(722g) Drop in "Additives" for Suspension Manipulation

The range of equilibrium colloidal interactions in aqueous suspensions is limited to less than a micron. We recently demonstrated a novel concept that can be used to drive non-equilibrium suspension interactions over millimeter length scales using chemical “beacons” that establish and maintain solute gradients (or chemical potential gradients) in-situ. These gradients drive diffusiophoretic migration of suspended particles, the direction of which is sensitive to the specific interaction between the out-fluxing solute from the beacon and the surface of the particles. Previous experimental work was restricted to fixed, cylindrical beacon structures to visualize and study the general scope of these so-called “soluto-inertial” (SI) interactions under a controlled, microfluidic environment. We now move on to a broader application of this phenomenon by showing that free falling, 3-dimensional beacon “particles” can be used to generate solute gradients in bulk suspensions and thereby attract (or repel) colloids to (from) specific locations in the suspension. As the beacon falls down, it leaves a solute wake behind it which then diffuses radially outwards to establish the concentration gradient required to drive SI colloidal migration. Theoretical predictions and experimental observations capture the scope of these interactions in terms of the size and falling speed of the beacon particles as well as their potential application in suspension flocculation and emulsion breakage. With this basic understanding, we believe that we can further extend the framework to design freely suspended colloidal beacons that will not only establish gradients but also migrate in response, potentially creating a new paradigm in the field of active matter.