Direct Measurements of Colloidal Solvophoresis under Imposed Solvent and Solute Gradients

Abstract

We describe a microfluidic system that enables direct visualization and measurement of diffusiophoretic migration of colloids in response to imposed solution gradients. Such measurements have proven difficult or impossible in macroscopic systems due to difficulties in establishing solution gradients that are sufficiently strong yet hydrodynamically stable. We validate the system with measurements of the concentration-dependent diffusiophoretic mobility of polystyrene colloids in NaCl gradients, confirming that diffusiophoretic migration velocities are proportional to gradients in the logarithm of electrolyte concentration. We then perform the first direct measurement of the concentration-dependent "solvophoretic" mobility of colloids in ethanol−water gradients, whose dependence on concentration and gradient strength was not known either theoretically or experimentally, but which our measurements reveal to be proportional to the gradient in the logarithm of ethanol mole fraction. Finally, we examine solvophoretic migration under a variety of qualitatively distinct chemical gradients, including solvents that are miscible or have finite solubility with water, an electrolyte for which diffusiophoresis proceeds down concentration gradients (unlike for most electrolytes), and a nonelectrolyte (sugar). Our technique enables the direct characterization of diffusiophoretic mobilities of various colloids under various solvent and solute gradients, analogous to the electrophoretic ζ-potential measurements that are routinely used to characterize suspensions. We anticipate that such measurements will provide the feedback required to test and develop theories for solvophoretic and diffusiophoretic migration and ultimately to the conceptual design and engineering of particles that respond in a desired way to their chemical environments.

Diffusiophoretic Focusing of Suspended Colloids

“…It is so common for DP to proceed up electrolyte gradients that a general sense has emerged that this is always the case, despite theory [12] and experiments [13] to the contrary. The fact that DP can occur in either direction raises the possibility of bidirectional DP and perhaps even focusing, akin to isoelectric focusing (IEF) in electrophoresis [14].…”

“…1 On the experimental side, it has been difficult to systematically measure DP mobilities, although methods have been improved from early techniques involving membrane deposition [2] or stop-flow diffusion cells [19] to microfluidic devices made in agarose gels [20]. We have recently developed a microfluidic system that allows systematic and quantitative measurement of DP mobilities [13]. Integrated hydrogel microwindow membranes [21] enable a wide range of solute gradients to be imposed without generating convective flows, and particle DP to be visualized directly and measured quantitatively.…”

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

“…Such theories have been developed for gradients of electrolytes (35,37) and nonelectrolytes (51), yet difficulties in establishing sufficiently strong, stable gradients have prevented systematic experimental measurements (as is routine for electrophoresis). Recent developments in microfluidics, however, have enabled more direct studies of DP (40,(52)(53)(54)(55)(56). In particular, we have recently developed a microfluidic device (56) that enables gradients to be directly imposed, and diffusiophoretic migration to be visualized and measured under various solute and solvent gradients (40).…”

“…These interactions exploit diffusiophoresis (DP)-the migration of suspended particles and droplets in response to gradients or fluxes of solute (35)(36)(37)(38) or solvent ("solvophoresis") (39,40). Such gradients arise spontaneously around…”

Micro-plumes for nano-velocimetry

“…In particular, colloids migrate diffusiophoretically in response to gradients in solute concentration c (Prieve et al Korotkova 1993), and microfluidic systems have proven quite useful for the control and study of diffusiophoresis (Abecassis et al 2008;Paustian et al 2015). Gradients arise for many reasons (Velegol et al 2016), implying that diffusiophoresis occurs in a wide range of micro-and nano-fluidic processes (e.g.…”