Boundary and concentration effects on the creeping motions of core-shell particles

Abstract

In this study, the creeping motions of core-shell particles at low-Reynolds numbers were analyzed and discussed in three cases: (i) a freely translating particle; (ii) a particle in a concentric hole; (iii) particles in a monodisperse suspension. The core and shell were assumed concentric and consist of immiscible liquids. The results revealed the particle motions were significantly influenced by the radius (or volume fraction) and viscosity of the shell and core, especially the shell viscosity. In the limiting situations, our general results for two-phase particles could be simplified to the studies of single-phase particles (solid, liquid or gas bubble) in literature. For the case of a freely suspending particle with the same volume fraction of shell and core, the modified coefficient was reduced by more than 30% as the dimensionless shell viscosity reduced from 100 to 0.01 at ηc∗=1; however, the coefficient was only slightly reduced by less than 1.5% as the dimensionless core viscosity reduced from 100 to 0.01 at ηs∗=1. For the translation of a particle in a concentric hole, the boundary effect of the cavity wall performed to significantly hinder the particle motion. When the volume fraction of the particle in the hole was 0.01 and 0.1, in the case of a21 = 1.25, ηc∗=1 and ηs∗=0.01, the boundary effect retarded the particle velocity to 2/3 and 1/3 of the free particle velocity, respectively. In a monodisperse system, the mean translational velocity of particles was hindered by the hydrodynamic interaction between the particles, and the hindrance effect increased as the particle concentration increased. Particle concentration and shell viscosity were the main properties affecting the fluid dynamics of the suspension.