Active sculpting of colloidal crystals

Active Particles Knead Three-Dimensional Gels into Porous Structures

Colloidal gels are prime examples of functional materials exhibiting disordered, amorphous, yet metastable forms. They maintain stability through short-range attractive forces and their material properties are tunable by external forces. Combining persistent homology analyses and simulations of three-dimensional colloidal gels doped with active particles, we reveal novel dynamically evolving structures of colloidal gels. Specifically, we show that the local injection of energy by active dopants can lead to highly porous, yet compact gel structures that can significantly affect the transport of active particles within the modified colloidal gel. We further show how passive interfaces play a topologically significant role in interacting with active particles in 3D. The results open the door to an unexplored prospect of forming a wide variety of compact but highly heterogeneous and percolated porous media through active doping of 3D passive matter, with diverse implications in designing new functional materials for active ground remediation.

Entropy production and collective excitations of crystals out of equilibrium: The concept of entropons

We study the collective vibrational excitations of crystals under out-of-equilibrium steady conditions that give rise to entropy production. Their excitation spectrum comprises equilibriumlike phonons of thermal origin and additional collective excitations called entropons because each of them represents a mode of spectral entropy production. Entropons coexist with phonons and dominate them when the system is far from equilibrium while they are negligible in near-equilibrium regimes. The concept of entropons has been recently introduced and verified in a special case of crystals formed by self-propelled particles. Here we show that entropons exist in a broader class of active crystals that are intrinsically out of equilibrium and characterized by the lack of detailed balance. After a general derivation, several explicit examples are discussed, including crystals consisting of particles with alignment interactions and frictional contact forces.

Translational and rotational dynamics of a self-propelled Janus probe in crowded environments

We computationally investigate the dynamics of a self-propelled Janus probe in crowded environments. The crowding is caused by the presence of viscoelastic polymers or non-viscoelastic disconnected monomers. Our simulations show that the translational as well as rotational mean square displacements have a distinctive three-step growth for fixed values of self-propulsion force, and steadily increase with self-propulsion, irrespective of the nature of the crowder. On the other hand, in the absence of crowders, the rotational dynamics of the Janus probe is independent of self-propulsion force. On replacing the repulsive polymers with sticky ones, translational and rotational mean square displacements of the Janus probe show a sharp drop. Since different faces of a Janus particle interact differently with the environment, we show that the direction of self-propulsion also affects its dynamics. The ratio of long-time translational and rotational diffusivities of the self-propelled probe with a fixed self-propulsion, when plotted against the area fraction of the crowders, passes through a minimum and at higher area fraction merges to its value in the absence of the crowder. This points towards the decoupling of the translational and rotational dynamics of the self-propelled probe at an intermediate area fraction of the crowders. However, such translational-rotational decoupling is absent for passive probes.

Universal reshaping of arrested colloidal gels via active doping

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 the 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 moderately sized 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.

Colloidal swimmers near curved and structured walls

We present systematic numerical simulations to understand the behavior of colloidal swimmers near a wall. We extend previous theoretical calculations based on lubrication theory to include walls with arbitrary curvature, and show how to extract from simulations a set of parameters crucial to accurately estimate the leading hydrodynamic contributions associated with the curvature of a wall. Our results show explicitly how introducing curvature to the wall not only affects the average incident angle the swimmer acquires when swimming near it, but it also leads to much broader angular distributions. This suggests an increasingly leading role of thermal fluctuations with curvature, which in turn results in significantly different motility of the swimmers. We also show how the backwards motion previously reported for pushers also extends to puller-like swimmers under the appropriate conditions. Finally, aiming at understanding the behavior of colloidal swimmers near a colloidal crystal, we also considered the case of a wall built from colloidal particles that are either free to rotate, representing a crystal held together by isotropic forces, or have their rotational degrees of freedom locked-in, representing a crystal held together by directional interactions. In both cases, we find that puller-like swimmers follow a stochastic run-and-tumble-like dynamics.