液液二相流

海洋、大気、陸水域など地球上の流れの大部分は密度成層乱流であり、異なる流体相間に浮力 (密度) 差を伴う混相乱流である。流れは「層」と同時に「相」を成している。多くは温度や塩分濃度差に起因する「液-液」あるいは「気-気」混相流であるが、気温と湿度からなる大気成層流・海洋の砕波・水面での気体交換過程などの「気-液」混相流や、濁度-水温などの「固-液」混相流のように異相の流体成分からなる混相流も存在する。後者では、浮力効果に加えて異相間の相互作用が重要となるが、本文では浮力が支配的な前者の場合に限定して、主に浮力と乱流輸送力の比をあらわす成層パラメーター: リチャードソン数、密度フルード数、について解説する。

We present a numerical simulation method based on phase-field model and immersed boundary method for permeation of oil-in-water (O/W) emulsion through fibrous filter in coalescer process. First, the effects of the wettability of fibers and filter porosity on the coalescing dynamics are investigated by the simulations of O/W emulsions permeating through modeled fibrous filters. The simulation results reveal that the highly hydrophilic fiber surface brings about the generation of small secondary droplets during detachment from the filter. In addition, the large pore spacing promotes the formation of larger droplets but increases the number of uncoalesced droplets. Then, to represent a realistic flow field inside the fibrous filters during simulation, a numerical method that coordinates the filter structure obtained by X-ray CT imaging is developed. The simulation demonstrates that the arrangement of closely attached fibers at the permeate side surface of the filter enhances the formation of large droplets.

We propose a microfluidic process to prepare monodisperse microcapsules possessing a large aqueous core and a hydrogel membrane shell through the formation of aqueous two-phase system droplets with dextran-rich core and tetra-PEG-rich shell and subsequent cross-linking reaction in the shell. This microfluidic approach can continuously produce the microcapsules from water-in-oil emulsion droplets under mild conditions without the use of radical initiators and external energy such as heat and ultraviolet light. In addition, the size and thickness of the microcapsules can be controlled separately by changing flow conditions upon microfluidic emulsification. In this paper, authors will introduce the detailed preparation procedure of the microcapsules.

Artificial lipid bilayers structurally mimic cell membrane. Such the artificial lipid bilayers have been used for analyzing membrane transport by incorporating transporters in the artificial lipid bilayers. The membrane transport analysis using the artificial lipid bilayer is advantageous in the sense that less amount of off-target molecules exists in the system compared with the method using the actual cell membrane. That feature makes the measurement highly sensitive and reliable. In this report, the author introduces one of the methods to analyze membrane transport using the artificial lipid bilayers, focusing on fluorescent measurement of membrane transport using artificial lipid bilayer microchambers.

Beautiful patterns such as snowflakes, wind ripples, and cloud patterns are ubiquitous in nature. Some of those are formed by interfacial instability. We focus on two pattern formations caused by the Rayleigh-Taylor instability. Rayleigh-Taylor instability is an interfacial instability between two liquids that occurs when a heavier liquid is on a lighter liquid. The first pattern we discuss is a fractal/cell pattern in a coffee cup. If a droplet of coffee solution is placed on milk, the coffee solution spreads on the surface of the milk. Then the Rayleigh-Taylor instability occurs at the interface between the coffee solution and milk, and the coffee solution starts to sink into the milk. We found that the fractal or the cell pattern is formed at the surface of the milk in this process. We showed that an aspect ratio between the radius of the container and the depth of the milk affects vertical flow and it leads to a transition between the fractal pattern and the cell pattern. We also showed that the fractal pattern is formed by a similar mechanism of a viscous fingering. The second pattern is a breakup of a droplet. A sinking droplet in a viscous solution spontaneously deforms to a vortex ring and then breaks up spontaneously. We experimentally investigated relations among breakup number, radii of droplets, viscosities and density differences between two solutions. We also proposed a phenomenological model considering the Rayleigh-Taylor instability. The phenomenological model provides a non-dimensional parameter derived from a radius of a droplet, viscosity and a density difference between two liquids. And, the model states that the breakup number is classified by the parameter. Our experimental results obey the parameter. It means that the competition between a driving force of gravity and the viscosity dissipation at the interface of two solutions determines the breakup number.