Interfacial fluid flows and deformations driven by solute concentration gradients

In natural and technological contexts, concentration gradients of solute molecules often arise by accident or by design. In the presence of a solid or fluid interface, these concentration gradients may drive fluid motion through phenomena including diffusio-osmotic flow and the Marangoni effect. Diffusio-osmotic flow is driven by solute–interface interactive forces distributed across a diffuse interfacial boundary layer, leading to sharp velocity gradients when tangential concentration gradients are introduced. Diffusio-osmotic flows on the surface of a colloidal particle are responsible for diffusiophoresis, the deterministic migration of a particle along a solute concentration gradient. Marangoni stresses refer to tangential gradients in interfacial tension e.g. due to adsorption of surfactant molecules, driving motion at fluid–fluid interfaces. These processes can be impactful on mass transfer of colloids and multiphase fluids: for example, diffusiophoresis has the potential to deliver particles at rates far greater than colloidal diffusion, generating recent interest (e.g. in low-cost separation technologies) since the advent of microfluidic experimental techniques. Marangoni stresses can generate convective instabilities that drive spontaneous mixing and disruption of the fluid–fluid interface, in some cases even driving spontaneous emulsification of one phase into the other to circumvent costly input of mechanical energy. Existing research on these topics deals primarily with simple solutes, neglecting self-assembly processes such as micellization, complexation, and formation of vesicles. In this dissertation, I first present an analytical calculation of the asymptotic deformations of a viscous fluid drop undergoing diffusiophoresis. Second, I present our quantification of colloidal diffusiophoresis in the presence of micellizing ionic surfactants and surfactant–polymer complexes, using a numerical transport model to identify signatures of key physical quantities—the diffusiophoretic mobility and solute diffusion coefficient—in our microfluidic experimental apparatus. Next, I turn to a novel hydrodynamic instability occurring at the oil–water interface between solutions of cationic and anionic surfactants, which we observed experimentally in certain ranges of solution pH and surfactant concentration. I explain the mechanism through a quasi-steady stability analysis and numerical simulations of the coupled chemical transport, reaction kinetics, and fluid mechanics, finding that complexation of oppositely charged surfactants drives a Marangoni instability. Finally, I present a model of interfacial convection based on diffuse interaction between a nonionic solute and a fluid–fluid interface, finding an instability criterion which is a generalization (in the relevant limits) of two previously reported convective phenomena: soluto-capillary convection for a free interface and diffusio-osmotic convection near a rigid wall.