Hydrodynamic front-like swarming of phoretically active dimeric colloids

Abstract

Self-propelled phoretic colloids have recently emerged as a promising avenue for the design of artificial swimmers. These swimmers combine purely phoretic interactions with intricate hydrodynamics which critically depend on the swimmer shape. Thermophobic dimer shaped colloids are here investigated by means of hydrodynamic simulations, from the single particle motion to their collective behavior. The combination of phoretic repulsion with hydrodynamic lateral attraction favors the formation of planar moving clusters. The resulting hydrodynamic assembly in flattened swarms is therefore very specific to these dimeric active colloids. PACS numbers: 66.10.cd, 87.17.Jj, 05.70.Ln, 02.70.Ns Synthetic microscale motors are attracting large attention due to their outstanding potential practical applications in fields like microfluidics or microsurgery [1][2][3]. When the propulsion mechanism is based on phoretic effects [4], such artificial microswimmers have the great advantage of behaving like passive colloids unless they are chemically [1,5,6], electrically [7,8], or thermally activated [9][10][11]. In particular, thermophoretic swimmers are built from two materials with well differentiated absorption coefficients, like gold and silica, where heterogeneous heating can produce a steady local temperature gradient around the colloid, which translates into its persistent self-propulsion. Thermophoretic swimmers can therefore be powered without any modification of the solvent, what makes them easily bio-compatible. Furthermore, devices engineered with this effect are expected to depict a very large versatility due to two additional facts. One is that thermophoresis has shown to be very sensitive to a large number of factors like pressure, average temperature or solvent composition; and two is that the heat sources, such as magnets or lasers, can be very precisely controlled in time and space.Large ensembles of phoretic swimmers are expected to share a large number of properties with other systems of active particles. Chemically active Janus colloidal particles have already shown clustering and selfassembled structures [12] as well as schooling behavior; and the formation of living crystals has already been observed for light powered micromotors [13,14]. Brownian simulations of thermophilic active colloids predicted the appearance of clustering and comet-like swarming structures [15,16]. Nevertheless, the mechanisms involved in the formation of these structures, the importance of the phoretic and hydrodynamic effects, and the behavior of various types of phoretic swimmers are still very relevant and largely unexplored questions. Until now, spherical Janus thermophoretic swimmers have been the only geometry investigated experimentally [9,17,18], although important effects related with particle shape can be expected. Catalytic dimer motors, moving in the direction of the catalytic cap, have already been synthesized [6]. These results indicate that diffusio-and thermophoretic motion of dimeric colloids in both directions ar...

Review: Interactions of Active Colloids with Passive Tracers

“…Here, the absorption differences between the two Janus hemispheres trigger a thermophoretic mobility in the thin interfacial layer and subsequently particle motion. Despite several groups working on this type of swimmer [62,63], to this moment, only few active-active interaction studies have been reported, showing that at decreased inter-particle distances, the motion is dominated by inter-particle interactions, and clusters form. We are not aware of any published consideration of passive particles [64], but interactions that confirm the observations in other motion mechanisms can be expected [65].…”

Collective behavior of self-propelled rods with quorum sensing

“…For phoretic SPPs, decreased propulsion at high densities due to a leveling of phoretic gradients has been observed in simulations [28][29][30]. Phoretic propulsion mechanisms occur for particles in externally imposed gradients of solute concentration, electric potential, or temperature [31,32]. When phoretic SPPs are close to each other, the gradients around the particles, and thus also the particle propulsion, decreases [31,[33][34][35].…”

Collective behavior of thermophoretic dimeric active colloids in three-dimensional bulk

“…Colloid shape has shown to importantly affect the phoretic particle properties [58][59][60], and also the induced flow field. Therefore, in the case of active phoretic dimers, the direction, shape, and intensity of the induced solvent velocity are determined by the surface properties of the swimmers and by their overall geometry [7,28,61]. Flow fields and stream lines around various thermophoretic dimers are shown in Fig.…”

“…Chemically active Janus colloidal particles have already shown clustering and self-assembled structures [22][23][24] as well as schooling behavior, and the formation of living crystals has already been observed for light-powered micromotors [25][26][27]. Interestingly, simulations of thermophobic dimers have recently shown the existence of a dynamic swarming behavior with front-like propulsion which occurs due to the specific combination of axial propulsion, phoretic repulsion, and hydrodynamic interactions [28]. There already exist various methods to perform simulations of active matter which account for the effects of propulsion, thermal fluctuations, hydrodynamics, and phoretic interactions [29][30][31][32][33].…”