Spiral diffusion of self-assembled dimers of Janus spheres
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Spiral diffusion of self-assembled dimers of Janus spheres John G. Gibbs,1 Amir Nourhani,1,2 Joel N. Johnson1 and Paul E. Lammert2 1
Department of Physics and Astronomy, Northern Arizona University, S San Francisco St, Flagstaff, AZ 86011, U.S.A.
2
Department of Physics, Pennsylvania State University, University Park, PA 16802, U.S.A.
ABSTRACT Janus spheres, micron-sized silica spheres half-coated with platinum, move rectilinearly away from the platinum side in aqueous hydrogen peroxide. Upon self-assembling, these colloidal particles can form dimers with different conformations that exhibit both rectilinear and rotational modes of motion depending upon the relative orientation of each Janus sphere. At the micron length-scale, stochastic rotational Brownian dynamics is of the order of deterministic dynamics, and their coupling results in effective diffusion, in addition to passive translational diffusion. For dimers with rotary motion, the dynamic coupling leads to spiral trajectories for an ensemble average of the displacement vector.
INTRODUCTION Artificial colloidal active matter [1] often resembles natural systems [2–4] that exhibit self-propulsion. The analogy is furthered by the collective [5, 6] and clustering behavior [7] often observed in both natural systems and their artificial counterparts. Hence, artificial systems can be useful models for investigating active matter. Artificial active colloidal swimmers operate via different mechanisms such as activation by externally applied forces, including magnetic [8–10] and electric fields [11], or self-propulsion by self-generated fields [12–14] including selfelectrophoresis [15] and self-diffusiophoresis [16]. While understanding the motion of individual swimmers is of fundamental importance, in order to develop technologies to take advantage of these micro- and nanomotors, we need to understand their interactions, self-assembly, and emergent phenomena as well as their modes of motion that result from these interactions. Moreover, at the micron and submicron length-scales, the effect of thermal fluctuations is sufficient to result in coupling between deterministic and stochastic dynamics, leading to emergent phenomena such as spiral diffusion [17, 18] for rotary swimmers. Here, we study the self-assembly of Janus spheres into dimers and the dynamics of the resulting microswimmer. A Janus sphere is comprised of a micron-sized dielectric silica sphere that is half-coated with a catalyst made of platinum (Pt). As shown in the left part of Fig. 1, this particle self-propels away from the platinum side in hydrogen peroxide by a self-phoresis mechanism [19, 20]. Upon moving and interacting, the Janus spheres may self-assemble into
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Figure 1. (Left) A Janus sphere consists of a silica microsphere half-coated with platinum (Pt).
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