(504e) Surfactant Driven Diffusiophoresis with Variable Mobility

Diffusiophoresis is the deterministic transport of colloids that is caused by a surrounding solute concentration gradient. For ionic solutes, the diffusiophoretic flux is driven by the gradient of the logarithm of the solute concentration, S. It is advantageous in many industrial applications where existing surfactant gradients could be used to drive particle transport. Micellization in surfactant gradients adds additional complexity to the system however, because the solute gradient consists of monomeric surfactant ions, dissociated counterions, and micelles. These components contribute to the collective diffusion coefficient of the solute. This collective diffusion coefficient varies versus surfactant concentration resulting in a diffusiophoretic mobility, M_DP that has a non-monotonic dependence on surfactant concentration (Warren et al., Soft Matter, 2019, 15, 278). As a result, the diffusiophoretic transport of colloids across surfactant gradients established in different concentration ranges but with the same gradient of lnS, should vary accordingly.

Here, microfluidic channel experiments are used to measure the diffusiophoretic transport of negatively charged polystyrene latex beads in sodium dodecyl sulfate (SDS) surfactant gradients that sample different concentration ranges with differing diffusiophoretic mobility. These are performed in an asymmetric channel geometry to provide increased sensitivity in measuring transport and mobilities. Numerical simulations of transport in the channel are used to guide experiment design and identify markers of variable diffusiophoretic mobility. Experimental results indeed indicate a change in sign of the mobility consistent with numerical predictions where different ranges of SDS gradients yield transport in opposite directions. This work elucidates the effects of a variable mobility on the bulk diffusiophoretic transport of colloids and will give insight into how to tune complex solute gradients to control both the magnitude and even direction of colloidal transport.