Controlled motion of Janus particles in periodically phase-separating binary fluids

We numerically investigate the propelled motions of a Janus particle in a periodically phase-separating binary fluid mixture. In this study, the surface of the particle tail prefers one of the binary fluid components and the particle head is neutral in the wettability. During the demixing period, the more wettable phase is selectively adsorbed to the particle tail. Growths of the adsorbed domains induce the hydrodynamic flow in the vicinity of the particle tail, and this asymmetric pumping flow drives the particle toward the particle head. During the mixing period, the particle motion almost ceases because the mixing primarily occurs via diffusion and the resulting hydrodynamic flow is negligibly small. Repeating this cycle unboundedly moves the Janus particle toward the head. The dependencies of the composition and the repeat frequency on the particle motion are discussed.Comment: 11 pages, 9 figure

Selective solvation in aqueous mixtures: Interface deformations and instability

We briefly review the effects of selective solvation of ions in aqueous mixtures, where the ion densities and the composition fluctuations are strongly coupled. We then examine the surface tension \gamma of a liquid-liquid interface in the presence of ions. We show that \gamma can be decreased drastically due to the electrostatic and solvation interactions near the interface. We calculate how the free energy is changed due to small surface undulations in the presence of an electric double layer. A surface instability occurs for negative \gamma, which can easily be realized for antagonistic ion pairs near the solvent criticality. Three-dimensional simulation shows how the surface instability is induced.Comment: 16 pages, 4 figure

Dynamics of Colloidal Particles in Soft Matters

We developed numerical methods for studying the dynamics of colloidal particles suspended in complex fluids. It is essential to employ a coarse-grained model for studying slow dynamics of these systems. Our methods are based on the "fluid particle dynamics (FPD)" method, which we have developed to deal with hydrodynamic interactions in colloidal systems in an efficient manner. We regard a solid particle as an undeformable fluid one. It has a viscosity much higher than the solvent, which smoothly changes to the solvent viscosity at the interface. This methods allows us to avoid troublesome boundary conditions to be satisfied on the surfaces of mobile particles. Since we express the spatial distribution of colloids as a continuum field, we can easily introduce the order parameter describing a complex solvent, e.g., ion distribution for charged colloids, director field for nematic liquid crystal, and concentration for phase-separating binary fluid. Then we solve coupled dynamic equations of three relevant parameters, i.e., particle positions, flow field, and the order parameter. We demonstrate a few examples of such simulations

Numerical study on the dynamics of colloidal particles immersed in nematic liquid crystal(Soft Matter as Structured Materials)

この論文は国立情報学研究所の電子図書館事業により電子化されました。近年,ネマティック液晶中にコロイド粒子が分散した系において,粒子が数珠状に配列するなど興味深い現象が発見され,多くの研究者達によって活発な研究がなされてきた.しかしながら,粒子に働く力は粒子表面のアンカリング効果によって変形を受けた液晶弾性場に起因するものであるため,本質的に多体効果であり,多粒子系において正しく求めることは難しい.また,液晶の特徴である流動性を取り入れて理論的・数値的研究を行うことも極めて困難である.我々は,コロイド分散系を扱うべく開発した流体粒子ダイナミクス法の分散媒に液晶配向場に関する秩序変数を与えたモデルを考案し,この系の数値シミュレーションを行った.Figure 1は,多数の粒子を含む液晶.コロイド混合系を温度クエンチし等方相からネマティック相へ転移させた後の時間発展の様子である.液晶配向場は粒子表面に対し垂直にアンカリングする.転移後,液晶場は多くの配向欠陥を形成するが,それは液晶の弾性エネルギーを減らすよう時間とともに減少し,それに従い粒子は複雑な配向欠陥を伴いながら凝集していく

Relaxation to steady states of a binary liquid mixture around an optically heated colloid

We study the relaxation dynamics of a binary liquid mixture near a light-absorbing Janus particle after switching on and off illumination using experiments and theoretical models. The dynamics is controlled by the temperature gradient formed around the heated particle. Our results show that the relaxation is asymmetric: The approach to a nonequilibrium steady state is much slower than the return to thermal equilibrium. Approaching a nonequilibrium steady state after a sudden temperature change is a two-step process that overshoots the response of spatial variance of the concentration field. The initial growth of concentration fluctuations after switching on illumination follows a power law in agreement with the hydrodynamic and purely diffusive model. The energy outflow from the system after switching off illumination is well described by a stretched exponential function of time with characteristic time proportional to the ratio of the energy stored in the steady state to the total energy flux in this state