Assembly of vorticity-aligned hard-sphere colloidal strings in a simple shear flow

Abstract

Colloidal suspensions self-assemble into equilibrium structures ranging from face-and body-centered cubic crystals to binary ionic crystals, and even kagome lattices. When driven out of equilibrium by hydrodynamic interactions, even more diverse structures can be accessed. However, mechanisms underlying out-of-equilibrium assembly are much less understood, though such processes are clearly relevant in many natural and industrial systems. Even in the simple case of hard-sphere colloidal particles under shear, there are conflicting predictions about whether particles link up into string-like structures along the shear flow direction. Here, using confocal microscopy, we measure the shear-induced suspension structure. Surprisingly, rather than flow-aligned strings, we observe log-rolling strings of particles normal to the plane of shear. By employing Stokesian dynamics simulations, we address the mechanism leading to this out-of-equilibrium structure and show that it emerges from a delicate balance between hydrodynamic and interparticle interactions. These results demonstrate a method for assembling large-scale particle structures using shear flows.colloids | shear-induced structure T he study of ordered structures and symmetries of equilibrium phases in condensed matter is central to our understanding of its various material properties (1). Once driven out of equilibrium, a material can exhibit richer phases with many unexpected structures (2-6). However, the dynamics of these nonequilibrium phases are much less understood relative to their equilibrium counterparts (7-9). Sheared hard-sphere colloidal suspensions provide a simple system for probing many out-of-equilibrium structures. For example, sliding layers in sheared colloidal crystals (10, 11), shear-induced crystallization of colloidal fluids (5), and formation of shear transformation zones-localized regimes of shear-induced structural rearrangement-in colloidal glasses (12) have been observed in such systems.The simplest and lowest ordered structure predicted to arise from sheared hard-sphere colloidal suspensions is a one-dimensional (1D) string structure, where particles link into strings under shear. This structure has received much attention since it was first found in a numerical simulation (13). Despite intensive study (13-23), however, there is still heated debate over whether such a structure exists in experiments (10,23,24) or is an artifact of certain numerical algorithms (20-23). In part, this controversy persists due to the lack of experimental techniques that provide direct visualization of the suspension structure. Since the string structure is predicted to exist in less concentrated suspensions below the crystallization threshold, previous scattering experiments, which average over large sample volumes, have not provided detailed information to unambiguously confirm or disprove the existence of this less regular structure (10,24). Here, by employing fast confocal microscopy, we show that sheared hardsphere colloidal suspensions form strings a...

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