Using active colloids as machines to weave and braid on the micrometer scale

Abstract

Controlling motion at the microscopic scale is a fundamental goal in the development of biologically inspired systems. We show that the motion of active, self-propelled colloids can be sufficiently controlled for use as a tool to assemble complex structures such as braids and weaves out of microscopic filaments. Unlike typical self-assembly paradigms, these structures are held together by geometric constraints rather than adhesive bonds. The out-of-equilibrium assembly that we propose involves precisely controlling the 2D motion of active colloids so that their path has a nontrivial topology. We demonstrate with proof-ofprinciple Brownian dynamics simulations that, when the colloids are attached to long semiflexible filaments, this motion causes the filaments to braid. The ability of the active particles to provide sufficient force necessary to bend the filaments into a braid depends on a number of factors, including the self-propulsion mechanism, the properties of the filament, and the maximum curvature in the braid. Our work demonstrates that nonequilibrium assembly pathways can be designed using active particles.colloidal machines | active colloids | self-assembly | braids and weaves B iology has long been known to use a combination of programmable interactions with specific energy input through small molecules (ATP) to create remarkable machines. Because we have not yet developed the ability to emulate these machines artificially, understanding assembly pathways at the microscopic scale is a fundamental challenge in both biology and nanotechnology (1). However, recently there have been major advances in colloidal science, making it possible to program interactions between individual units. This has led to robust assembly of complex structures, approaching the complexity of biological systems. But biological systems also use specific energy input of small molecules to create machines. How can we do this with soft materials?A parallel recent development has been to use chemical reactions on the surface of colloids to create motility mechanisms. This has led to the growth of the field of active matter, which aims to design, control, and understand self-propelled particles (2-12). These active particles provide significant yet untapped potential: by combining this mechanism for energy input with controllable interactions, it becomes possible to imagine the creation of colloidal-scale machines that can do work and perform tasks.Here we present a proof-of-principle demonstration of how this could work. We show one way in which active colloids can be harnessed to drive the assembly of weaves and braids, turning long filaments into complex structures, using only active forces [e.g., hydrogen peroxide-propelled colloids (2-4)] and nonspecific short-range attractive forces (e.g., depletion forces). The typical self-assembly paradigm encodes information through the specific interactions between building blocks that in turn promote assembly. For example, a given amino acid sequence consistently folds into a specific pro...

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