughness on wall surfaces significantly affects the behavior of bubble nucleation and its growth during boiling. In this study, molecular dynamics simulations of bubble growth on a wall with periodic roughness were performed in order to examine the effect of nanoscale roughness on the contact line motion with bubble growth from a molecular-scale perspective. The contact line was pinned at the edge of the bumps, and its depinning behavior depended on the fluid density in the grooves. In this study, the depinning behavior is classified into three types based on the fluid density in the grooves. The first type was that the contact line was newly formed when the gas-liquid interface contacted the edge of neighboring bump, which occurred when the fluid density in the groove was same as that of the liquid phase. The second type was that the liquid molecules near the contact line detached from the top surface of the bump, which occurred when the fluid density in the groove was same as that of the gas phase. The third type was that the fluid density in the groove fluctuated between those of the liquid and gas phases, and the fluid in the groove was regarded as a part of the gas-liquid interface.