Size dependent efficiency of photophoretic swimmers.

Andreas P Bregulla,Frank Cichos

doi:10.1039/c5fd00111k

Abstract

We investigate experimentally the efficiency of self-propelled photophoretic swimmers based on metal-coated polymer particles of different sizes. The metal hemisphere absorbs the incident laser power and converts its energy into heat, which dissipates into the environment. A phoretic surface flow arises from the temperature gradient along the particle surface and drives the particle parallel to its symmetry axis. Scaling the particle size from micro to nanometers, the efficiency of converting optical power into motion is expected to rise with the reciprocal size for ideal swimmers. However, due to the finite size of the metal cap, the efficiency of a real swimmer reveals a maximum depending sensitively on the details of the metal cap shape. We compare the experimental results to numerical simulations.

Highlights

The eld of arti cial micro-swimmers has recently attracted much interest
The diversity of the driving mechanism with the simplicity of the design and controllability of their motion make them a valuable model system to study processes far from equilibrium. When such self-propelled objects are starting to interact at higher densities, coherent collective motion is observed in which the swimmers align, and form ocks, swarms or other complex patterns.[6,9,10,11,12]. Most of these self-propelled particles are driven by an osmotic pressure difference across the particles surface caused by symmetry breaking of e.g. temperature or concentration pro les along the particle surface realized by an asymmetric material composition
Since the phoretic velocity is the product of the thermodiffusion coefficient DT and the temperature gradient along the particle surface VkT, either both are independent of the particle size or their size dependencies exactly cancel each other out

Summary

Introduction

The eld of arti cial micro-swimmers has recently attracted much interest. Over the last decade, numerous types of such arti cial swimmers driven by self-thermophoretic,[1,2,3] self-diffusiophoretic[4,5,6] or self-electrophoretic[7,8] surface ows have been proposed. For diffusiophoretic Janus type particles (driven by concentration gradients caused by catalytic reactions), theoretical studies on the size dependent efficiency[18] support this dependence but reveal a deviation for small particle sizes Measurements of their size dependent velocity[19] exist but without considering the efficiency of the propulsion and their dependence on size. We will explore the size dependence of the photophoretic swimming efficiency by measuring the dependence of the phoretic velocity and the optical properties on the particle radius, ranging from R 1⁄4 100 nm up to 625 nm Throughout all of these studies, the gold cap thickness Dr 1⁄4 50 nm of the Janus particle is kept constant (see Fig. 1 for details). As the surface temperature pro le is difficult to access experimentally, we rely on nite element simulations (FEM) for this part

Experimental

Size dependence of the photophoretic velocity

Size dependence of the absorbed power

Size dependence of the temperature pro le

Size dependent propulsion efficiency

Conclusion