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.olloidal suspensions and emulsions of 10-nm to 10-μm particles play a central role in a wide variety of industrial, technological, biological, and everyday processes. Everyday goods, including shampoos, inks, vaccines, paints, and foodstuffs as well as industrial products such as drilling muds, ceramics, and pesticides, rely fundamentally on stably suspended microparticles for their creation and/or operation. This incredible versatility derives from the extensive variety of properties (e.g., mechanical, optical, and chemical) attainable in suspension through a generic set of physicochemical strategies (1-4). A proper understanding of the stability and dynamics of suspensions in general thus underpins both fundamental science and technological applications.The properties and performance of suspensions depend preeminently on the effective interactions between particles. The celebrated Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (5-7) balances electrostatic interactions (typically repulsive) between charged colloids-as screened by ions in the surrounding electrolyte-against van der Waals attractions, and successfully predicts the stability, phase behavior, and response of electrostatically stabilized suspensions. Additional (non-DLVO) forces can also be used to stabilize or destabilize colloidal suspensions. Grafted or adsorbed macromolecules provide short-range steric repulsions that stabilize suspended particles against van der Waals-induced flocculation (8-11). By contrast, nonadsorbed macromolecules that remain dispersed in solution introduce entropic depletion attractions whose strength and range is set by the size and concentration of depletants (12, 13). Such depletion interactions scale with thermal energy (k B T), and thus enable tunable and reversible attractions (14, 15). Clever design of shaped or patterned coll...

We present a powerful and versatile technique that enables exquisite spatial and temporal control over local solution chemistry in microfluidic devices. Using a microscope and a UV lamp, we use projection lithography to photopolymerize thin (10-25 m) hydrogel membrane ''microwindows'' (HMMs) into standard microfluidic devices. These microwindows are permeable to solute and solvent diffusion and to electric fields, yet act as rigid walls from the standpoint of fluid flow. Reservoirs of solution may thus be rapidly imposed, switched, and maintained on one side of a HMM using standard microfluidic techniques, provoking changes in solution conditions on the other side without active mixing, stirring, or diluting. We highlight three paradigmatic experimental capabilities enabled by HMMs: (1) rapid dialysis and swapping of solute and/or solvent, (2) stable and convection-free localized concentration gradients, and (3) local electric permeability. The functional versatility of hydrogel microwindow membranes, coupled with the ease and speed of their fabrication and integration into simple microchannels or multilayer devices, will open a variety of novel applications and studies in a broad range of fields.