Dynamic self-assembly of microscale rotors and swimmers

Abstract

Biological systems often involve the self-assembly of basic components into complex and functioning structures. Artificial systems that mimic such processes can provide a well-controlled setting to explore the principles involved and also synthesize useful micromachines. Our experiments show that immotile, but active, components self-assemble into two types of structure that exhibit the fundamental forms of motility: translation and rotation. Specifically, micron-scale metallic rods are designed to induce extensile surface flows in the presence of a chemical fuel; these rods interact with each other and pair up to form either a swimmer or a rotor. Such pairs can transition reversibly between these two configurations, leading to kinetics reminiscent of bacterial run-and-tumble motion.Self-assembly is a hallmark of biological systems as organisms construct functional complex materials and structures from simpler components. The use of selfassembly in the fabrication of new materials is attractive owing to its inherent versatility and potential for mass production [1]. Research has focused primarily on systems where the formation of macrostructures does not require the continuous input of energy [2-5], a process known as equilibrium self-assembly. Another route to self-assembly is dynamic: where structures persist only while energy is being supplied to the system [6]. Artificial micron-scale motors, immersed in a fuel laden fluid, have been shown to spontaneously self-organize into crystal structures [7], and form into asters of two or more particles [8]. These studies have active components, producing a local fluid flow which leads to motility. A striking aspect of these active systems is their potential for emergent dynamics, whereby individuals and groups have qualitatively different behaviour [9], such as flocking [10], formation of flow structures on larger scales than the individual components [11], or collective flows which are faster than those induced by individual active particles [12].Here we describe how two of the simplest machines can be formed by dynamic self-assembly from a single type of building block. Active, but immotile, rods spontaneously self-assemble into structures that exhibit the two fundamental types of motion: rotation and translation ( Fig. 1d-g). This occurs without imposing any external field. Assemblies can transition between rotor and swimmer, leading to behaviour reminiscent of the run-and-tumble motion of E. coli [13]. The type and direction of motion is determined by the configuration of rods in an assembly, thereby linking the self-assembly to the emergent motility. This is a rare example of artificial dynamic self-assembly and therefore represents an important step towards making more complex micron-scale machines.

Clustering-induced self-propulsion of isotropic autophoretic particles

“…Hydrodynamic and chemical interactions of such self-propelled particles may lead to complex collective dynamics including clustering [23][24][25] or richer patterns [26][27][28][29], which can often be linked to instabilities of homogeneous suspensions [30,31]. Individually non-motile particles can further achieve self-propulsion by forming stable chemically-inhomogeneous clusters [32,33], a phenomenon which was recently also observed in experiments [34,35].…”

Dynamical density functional theory for circle swimmers

“…Possibly, the latter quantities could also be extracted using our approach and explicit swimmer model. Apart from that, in the future also the dynamic behavior of pure active swimming rotors [149][150][151] could be considered in an analogous statistical approach, including the induced hydrodynamic interactions between the rotors. Another extension concerns the treatment of crystallization effects [78] for active microswimmers taking into account hydrodynamic interactions.…”

Guiding microscale swimmers using teardrop-shaped posts

“…The artificial swimmers used in this study are 300 nm diameter nanorods consisting of two solid segments, one gold (Au) and one platinum (Pt), fabricated by electrochemical deposition in anodized aluminum oxide (AAO) membranes [21,22]. The rods are 2.2 ± 0.4 µm in length.…”