Long-range, selective, on-demand suspension interactions: Combining and triggering soluto-inertial beacons

Abstract

Structures and particles that slowly release solute into solution can attract or repel other particles in suspension via diffusiophoresis, a process we termed “soluto-inertial (SI) interactions.” These SI interactions involve “beacons” that establish and sustain nonequilibrium solute fluxes over long durations. Here, we demonstrate the versatility of the SI concept and introduce distinct strategies to manipulate solute gradients and, hence, suspension behavior using beacons with different physicochemical properties. First, 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 a way that allows directed, yet selective, colloidal migration to specific target sites. Moreover, this multibeacon harmony enhances migration velocities, and delays the equilibration of the SI effect. The different SI techniques highlighted here suggest previously unidentified possibilities for sorting and separating colloidal mixtures, targeting particle delivery, and enhancing rates of suspension flocculation.

Physicochemical hydrodynamics of droplets out of equilibrium

“…Here the Peclet number is defined with the mobility M of the droplet, the surface emission rate α [84], and the droplet radius R, namely P e = M αR 2 /D 2 . The crucial mechanism is that nonlinear mixing of the dissolved substance or the reagent establishes gradients in concentration, which then are converted into flow by Marangoni forces or diffusiophoresis [66,[92][93][94][95]. The bifurcation diagram of this process and the resulting swimming velocities are shown in figure 1e, along with the concentration fields around the droplets for various Peclet numbers, see insets of figure 1e.…”

Non-Faradaic Electric Currents in the Nernst-Planck Equations and Nonlocal Diffusiophoresis of Suspended Colloids in Crossed Salt Gradients

“…In itself this is surely a fascinating phenomenon, but importantly for diffusiophoresis the nonvanishing current means there is a nonlocal contribution to diffusiophoresis that can only be elucidated by solving the whole ion transport problem. Since it seems quite possible to engineer crossed gradients with dissolving salt crystals [3], ion-exchange resins [5], suitably chosen "soluto-inertial beacons" [7,24], or by using CO 2 to generate carbonic acid gradients [10], and perhaps in combination with merging streams with different salinities [1], these observations may provide novel opportunities for particle manipulation and sorting.…”

“…I note that in this situation one loses the logarithmic sensitivity exhibited in binary electrolytes [2,15,48], so that in relying solely on transient gradients, the distance over which particles move is limited by the relaxation time for these gradients. This can be alleviated engineering steady-state gradients as in the present case study, using soluto-inertial beacons [7,24] or other semipermanent sources and sinks, or by designing microfluidic devices in which long-lived gradients can be established [1,6,10].…”

Non-Faradaic electric currents in the Nernst-Planck equations and 'action at a distance' diffusiophoresis in crossed salt gradients

“…In itself this is surely a fascinating phenomenon, but importantly for diffusiophoresis, the presence of electric fields in bulk regions where there are no local concentration gradients implies that particles should move in those regions, as a kind of diffusiophoretic 'action at a distance'. Since it seems quite easy to engineer crossed gradients either in microfluidics devices or with * patrick.warren@stfc.ac.uk suitably chosen 'soluto-inertial beacons' as sources and sinks [6,21], these observations provide novel opportunities for particle manipulation and sorting.…”

“…I note that in this situation one loses the logarithmic sensitivity exhibited in binary electrolytes [2,13,41], so that the distance over which particles move is limited by the relaxation time for the ion densities. This can be al-leviated by using soluto-inertial beacons [6,21], or microfluidic devices in which long-lived gradients can be established [1,5,9].…”