Micelle And Soft-Particle Dynamics

Abstract

Soft matter systems deal with particles or collections of particles that respond to applied forces with a non-linear response. Examples include polymers, colloids, vesicles, gels, emulsions, micelles and capsules. It is therefore unsurprising that these soft particles are ubiquitous: present in our everyday life, biologically important, and found in many industrial processes. To these systems, flow plays an essential role technologically (e.g. electrospinning, microfluidics, and directed self-assembly), biologically (e.g., blood flow), and industrially (e.g. polymer extruders and pumping of nuclear waste). Utilizing non-equilibrium coarse-grained molecular dynamics (CGMD), we have explored time and length scales of self-assembled polymeric micelles and polymer grafted nanoparticles (PGNs) beyond the computational realm of atomistic MD. We first showed that concentrated polymeric micelle suspensions strongly organize into 2D patterns normal to the flow direction. The precise lattice formed was found to be dependent upon micelle concentration and the rate of the applied shear, in agreement with literature in the high shear regime. It was also shown, for the first time, that individual micelles exhibit rich dynamical behavior under flow, including tank-treading, trembling, tumbling, stretching and contraction, and rotation. Current work has addressed the latter, the dynamical behavior in the dilute regime of polymeric micelles and PGNs - that is, individual soft particles subjected to shear flow. Understanding these dynamics is interesting fundamentally, but also in the aforementioned applications where soft particles are closely packed. This work has shown that the presence of these soft particles induces a strong disturbance to the otherwise linear shear field, and the shape and extent of this disturbance is a function of a characteristic relaxation time (corona length) and rate of applied shear. Finally, the shape fluctuations, flow-alignment, rotation, and tank-treading of these particles have also been characterized in detail and are compared to the more well-understood dynamics of vesicles and star polymers.