Clustering of Janus particles in an optical potential driven by hydrodynamic fluxes

Abstract

Self-organisation is driven by the interactions between the individual components of a system mediated by the environment, and is one of the most important strategies used by many biological systems to develop complex and functional structures. Furthermore, biologicallyinspired self-organisation offers opportunities to develop the next generation of materials and devices for electronics, photonics and nanotechnology. In this work, we demonstrate experimentally that a system of Janus particles (silica microspheres half-coated with gold) aggregates into clusters in the presence of a Gaussian optical potential and disaggregates when the optical potential is switched off. We show that the underlying mechanism is the existence of a hydrodynamic flow induced by a temperature gradient generated by the light absorption at the metallic patches on the Janus particles. We also perform simulations, which agree well with the experiments and whose results permit us to clarify the underlying mechanism. The possibility of hydrodynamic-flux-induced reversible clustering may have applications in the fields of drug delivery, cargo transport, bioremediation and biopatterning.

Transient coarsening and the motility of optically heated Janus colloids in a binary liquid mixture

“…We set C = 4, s = 0.001, D = 100, and D c = 3D. We assume that the typical molecular size v 0 (see eqn (17)) is equal to the amplitude x 0 À = 0.1 nm of the bulk correlation length (above T c for the lower critical point). The CHC theory units are then r 0 = 0.35 nm for a length and t 0 = 10 À7 s for time (see Appendix A).…”

“…[14][15][16], which are concerned with shallow quenches, this parameter was set to 1, i.e., the rescaling factor c 0 was equal to c. In the present study we assume C = 4, which is appropriate for deep quenches, as considered here, for which the bulk correlation length x À (T 1 ) at T 1 is comparable to the size of the mesh in the numerical calculations. We assume that the strength z We recall that for the system with a homogeneous temperature T, the mean field bulk correlation length (above T c in the system with a lower critical point) is given by x À (T) = x 0 À |t| À1/2 , where the amplitude x 0 À ¼ ffiffiffiffiffiffiffiffiffi ffi c=A p follows from the free energy given by eqn (17). Using the relations T( , which typically is of the order 10 À14 m 2 s À1 , 48 renders t 0 C 10 À7 s for the time unit.…”

“…Using the relations T( , which typically is of the order 10 À14 m 2 s À1 , 48 renders t 0 C 10 À7 s for the time unit. The generic preference of the colloidal surface for one of the two components of the binary mixture is taking account by considering a surface energy contribution 1 2 a s Ð S c 2 dS À h s Ð S cdS which has to be added to the free energy functional in eqn (17). 34 Here, S refers to the surface of the colloid, a s is a surface enhancement parameter, and h s is a symmetry breaking surface field.…”

“…We find that the coarsening dynamics around immobilized Janus particles is more involved than the one observed for a self-propelling particle: the coarsening patterns which form around a fixed particle at later times are not observed around a mobile particle. Such patterns might, however, be relevant for Janus particles in optical potentials, which have been recently studied in the context of, e.g., clustering 17 or of microscopic engines powered by the local demixing of a critical binary liquid mixture. 18 The transient dynamics has consequences for the self-propulsion.…”

Realization of a motility-trap for active particles

“…Compared to Brownian particles, the application of optical traps is not straightforward, partly because APs are typically composed of nonuniform dielectric materials, including partial metallic coatings, which lead to non-isotropic optical forces and strong light scattering effects. Accordingly, present trapping mechanisms of APs require the presence of pores and walls [24][25][26][27][28][29][30][31] , external forces 22,23,[32][33][34][35][36][37][38][39] , or the presence of other particles 40,41 .…”