Transport and rheological phenomena at fluid-fluid interfaces
Keywords: Chemical Engineering.

Fluid-fluid interfaces are commonly found in nature and in various industrial processes, and can be vastly important in systems involving thin films and microscopic fluid volumes, such as emulsions and foams. In this thesis, we study two different problems involving fluid-fluid interfaces. We begin by focusing on simple interfaces, which can be fully characterized by a surface tension. Although their free energy is prescribed by a single scalar value, simple interfaces can support stresses arising from nonzero curvatures or surface tension gradients, which can lead to complex, nonlinear fluid motions. We then study complex interfaces, which are formed upon the adsorption of surface-active molecules that form viscoelastic microstructures. They are capable of supporting nonisotropic stresses arising from non-linear rheological properties, such as shear and dilatational elasticities, which can significantly alter the configuration and stability of static emulsion systems. Solutocapillary Marangoni flows occur in thin liquid films when the evaporation of a volatile solvent induces concentration inhomogeneities that give rise to spatial gradients in surface tension. Perhaps the most well-known example is the "tears of wine" phenomenon, in which Marangoni flows are driven by a concentration imbalance of water and ethanol. However, these flows can also occur in drying paint films, foams, and ocular surfaces. This thesis focuses on understanding solutocapillary phenomena in ultrathin (< 1μm) liquid films with trace amounts of a solute impurity (< 1%). Experiments are conducted with low-molecular-weight silicone oil (PDMS) mixtures composed of a volatile solvent and a nonvolatile solute. A thin oil film is formed atop a spherical substrate and allowed to drain and evaporate. The evolution of this system is studied by varying the solvent volatility and the bulk solute volume fraction. Our results reveal that both Marangoni stresses and stabilizing van der Waals interactions can induce film regeneration, and their relative contribution is modulated by the bulk solute concentration. Furthermore, we reveal that increasing the rate of evaporation enhances the volumetric flow rate from thicker, solvent-rich areas towards thinner, solute-rich regions of the film. Complex interfaces are studied in the context of asphaltenes, a class of high molecular weight compound found in crude oil. They adsorb onto toluene-water interfaces and induce a spontaneous emulsification phenomenon, whereby a stable water-in-oil emulsion forms without the need of an external energy input. Over time, the adsorbed asphaltenes form rigid viscoelastic networks that stabilize the emulsion. Controlling and understanding spontaneous droplet formation in the presence of asphaltenes is particularly useful for crude oil refining, since stable emulsions hamper the efficiency of downstream processing operations. We investigate the addition of co-polymers designed as crude oil flow improvers, and find that they competitively adsorb onto the oil-water interface and reduce emulsion formation. We also determine the mechanism of spontaneous emulsification in our systems and conclude that an emulsion forms via the diffusion of molecular water into the oil phase and subsequent binding with asphaltene aggregates, leading to the nucleation of micron-sized water droplets. Finally, we study the interfacial shear and dilatational viscoelastic response of our interfaces and find that the rate of formation of an interfacial microstructural network is inversely correlated with the extent and rate of spontaneous emulsification