Steering colloidal particles over millimeter distances with soluto-inertial beacons

Abstract

The last decades have seen the impressive rise of nanotechnology and nanomedicine, and the concomitant integration of nanometer-sized agents (1) and laboratory-on-a-chip devices (2). The imagination can hardly keep up with the variety of nanoparticles that are currently synthesized by scalable technologies, not to speak of the sheer inexhaustible range of properties that can be imparted by grafting complex molecules to their surface. These developments have revolutionized materials design and enabled the formulation of sophisticated therapeutics. There is, however, a limit to the range over which these particles can be steered in solution, and many applications rely on thermal processes that restrict control or reaction speed for the intended purpose, whether it is the formation of a large defect-free crystal or the adsorption of a diluted component to a template. Gravity and centrifugation can induce deposition, but are not very specific in their effect and direction. Electric fields offer more tunability, but have a limited range in aqueous solutions. Mother Nature clearly found a wealth of solutions to operate beyond the micrometer-length scale, to communicate over large distances, and speed-up rare events, and one might wonder if there are simple principles that could be adopted in an artificial setting. In PNAS, Banerjee et al. (3) demonstrate an interaction that extends over hundreds of micrometers, lasts for several minutes, and is highly specific to the type of dissolved particle. The principle relies on a mediating solute that is emitted by a socalled beacon. The beacon creates a gradient in solute concentration that induces polystyrene particles and decane droplets to move in opposite directions.It is tempting to make comparisons with chemotaxis, the ability of bacteria to move toward higher concentrations of nutrients or lower concentrations of toxins. However, the colloidal particles in the experiments by Banerjee et al. (3) migrate without any activated process, and are purely driven by a wellcontrolled thermal process known as diffusiophoresis. The velocity of the colloids depends on their surface properties, which determine the interaction with the solute. An attraction between solute and colloids drives the colloids up the solute concentration gradient and a repulsion down gradient, as if the colloids had a certain sense of smell.To demonstrate the physical process itself, Banerjee et al. (3) create a three-channel device, in which the outer channels act as a solute reservoir that maintains a stable solute gradient over the inner channel, containing colloidal particles. Banerjee et al. choose the surfactant SDS as the solute, and use either fluorescent sulphonated polystyrene colloids or decane drops as colloidal particles. Polystyrene colloids clearly migrate down SDS concentration gradients, whereas decane drops migrate up the gradient, over about 100 μm in 100 s (Fig. 1A). Banerjee et al. mention that concentration gradients arise spontaneously around membranes, reactive surfaces, electrodes, ...

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How fast do microdroplets generated during liquid-liquid phase separation move in a confined 2D space?

“…The creation and movement of microdroplets in confinement hold promise for improved fluid transport in enhanced oil recovery processes, [1][2][3] novel chemical analysis techniques, 4 drug and gene delivery systems, 5 smart colloid transportation, [6][7][8][9][10][11] and chemical synthesis. 12,13 Recently, the autonomous motion of microdroplets has attracted research interest, due to the potential to imitate numerous forms of life-like and robotic behaviours.…”

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