Diffusion of chiral Janus particles in a sinusoidal channel

We investigated the transport diffusivity of artificial microswimmers, a.k.a. Janus particles, in the absence of external biases. We considered the case of chiral Janus particles moving either in the bulk or in sinusoidal channels with reflecting walls. Their self-diffusion constants turned out to depend on both the strength and the chirality of the self-propulsion mechanism. More importantly, in a periodic channel self-diffusion can be controlled by tailoring the compartment geometry. Copyright c © EPLA, 2015 Introduction. – Over the last decade the problem of controlling transport of regular Brownian particles in narrow corrugated channels has attracted the attention of many investigators with the purpose of better understanding biological processes in the cell or designing artificial microand nano-devices [1,2]. In a recent development [3] regular Brownian particles have been replaced with a special type of diffusive tracers, namely, with active or self-propelled artificial microswimmers. Since such particles operate by harvesting energy from their environment, mostly in a nonequilibrium steady state, their autonomous transport is generally enhanced [3]. Self-propulsion is the ability of most living organisms to move, in the absence of external drives, thanks to an “engine” of their own [4]. Optimizing self-propulsion of microand nano-particles (artificial microswimmers) is a growing topic of today’s nanotechnology [5–8]. In artificial microswimmers [9,10] self-propulsion takes advantage of the local gradients asymmetric particles can generate in the presence of an external energy source (selfphoretic effects). Such particles, called Janus particles (JP), consist of two distinct “faces”, only one of which is chemically or physically active [11]. Thanks to their functional asymmetry, JP’s can induce either concentration gradients (self-diffusiophoresis) by catalyzing a chemical (a)E-mail: [email protected] reaction on their active surface [12–14], or thermal gradients (self-thermophoresis), e.g., by inhomogeneous light absorption [15] or magnetic excitation [16]. The self-propulsion mechanism acting on an pointlike particle can be modeled in terms of an effective force and, possibly, an effective torque, which result from local gradients in the suspension fluid surrounding the particle. In contrast with the case of externally applied macroscopic gradients, such phoretic forces and torques cause no longrange flow patterns in the fluid; as we do not explicitly account for the fluid flow disturbances, phoretic forces and torques are taken here as independent model parameters (for more details see the discussion in ref. [17]). In the absence of a torque, the line of motion is directed parallel to the self-phoretic force and the JP propels itself along a straight line, until it changes direction after a mean persistence length, lθ, due to gradient fluctuations [18] or random collisions against other particles or geometric boundaries [19]. In the presence of asymmetries in the propulsion mechanism, the self-phoretic force and the line of motion are no longer aligned and the microswimmer tends to execute circular orbits with radius RΩ, as if subject to a torque with chiral frequency Ω [20,21] (fig. 1). Active chiral motion has long been known in biology [21–23] and more recently observed in asymmetrically propelled microand nano-rods: A torque can be intrinsic to the propulsion mechanism, due to the presence of