Shape control and compartmentalization in active colloidal cells

Abstract

Small autonomous machines like biological cells or soft robots can convert energy input into control of function and form. It is desired that this behavior emerges spontaneously and can be easily switched over time. For this purpose we introduce an active matter system that is loosely inspired by biology and which we term an active colloidal cell. The active colloidal cell consists of a boundary and a fluid interior, both of which are built from identical rotating spinners whose activity creates convective flows. Similarly to biological cell motility, which is driven by cytoskeletal components spread throughout the entire volume of the cell, active colloidal cells are characterized by highly distributed energy conversion. We demonstrate that we can control the shape of the active colloidal cell and drive compartmentalization by varying the details of the boundary (hard vs. flexible) and the character of the spinners (passive vs. active). We report buckling of the boundary controlled by the pattern of boundary activity, as well as formation of core-shell and inverted Janus phase-separated configurations within the active cell interior. As the cell size is increased, the inverted Janus configuration spontaneously breaks its mirror symmetry. The result is a bubble-crescent configuration, which alternates between two degenerate states over time and exhibits collective migration of the fluid along the boundary. Our results are obtained using microscopic, non-momentum-conserving Langevin dynamics simulations and verified via a phase-field continuum model coupled to a Navier-Stokes equation.active matter | emergent pattern | confinement | colloids A ctive matter describes particulate systems with the characteristic that each "particle" (agent) converts energy into motion (1, 2). Active matter covers a range of length scales that include molecular motors in the cytoskeleton (3-5), swimming bacteria (6-8), driven colloids (9, 10), flocks of birds and fish (11)(12)(13)(14), and people and vehicles in motion (15). Over the last decade, studies of active matter have demonstrated behavior not seen in equilibrium systems, including giant number fluctuations (16, 17), emergent attraction and superdiffusion (18)(19)(20), clustering (21, 22), swarming (23-27), and self-assembled motifs (28, 29). These systems provide interesting theoretical and engineering challenges as well as opportunities to explore and target novel behaviors that proceed outside of thermodynamic equilibrium.Of particular interest are systems found in nature or inspired by natural phenomena. Biological systems usually operate in confined regions of space--think of intracellular space, interfaces and membranes, and the crowding of cells near surfaces. The role of hydrodynamics in confinement has been studied for biological swimmers, such as bacteria and sperm, showing accumulation at the walls (30-32) and upstream swimming along surfaces (33) or in a spiral vortex (34-36). Attraction to walls has also been reported in the absence of hydrodynamics for disks (37...

Targeted assembly and synchronization of self-spinning microgears

“…Colloidal spinners provide a platform to explore active spinning matter and test theoretical predictions [34][35][36][37][38][39] , although the competition in our system of long-range diffusiophoretic interactions with hydrodynamics may significantly enrich the dynamics. The interplay between phase synchronization and spatial organization has the potential to achieve a new form of self-organization, without equilibrium counterparts, and not observed for collections of translational self-propelled particles 40 .…”

Dynamics of inert spheres in active suspensions of micro-rotors

“…Self-organization in rotor suspensions have been analyzed mostly computationally [29][30][31][32][33], though not all studies consider the effects of the immersing liquid on the rotor motions.…”

Chemically Controlled Spatiotemporal Oscillations of Colloidal Assemblies

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We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters,the size of whichc hanges periodically.T he system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide.The colloid particles first attract each other to form clusters.A fter as hort delay,t hese clusters abruptly disperse and oscillation begins,a lternating between clustering and dispersion of particles.After acluster oscillation initiates,the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase-shifted synchrony. [4][5][6][7][8][9] These synthetic collective systems,when combined with other biomimetic functions,c ould lead to advances in chemical sensing, signal processing, drug delivery,and environmental remediation. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters.D ensely packed Ag 3 PO 4 particles form an on-oscillating core with an oscillating shell composed largely of silica microparticles.

Self-organization takes the form of dynamic patterning and structures in living systems over arange of length scales,from fish schools to bacteria colonies and structural protein assemblies.

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