Universal reshaping of arrested colloidal gels via active doping

Abstract

Colloids that interact via a short-range attraction serve as the primary building blocks for a broad range of self-assembled materials. However, one of the well-known drawbacks to this strategy is that these building blocks rapidly and readily condense into a metastable colloidal gel. Using computer simulations, we illustrate how the addition of a small fraction of purely repulsive self-propelled colloids, a technique referred to as active doping, can prevent the formation of this metastable gel state and drive the system toward its thermodynamically favored crystalline target structure. The simplicity and robust nature of this strategy offers a systematic and generic pathway to improving the self-assembly of a large number of complex colloidal structures. We discuss in detail the process by which this feat is accomplished and provide quantitative metrics for exploiting it to modulate self-assembly. We provide evidence for the generic nature of this approach by demonstrating that it remains robust under a number of different anisotropic short-ranged pair interactions in both two and three dimensions. In addition, we report on a novel microphase in mixtures of passive and active colloids. For a broad range of self-propelling velocities, it is possible to stabilize a suspension of fairly monodisperse finite-size crystallites. Surprisingly, this microphase is also insensitive to the underlying pair interaction between building blocks. The active stabilization of these moderatelysized monodisperse clusters is quite remarkable and should be of great utility in the design of hierarchical self-assembly strategies. This work further bolsters the notion that active forces can play a pivotal role in directing colloidal self-assembly.

Microscopic origin of tunable assembly forces in chiral active environments

“…24. Also closely related, a series of works by Mallory et al 14,25,26 demonstrate that nonequilibrium perturbations arising from an active bath can provide a self-organizing force. They investigate, for example, a setting in which the active particles are designed with an inherent asymmetry that produces directional flows and hence kinetically induced aggregation.…”

Electroosmotic flow spin tracers near chemical nano/micromotors

“…6 As a result, a nano/micromotor attracts or repels nearby tracers and forms exclusion zones, 6,7 colloidal crystals, 8,9 or colloidal gels. 10,11 Beyond attraction and repulsion, torques are another primary interaction mechanism, and play a critical role in inducing collective behaviors among rod-shaped natural microswimmers. 12 Yet, torques by synthetic nano/micromotors are rarely reported.…”

“…6 As a result, a nano/micromotor attracts or repels nearby tracers and forms exclusion zones, 6,7 colloidal crystals, 8,9 or colloidal gels. 10,11…”

Dynamic overlap concentration scale of active colloids

“…An interest motivated, in part, by the tantalizing prospect that mechanical equations-of-state (EoS) will play as important of a role in active matter as they do in equilibrium theory. In fact, pressure has already proven to be a salient metric for rationalizing the often-complex behavior of active suspensions ranging from instabilities exhibited by expanding bacterial droplets [16], vesicles filled with active particles [17][18][19][20], active depletion [21][22][23], and the dynamics of colloidal gels [24][25][26], membranes [27], and polymers [28][29][30] immersed in a bath of active colloids. The burgeoning of research on the nature of pressure in active systems has not only contributed to progress in understanding the phenomenology of active systems, but has also played a central role in assessing the validity of new theoretical concepts for nonequilibrium systems [31][32][33][34][35][36][37][38][39][40][41].…”