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Soluto-inertial phenomena: Designing long-range, long-lasting, surface-specific interactions in suspensions

Significance Liquid suspensions of micron-scale particles and drops play a ubiquitous role in a broad spectrum of materials of central importance to modern life. A suite of interactions has long been known and exploited to formulate such suspensions; however, all such interactions act over less than a micron in water—and often much less. Here we present a concept to design and engineer nonequilibrium interactions in suspensions, which are particle surface-dependent, may last for hundreds of seconds, and extend hundreds of times farther than is currently possible. The conceptual versatility of the results presented here suggests new capabilities for manipulating suspensions, sorting particles, and synthesizing novel materials and particles. Equilibrium interactions between particles in aqueous suspensions are limited to distances less than 1 μm. Here, we describe a versatile concept to design and engineer nonequilibrium interactions whose magnitude and direction depends on the surface chemistry of the suspended particles, and whose range may extend over hundreds of microns and last thousands of seconds. The mechanism described here relies on diffusiophoresis, in which suspended particles migrate in response to gradients in solution. Three ingredients are involved: a soluto-inertial “beacon” designed to emit a steady flux of solute over long time scales; suspended particles that migrate in response to the solute flux; and the solute itself, which mediates the interaction. We demonstrate soluto-inertial interactions that extend for nearly half a millimeter and last for tens of minutes, and which are attractive or repulsive, depending on the surface chemistry of the suspended particles. Experiments agree quantitatively with scaling arguments and numerical computations, confirming the basic phenomenon, revealing design strategies, and suggesting a broad set of new possibilities for the manipulation and control of suspended particles.