Nonequilibrium phenomena can be ubiquitously seen in nature and daily lives. Comparing with equilibrium states where physical properties are defined by thermodynamical conditions, dynamical paths are the most important for the nonequilibrium phenomena. Thus, visualization technique is one of the most crucial methods for understanding of the nonequilibrium phenomena. Here we introduce two equilibrium phenomena. The first one is a collective collapse of a foam. When a liquid film is punctured, a liquid droplet is emitted. In addition, the emitted droplet punctures a distanced liquid film and then the collapse occurs collectively. The other example is a gravitational instability of fluids. We use a physical gel as a lower layer in order to sustain a heavy upper layer. When we heat the sample rapidly, the physical gel is fluidized. Then the instability starts to occur. It makes us to observe the instability from the beginning without any artificial perturbation.

The enhancement of bubble rising velocity was experimentally investigated by mechanically applying an oscillating pressure to a single small air bubble (e.g., 1 mm3) in a viscoelastic fluid. For shear-thinning fluids, the cyclic change in bubble diameter induced by the oscillating pressure generates a continuous strong local flow near the bubble surface. The motion of a tiny air bubble in an aqueous solution of cetyl trimethyl ammonium bromide and sodium salicylate, which forms wormlike micelles, was observed using a high-speed polarization camera. Complex local flow is observed around the bubble surface because the bubble shape deforms repeatedly at 100 Hz. The camera can be used to measure the retardation distribution, which is related to the stress field around the bubble. Different retardation and orientation angle distributions were observed during the contraction and expansion phases.

Turbulence attenuation in turbulent boundary layer flow due to surfactant additives was explained by using flow visualization of dye-injection and PIV measurements.

An experimental study was performed to investigate the effects of the extensional rheological properties of polyethyleneoxide (PEO) solutions on vortex deformation in turbulent flow and turbulent statistics. To focus on the extensional properties, a self-standing two-dimensional turbulent flow was used as an experimental setup, and the flow was observed through interference patterns and particle image velocimetry. Vortex shedding and the resulting deformation in the 2D flow were categorized into three types, which was defined by the shedding frequency and relaxation time of the polymer solution. Turbulent energy was suppressed by PEO additives. However, the suppression was not linearly varied with PEO concentrations. The results imply that the characteristic turbulent energy peak appearing upon PEO addition is influenced by the relaxation process of the extended polymers in the flow.

  A suspension in which a phase-change material is dispersed in water in the form of fine particles by the action of a surfactant is called a phase-change emulsion. Phase-change emulsions are expected to be used not only as a latent heat storage material but also as a highly efficient heat transport medium because they exhibit high heat exchange performance under conditions accompanied by a solid-liquid phase change of the dispersoid. It was suggested that the high heat exchange performance associated with the solid-liquid phase change of the dispersoid is due to the effect of convection being promoted by the density change accompanying the phase change of the dispersoid, in addition to the effect of latent heat of fusion. This paper introduces some results on the unique temperature field and convection field exhibited by the phase-change emulsion that naturally convects in a rectangular vessel with a vertical heating/cooling wall when the solid-liquid phase change of the dispersoid is accompanied.

Since "temperature-sensitive magnetic fluid" has a large temperature dependence of magnetization in the range of 100 degrees or less, it is expected to be applied to heat transport devices. Ferromagnetic particles in ferrofluids are known to form chain clusters, and this cluster structure promotes heat transfer while causing clogging. Understanding and controlling the formation characteristics of this cluster is important for the practical application and high functionality of the device. However, since the magnetic fluid is black and opaque and the magnetic particles are nano-sized, it is difficult to obtain an experimental local flow field. Therefore, the authors are trying to understand the flow characteristics by visualizing and measuring the behavior of magnetic flow fields and clusters by encapsulating magnetic nanoparticles in larger polymer microparticles and applying fluorescent labeling. We introduce recent attempts on magnetic microparticles synthesized by the authors, measured magnetic clusters, and flow fields.