Complex dynamics of sheared active particle suspensions

Abstract

Active systems, whether natural or artificial, have a unique ability to extract energy from their surroundings, driving themselves out of equilibrium. This capability gives rise to a variety of non-equilibrium phenomena such as swarming and clustering, creating potential uses for new materials and technologies. Among these active systems, we are particularly interested in the complex dynamics and the rheological behaviors of active colloidal suspensions and chiral active polymer chains when they are exposed to shear flow. In this sense, active Brownian dynamics (ABP), one of the most common computational methods, is used to study the complex dynamics of these active systems. In addition, the Multiparticle Collision Dynamics (MPCD) method is chosen to simulate in a computationally efficient way how the solvent dynamics, especially the resulting hydrodynamic effects, around these active systems behave. Phase transitions and collective dynamics of active colloidal suspensions are fascinating topics in soft matter physics, particularly for out-of-equilibrium systems, which can lead to rich rheological behaviours in the presence of steady shear flow. The role of self-propulsion in the rheological response of a dense colloidal suspension is investigated by using particle-resolved Brownian dynamics simulations. First, the combined effect of activity and shear in the solid on the disordering transition of the suspension is analyzed. While both self-propulsion and shear destroy order and melt the system if critical values are exceeded, self-propulsion largely lowers the stress barrier needed to be overcome during the transition. We further explore the rheological response of the active sheared system once a steady state is reached. While passive suspensions show a solid-like behaviour, turning on particle motility fluidises the system. At low self-propulsion, the active suspension behaves in a steady state as a shear-thinning fluid. Increasing the self-propulsion changes the behaviour of the liquid from shear-thinning to shear-thickening. We attribute this to clustering in the sheared suspensions induced by motility. This new phenomenon of motility-induced shear thickening (MIST) can be used to tailor the rheological response of colloidal suspensions. Expanding the active Brownian dynamics simulation for particles of more complex shapes such as active polymers, we explore the complex formation of an active flexible polymer chain in linear shear flow in two spatial dimensions. The chiral head monomer is active and circling, while all other monomers are passive following both the motion of the head polymer and the shear flow. By the combination of activity, chirality and shear rate, a wealth of different states are found including the formation of a linear/complex folding and a spiralling state with both head-in and head-out morphologies. As the vorticity of the applied shear competes with the circling sense of the head, the chirality of the whole complex can be tuned by activity. Our results are relevant to characterize the response of living and artificial chiral active polymer chains to complex flow fields. These initial Brownian dynamics simulations of active polymer chains under shear flow did not account for hydrodynamic interactions (HI), which simplified the system to a dry environment influenced only by Gaussian-colored noises. Recognizing the potential impact of HI on the conformational and dynamical properties of these polymers, we advanced our study by incorporating shortly the hydrodynamic interactions using a hybrid Molecular Dynamics-Multiparticle Collision Dynamics (MD-MPCD) approach. In this way, we discuss the polymer dynamics and conformations for more realistic scenarios, observed in experiments. This hybrid approach is an improvement that captures the hydrodynamic effects of the solvent as well as thermal fluctuations. The swelling effect induced by HI is critical for these transitions, causing delayed conformational changes on the state diagram and preventing certain states already observed in the absence of HI. The results show that hydrodynamic interactions enhance linear folding and head-in spiraling states while suppressing complex folding and head-out spiraling. We could analyze simply the complex interplay between self-propulsion, shear, and hydrodynamics in active chiral polymer systems under shear flow, which will provide more realistic insights for future research and applications.