Transport of Colloidal Particles by Diffusiophoresis

Abstract

In this thesis, we study the entrainment of fluorescent particles in dead-end pores by diffusiophoresis as well as the nearly complete cleaning of dead-end pores by 'pulsed' diffusiophoresis. Diffusiophoresis is a phenomenon by which particles migrate along solute concentration gradients (Shin et al. 257). Generally, in an aqueous environment, a particle acquires a surface charge. In order to preserve electroneutrality, an excess of counter-ions (namely ions with a charge opposite in sign to that of the particle) surround the particle. They populate what is called the Debye layer beyond which the particle charge is effectively screened (Prieve et al. 250). In our system, we expose fluorescent particles to a sodium chloride concentration gradient. Owing to the difference in diffusivities of the sodium and chlorine ions, a local electric field arises so as to prevent the development of a diffusion current (Prieve et al. 248). In turn, the excess counter-ions within the Debye layer experience an electric force. Moreover, there exists an imbalance in the osmotic pressure along the particle's surface due to the concentration gradient. Both these effects, termed electrophoresis and chemiphoresis (respectively), lead to fluid flow along the particle's surface and the migration/propulsion of the particle (Prieve et al. 247). Diffusiophoretic motion is nearly "two orders of magnitude faster than" that due to Brownian effects (Abecassis et al. 785).

Through high exposure imaging, particle image velocimetry (PIV), and particle tracking, we present a novel understanding of an unresearched phenomenon. We determine that particles that are ultimately captured in dead-end pores travel nearly at/along a main channel wall off of which protrude dead-end pores. Subsequently, upon capture in a pore, the particle moves along the pore's left lateral wall before eventually migrating towards its center. At all sampled Reynolds numbers, the velocity signature of a captured particle is unique: the particle hops from pore to pore and successfully enters/penetrates a pore once its x-velocity is negligible and its diffusiophoretic velocity (most significant in the y-direction) dominates. By means of a complete two-dimensional simulation of the system, we reproduce these features. We also distinguish characteristics of the three-dimensional streamlines that deviate from the two-dimensional study of the driven cavity problem; in particular, we find that flow penetration in the cavity is greater for the three-dimensional case. Our simulation results suggest that the primary eddy from the two-dimensional cavity either resides deeper in the pore or is much smaller in magnitude for the three-dimensional case.

In this thesis, we also present an original method by which to achieve quasi-complete cleaning of dead-end pores. In this proof of concept study, by restoring a solute concentration gradient in time, we prolong the effective timescale of diffusiophoresis. In brief terms, diffusiophoresis only acts as an effective transport mechanism in the presence of a solute concentration gradient. Accordingly, through the simple re-establishment of a concentration gradient within the pore, we promote 'persistent' migration of particles from the dead-end pore. This technique shows much promise in industrial applications. From cosmetics to the oil/gas industry, the recovery or removal of particulate matter from dead-end pores is a pressing issue.