Mesoscopic simulations of shear-induced phase transition in active colloidal suspensions

Phase transition and collective dynamics of dense active colloidal suspensions is a fascinating topic in soft matter physics, in particular for out-of-equilibrium systems. These systems are challenging to describe with conventional methods due to the coexistence of different length-time scales and the lack of molecular-level details. The issue is to include all together the effects of thermal fluctuations, hydrodynamic interactions and spatio-temporally varying forces.

As an initial step, Brownian dynamics is used to describe active colloids interacting via a repulsive Yukawa potential for the melting of colloidal suspensions. The activity of the system has been introduced by a self-propulsion in the presence of a steady shear flow. In this way, the interplay between activity and shear could be analysed without hydrodynamics.

Concurrently, a mesoscopic approach, as the most suitable choice to model the solvent of colloidal suspensions, is followed. In this regard, the Multiparticle Collision-Anderson Thermostat (MPC-AT) algorithm is preferred due to its capability to preserve also the angular momentum with respect to other MPC algorithms. A conventional molecular dynamics method is then used to describe the colloids, leading, in combination with MPC-AT for the fluid particles, to a multi-scale approach for the same simulation.

The structural changes are followed up through the global bond-orientational order parameter, by changing the strength of self-propulsion and the shear rate. Thereby, we characterize the non-equilibrium state diagram, showing the hexagonal crystal to melt transition. Further, we quantify the effect of hydrodynamic interactions, which effectively shifts the transition line.