Multiparticle collision dynamics simulations of hydrodynamic interactions in colloidal suspensions: How well does the discrete particle approach do at short range?

The multiparticle collision dynamics (MPCD) simulation method is an attractive technique for studying the effects of hydrodynamic interactions in colloidal suspensions because of its flexibility, computational efficiency, and ease of implementation. Here, we analyze an extension of the basic MPCD method in which colloidal particles are discretized with a surface mesh of sensor nodes/particles that interact with solvent particles (MPCD + Discrete Particle or MPCD + DP). We use several situations that have been described analytically to probe the impact of colloidal particle mesh resolution on the ability of the MPCD + DP method to resolve short-ranged hydrodynamic interactions, which are important in crowded suspensions and especially in self-assembling systems that create high volume fraction phases. Specifically, we consider (A) hard-sphere diffusion near a wall, (B) two-particle diffusion, (C) hard-sphere diffusion in crowded suspensions, and (D) the dynamics of aggregation in an attractive colloidal suspension. We show that in each case, the density of sensor nodes plays a significant role in the accuracy of the simulation and that a surprisingly high number of surface nodes are needed to fully capture hydrodynamic interactions.

Partition of nanoswimmers between two immiscible phases: a soft and penetrable boundary.

The behavior of run-and-tumble nanoswimmers which can self-propel in two immiscible liquids such as water-oil systems and are able to cross the interface is investigated by dissipative particle dynamics. At the steady-state, the partition ratio (φ) of nanoswimmers between the two immiscible liquids is obtained, and it depends on the active force (Fa), run time (τ), and swimmer-solvent interactions. The partition ratio φ is found to grow generally with increasing Fa2τ. At sufficiently large Fa, it is surprising to find that hydrophilic nanoswimmers prefer to stay in the oil phase rather than in the water phase. The partition ratio is also influenced by the hydrophobicity of swimmers in the oil phase. Two simple models are proposed to describe the partition ratio, including a near-equilibrium model and a kinetic model. Surface accumulation appearing at an impenetrable interface is also observed at the fluid-fluid interface for small Fa but it vanishes for sufficiently large Fa.

Favorable partition of nanoswimmers toward a confined slit.

The partition of nanoswimmers between a narrow channel and a reservoir is explored by dissipative particle dynamics. In contrast to passive colloids, nanoswimmers prefer to stay in the slit rather than in the reservoir for sufficiently large active force (F_{a}) or run time (τ). The partition ratio (φ) increases with F_{a} and τ. Interestingly, as the slit height decreases, φ grows accordingly until the confinement effect dominates. Three types of the concentration profile in the slit are identified as a consequence of the competition between surface accumulation and entropic barrier.