Controlling Phoretic Swimmer Trajectory
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Controlling Phoretic Swimmer Trajectory Stephen Ebbens,1 Alireza Sadeghi,1 Jonathan Howse1, Ramin Golestanian2 and Richard Jones3 1 Department of Chemical and Biological Engineering, University of Sheffield 2 The Rudolf Peierls Centre for Theoretical Physics, University of Oxford 3 Department of Physics and Astronomy, University of Sheffield ABSTRACT Individually propulsive catalytic Janus particle swimmers are observed to self-assemble into aggregate swimmers with a wide variety of translational and rotational velocities. The trajectory for a given doublet is shown to be determined by the frozen in relative orientation of the particles. The new swimmers suggest applications as transport and mixing devices, and will allow study of the interplay between propulsion and Brownian phenomena. Furthermore this random assembly process can be controlled using external magnetic fields to orientate individual ferromagnetic swimming particles so as to favor the production of swimmers with particular desirable configurations resulting in linear trajectories. This approach also produces swimmers that can be orientated, and so “steered” by external fields. INTRODUCTION The development of nano and micron scale autonomous synthetic swimmers has received much attention due to the potential for such devices to perform fluidic transport/assembly tasks and deliver pharmaceutically active ingredients. As an example, micron sized Janus particles formed by coating one half of a polystyrene bead with a thin layer of Platinum, have been observed to undergo propulsion by asymmetrically decomposing hydrogen peroxide fuel into water and oxygen.[1] The development of these and other similar rod shaped swimming devices has been recently extensively reviewed.[2] Here we demonstrate that these individually propulsive particles can also self-assemble into larger propelling agglomerates. The assembly process couples the particles “motors” to result in a rich variety of translational/rotational behavior. We have also used fluorescent microscopy to show that the self-assembled agglomerates propulsive force direction is determined by the relative orientation of the platinum patches on the individual constituents. We also show that if the individual swimmers have a magnetic core, and are magnetized in a specific orientation relative to the platinum cap, it is possible to control their assembly to favor particular relative orientations. The resulting agglomerate swimmers have linear trajectories, due to their controlled assembly configuration, and can also be steered by external magnetic fields. RESULTS The range of trajectories exhibited by self-assembled phoretic swimmer doublets in peroxide fuel solutions, together with a control doublet in water are shown in Figure 1. The trajectories of the fueled, propelling, doublets range from tight circles (1b) to linear translation
(1f). The doublets continue to move with a constant ratio of translational and angular propulsion velocities for several days until fuel is exhausted. The doublet trajectories also
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