Extremely Long-Range Light-Driven Repulsion of Porous Microparticles

Abstract

The repulsive surface forces, such as electrostatic or steric, acting between particles explain why they remain well separated in aqueous electrolyte solutions and are responsible for the stability of colloidal dispersions. However, the effective range of these interactions is always well below hundreds of nanometers and typically can be controlled by advanced manipulations such as tuning the electrolyte concentration or modifying the particle surface or, in some more specific cases, via subjecting the suspension to an external electric or magnetic field. Here we employ solutions with small additives of a photosensitive ionic surfactant to investigate if a repulsive interaction of microsized particles sedimented at the solid surface can be remotely controlled simply by illuminating it with an appropriate wavelength. We show that interactions of conventional impermeable particles remain practically unaffected by light, but, in contrast, for porous particles, we observe a long-range repulsion, several orders of magnitude longer than any conceivable equilibrium surface force. This repulsion emerges due to the diffusio-osmotic flow generated near the porous particles that in this scenario are playing a role of micropumps. The diffusio-osmotic repulsion of porous particles can be used for a remote control of their two-dimensional assemblies at the solid wall, and in particular, we demonstrate that by simply using two different illumination wavelengths it is possible to reversibly switch the state of porous particle dispersion from densely packed surface aggregates to a periodic lattice of particles separated by distances on the order of tens of micrometers.

Light-induced manipulation of passive and active microparticles

“…c, d Examples of the optical micrographs showing a lattice of porous particles separated by distances of several tens of m (c) and aggregation of porous particles due to their DO attraction (d) (adapted from Ref. [ 31 ]) …”

“…We stress that in this case the velocities of particles could be significantly augmented compared to discussed in Sect. 2.1 , where the DO flow was induced by focused laser beam [ 31 ].…”

“…2.1 since in this situation ∂Γ/∂x = 0. However, a light-induced DO flow can be generated when sedimented particles are porous [31]. At equilibrium such particles absorb the ionic species [36][37][38][39], but irradiation with blue light induces a rapid escape of cis-isomers out the pores.…”

“…The DO flow can also be induced by light allowing to manipulate inert (i.e. nondissolvable) particles [ 30 ], and it has recently been reported that some type of particles, such as porous, can even participate in its generation mechanism, acting as micropumps and thus representing a novel type of active colloids [ 31 ].…”

“…Previous studies of light-driven DO, usually referred to as LDDO, have addressed the motion and aggregation of either passive or active particles under illumination of focused or uniform light [ 30 , 31 ]. Here, we show how to induce changes in dynamic pattern of colloidal ensemble of nonporous (passive) and porous (active) particles, as well as temporary steady states, by using the irradiation with light of combined wavelength and simple adjustment of its parameters, such as wavelengths, intensity and its distribution, and the size of the laser beam.…”