Marked changes in behavior of flowing interfaces induced by particles that were observed by the author are reviewed. The first phenomenon is related to the trap of particles on a liquid–gas interface, which is widely used to the formation of the colloidal crystals: a formation of a 3D-structured monolayer in a glass capillary is presented. The formation mechanism is also explained by means of the particle and the wall wettability. The second phenomenon is related to the long-ranged hydrodynamic interaction of particles: a case of droplets entering “concentration interfaces” of dyed water of particle suspension is presented. The droplet motion shows that the interface exerts the repulsive force if the interface is consisted of particle suspension. The magnitude of the repulsive force, which stems from the overlap of the Stokeslets generated by the migration of each particle, is estimated and showed that it is comparable to the force exerted on the droplets by the surrounding flow.

Drop evaporation, a common fundamental process in numerous industrial applications, involves interplay of several thermofluidic phenomena such as vapor diffusion, heat transfer, flow motion and wetting. It is possible to “see” and investigate such complex and beautiful phenomena using appropriate method. During the last decade, infrared (IR) thermography has become a popular tool to visualize local temperature distributions in evaporating drops without any thermal or flow disturbance. However, thermal images directly obtained by the IR camera can often be meaningless. It usually requires post-processing, accounting for the IR radiative properties of the system, to produce more accurate temperature data. In this article, calculations of the temperature at the liquid-vapor interface from raw IR images is demonstrated. Applying this method to ethanol drops evaporating in humid air, an implication of condensation of surrounding water vapor into the drop was found and later confirmed by gas chromatograph analysis. IR camera was also used to reveal thermocapillary flows in pure water drops induced by localized heating, implying a new technique for micromixing.

Mass transportation of fine particles in bubbly flow are used in a variety of applications such as PM_2.5 filtration and suppression of fission products. The present study aims to measure such transportation phenomena from the concentration field of dissolved particles from a bubble by using interferometric fringe measurement. In this paper, firstly, the Maki-type interferometer, which is a modified Mach-Zehnder interferometer was introduced. Using this interferometer, the mass transportation at the bubble interface was visualized. Next, the basic theory of the Fourier transform method for interference fringe measurement was explained. By using the method, the disturbance of interference fringe by particle dissolution was converted into the projection of phase information due to refractive index difference of solution. Finally, by using the Abel inverse transform, the reconstruction of projection information into cross-sectional information was explained. The series of techniques explained are applicable to new areas for interferometric technology, such as visualization of multiphase mass transportation and mixing phenomena.

In this paper, we introduce pulse echography techniques for the monitoring of bubbly flows and demonstrations of each technique. These techniques can be classified as two categories according to the size of bubbles; over mm-sized bubbles and sub-mm-sized bubbles. Analyzing methods to extract information of the gas-phase in bubbly flows from echo waveforms are different in these two categories. The information includes the void fraction, interface, velocity, and volume flow rate of gas-phase in the flow. In the addition, for the extension of these techniques to the three-phase flows, we suggest a technique to distinguish the phases of material, such as bubbles and solid particles, in the liquid.

Gas-liquid two-phase flow is observed in various engineering disciplines including chemical and nuclear power plants. It is known that several two-phase flow regimes exist due to the presence of deformable gas-liquid interface. For the improved safety and optimization in operating plants, it is crucial to understand the behavior of the gas-liquid interface for the advancement of two-phase flow models. In this review article, extraction of the bubble features in bubbly flow regime using the object detection algorithms based on convolutional neural networks (CNN) is briefly reviewed. The generation of synthetic bubbly flow images using GAN (BubGAN) applied towards upward two-phase flow in circular channel is also presented. Since void fraction and interfacial area concentration are one of the geometric parameters in the two-phase flow models, including two-fluid model, adaptation of deep neural network has a potential to evaluate the two-phase flow parameters instantaneously by calculating these parameters from the extracted information.

The Lattice Boltzmann Method (LBM) is one of the methods in Computational Fluid Dynamics (CFD). One of the fields closely related to the progress of LBM is multiphase flow. LBM is a mesoscopic numerical method based on the analogy of gas-molecule kinetics, and it has been demonstrated to be suitable for simulating interfacial dynamics and multi-phase systems with phase transitions. Because of its mesoscopic characteristics, LBM is considered to be one of the most promising solutions for the multiscale phenomenon of boiling. In this paper, as one of the examples of the application of LBM, we present the methodology based on the pseudo-potential model and some recent results for the boiling.