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 introduce a concept to design and engineer non-equilibrium interactions in suspensions, which are particle surface-dependent, may last for hundreds of seconds, and extend hundreds of times farther than is currently possible.
First, we show that these interactions rely on diffusiophoresis which refers to the migration of colloids under concentration (or chemical potential) gradients. Three ingredients are required to design the soluto-inertial interactions – i) chemical “beacons” that establish and sustain solute fluxes, ii) particles that respond to the fluxes and migrate via diffusiophoresis, and iii) solute that mediates the interaction between the suspended colloids and the beacon.
In subsequent chapters, we highlight the versatility of the SI concept and introduce distinct strategies to manipulate solute gradients, and hence suspension behavior, using beacons with different physicochemical properties. We demonstrate on-demand particle migration using beacons that can be actuated with a trigger. We then show the synergy between multiple, distinct beacons that modify solute fluxes in solution in a way that allows directed, yet selective colloidal migration to specific target sites. Moreover, we
provide a general conceptual framework to enable direct prediction of the range, duration and strength of the SI effects in suspension, for a given beacon-solute pair.
While most of the work presented here involves suspension interactions mediated by fixed, cylindrical beacons in well controlled microfluidic devices, we conclude by showing manipulation of bulk suspensions by freely falling, spherical beacons. The beacon forms a solute wake as it sediments, which subsequently diffuses out, inducing diffusiophoretic migration of nearby suspended objects. Motivated by this simple observation, we eventually envision freely suspended beacons, that will not only establish solute fluxes, but also
migrate in response, thereby suggesting new possibilities in active matter.
The conceptual versatility of the soluto-inertial interactions presented here suggests new capabilities for sorting and separating colloidal mixtures, targeting particle delivery to specific sites, enhancing rates of suspension flocculation, and synthesizing novel materials.