Nonequilibrium Dynamics of Soft Materials

Han, Ming

doi:https://doi.org/10.21985/n2-r7kv-hr11

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Nonequilibrium Dynamics of Soft Materials

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Soft materials such as colloids and polymers often exhibit a variety of mesoscopic structures that are governed merely by weak physical interactions. Due to these intermediate structures, they can be easily taken out of thermal equilibrium by introducing external stimuli such as a shear flow and electromagnetic fields. This thesis is concerned with various nonequilibrium phenomena in soft materials, which are either of practical relevance or fundamental importance. We begin by examining nonequilibrium collective behaviors of colloids under external driving forces (Part 1). A microchannel flow can force a colloidal gel to undergo a continuous transition from shear-induced crystallization to melting. Within the transition regime, we discover a cyclic effect in colloidal dynamics, which relies on a combination of nonuniform shear and the Plateau–Rayleigh instability. Unlike a shear flow that drives passive motion, an AC electric field can stimulate the self-propulsion of metal-dielectric Janus colloids. We demonstrate that by simply varying the field frequency, one can reconfigure these Janus colloids into multiple active states. Beyond such design of active materials, we further evaluate the effective temperature concept in an active colloid mixture. We demonstrate that constant interparticle collisions can play the role of temperature, which control both the kinetics and phase behavior of the active-matter system. In fact, a complete mapping between this system with a thermal binary fluid is also found. We then continue to study nonlinear electrokinetics (Part 2), the self-propulsion scheme for individual active colloids. To resolve electrophoretic flow and the underlying charge dynamics, we developed an efficient algorithm that can explicitly calculate both polarization and hydrodynamic effects. Employing such method, we reveal the mechanism of the unexpected flow reversal widely reported in NEK. It arises from strong surface ionic currents facilitated by ion-induced polarization. We also use this method to study the dielectrophoretic rotation of patchy colloids, which were employed for reconfigurable directed-assembly. Lastly, we turn our attention to polymeric gels (Part 3). They stay out of equilibrium via kinetic arrest. We demonstrate that a hydrogel of peptide-DNAs can exhibit a reversible hierarchical structure, where DNA-rich bundles are segregated from DNA-depleted fibrous networks. This hierarchical self assembly relies on monomer exchange that is only allowed by weak interactions. To characterize polymeric gels in general, we invent a spectral machine learning (ML) method, which over beats all existing real-space MLs in both performance and interpretability.

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