(51d) Diffusiophoresis Driven By Gradients of Macromolecules and Surfactant Systems

Diffusiophoresis is the deterministic transport of a colloid in a solute gradient. The energy input to drive the transport is the energy required to create the solute gradient field. Diffusiophoresis of charged colloids in gradients of ionic solutes is sensitive to the difference between the anion and cation diffusivities. The current understanding of diffusiophoresis is limited to solute gradients of simple electrolytes or ionic surfactants. Yet, diffusiophoresis is expected to be important in applications of complex fluid formulations, which often involve large, charged macromolecules or charged complexes of macromolecules and surfactants. Often in such fluid formulations, association of anionic surfactants with nonionic, water-soluble polymers forms charged complexes. These complexes are expected to behave similarly to charged macromolecules. Both charged macromolecules and macromolecule complexes have a greater difference between the anionic and cationic species diffusivities. It is hypothesized that because of this greater difference in diffusivities, diffusiophoretic transport of colloids in gradients of these charged macromolecules and macromolecule complexes will be enhanced. Furthermore, because they have relatively low diffusivities, a gradient will persist for longer times, possibly prolonging the duration of diffusiophoretic transport in a transient gradient.

In this work a psi-shaped microfluidic device with three inlets that converge into one continuous channel is used to study diffusiophoretic transport driven by gradients of charged macromolecules, surfactants or complexes. The inlets allow for spatial control of the solute gradient and the overall geometry of the channel enables quantitative comparison of transport rates among the different solute systems. Results from the microfluidic diffusiophoresis experiments in gradients of Pluronic P123 water-soluble tri-block copolymers and sodium dodecyl sulfate (SDS) surfactants demonstrate that the initial rate of colloid transport and the duration of diffusiophoresis are indeed enhanced and prolonged significantly compared to gradients of SDS alone. Experimental results of colloid transport in gradients of just charged macromolecules, namely synthetic polyelectrolytes, will be reported to elucidate the effect of similarly charged solute systems with large differences in cation and anion diffusivity on diffusiophoretic transport. This probes the key qualitative difference between charged macromolecules and charged complexes: The composition and therefore the charge of a macromolecule/surfactant complex depends on the local composition and will vary across the gradient. In contrast, a charged macromolecule has a constant charge across the gradient. The effect of this difference on diffusiophoresis will be reported. The implications of this work on the theoretical understanding of diffusiophoresis will also be discussed, specifically addressing how macromolecule adsorption to the colloid surface and large differences in cation and anion diffusivities affect the diffusiophoretic mobility of the system.