Phase separation of binary mixtures containing particles

Wetting-induced depletion interaction between particles in a phase-separating liquid mixture

@Inclusion of solid particles drastically affects the pattern evolution of phase separation of a binary fluid mixture, via preferential wetting of one of the phases to the particles. Here we study this problem by numerical simulation, which incorporates interparticle hydrodynamic interactions properly. When particles favor one of the components of a mixture, wetting layers are quickly formed on the particle surfaces and all particles are eventually included into the more wettable phase. For immobile particles, domains of the more wettable phase are pinned to the particles and the domain growth is thus suppressed. For this case, the domain size at a certain phase-separation time decreases monotonically with increasing the particle concentration. For mobile particles, on the other hand, the reentrant morphological transformation is observed as a function of the particle concentration: With an increase in the particle concentration, the domain morphology of the more wettable phase sequentially changes from network, droplet to network. We found that the final morphological transition is induced by wetting-induced depletion interaction: Strong attractive interactions act among particles when the total volume of the more wettable phase is not enough to cover all the particles by wetting layers.

Fig. 1: Pattern evolution of a symmetric liquid mixture containing immobile particles (white circles) (a) Np=100, (b) Np=196, (c) Np=200

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.

Fig. 2: Snapshots of a Janus particle in periodically phase-separating binary fluids. The average concentrations of the more wettable phase are (a)0.3, and (b)0.5
AnimationsFvolume fractions $\bar{\phi}=0.3$, $\bar{\phi}=0.5$, $\bar{\phi}=0.7$B