Self-Propulsion Mechanism of Active Janus Particles in Near-Critical Binary Mixtures

Abstract

Gold-capped Janus particles immersed in a near-critical binary mixture can be propelled using illumination. We employ a non-isothermal diffuse interface approach to investigate the self-propulsion mechanism of a single colloid. We attribute the motion to body forces at the edges of a micronsized droplet that nucleates around the particle. Thus, the often-used concept of a surface velocity cannot account for the self-propulsion. The particle's swimming velocity is related to the droplet shape and size, which is determined by a so-called critical isotherm. Two distinct swimming regimes exist, depending on whether the droplet partially or completely covers the particle. Interestingly, the dependence of the swimming velocity on temperature is non-monotonic in both regimes.The study of self-propelling synthetic colloids is an area of intense active research [1,2]. The out-of-equilibrium directed motion of these colloidal microswimmers is maintained by a constant energy input which originates from their own activity. The directed swimming, coupled to the particle's rotational diffusion, leads to a significant increase in the effective diffusion coefficient [3][4][5] and to complex collective behavior, such as dynamical phaseseparation [6][7][8] and clustering [9][10][11]. Optimization of the microswimmers design is essential for realizing applications such as targeted cargo and drug delivery, parallel assembly and scavenging of contaminants [1,12,13].The design of synthetic swimmers requires an understanding of the underlying mechanisms for the self-propulsion, e.g.self-diffusiophoresis [3, 14-18], self-induced electrophoretic flow, [19,20] and selfthermophoresis [4,21]. In many realizations, the particle motion is attributed to a microscopically thin boundary layer adjacent to the solid-fluid interface, which interacts with a self-generated field, such as electrical potential, solute concentration and temperature. Body forces within this layer give rise to an apparent slip velocity at the surface [22] while the fluid outside the interfacial layer is considered force-free. Thus, the particle motion is completely determined by the slip velocity distribution on its surface [17,23]. However, this simple picture breaks down when the self-generated field extends to a region with a size similar to that of the particle. In this Letter we explore such a scenario of self-diffusiophoresis due to a local solvent demixing, leading to a complex swimming behavior arising from the coupling of the self-generated chemical potential gradients and the fluid motion.We focus on a recently realized new class of swimmer consisting of Janus colloids immersed in a near-critical binary mixture. Local heating of the colloid surface and the ensuing solvent demixing propels these particles, which exhibit fascinating individual and collective behaviour [5,6,[24][25][26]. A similar system was studied by Araki and Fukai [27] but in their simulations heating is periodically applied to the whole mixture. In this work we study the self-propulsion mec...

Phototaxis of synthetic microswimmers in optical landscapes

“…When illuminating the sample cell with light, which is only absorbed by the capped region, the solvent locally demixes. This leads to an inhomogeneous concentration field around the particle, which results in light-controlled active particle motion due to self-diffusiophoresis 23 57 58 . Only when the intensity exceeds a threshold value I 0 (see arrow in Fig.…”

Emergent behavior in active colloids

“…Heating the cap with laser light demixes the binary liquid and initiates selfdiffusiophoretic motion. Recent theoretical calculations have been performed in [88,89].…”

Run-and-tumble-like motion of active colloids in viscoelastic media

“…When the cell is illuminated with laser light (λ=532 nm), it is absorbed by the carbon caps, which results in local demixing of the fluid (for our intensities this demixing region is at least one order of magnitude smaller than the particle size [31]). This leads to a selfdiffusiophoretic motion whose propulsion velocity is controlled by the incident light intensity as shown by the green symbols in figure 1(b) [7,31,33]. To rule out a possible temperature dependence of the rheological properties of the viscoelastic mixture, which would influence the viscous drag experienced by APs, we have measured the shear-rate dependent viscosity of our system between 25°C (bath temperature) and 31°C, the latter being close to T C (for further details see appendix).…”