Emulsions are liquid-liquid mixtures that are subject to sedimentation driven by gravitational forces. Examples of dilute-dispersed phase emulsions that are relevant for biological applications, or derived from bio-materials, include pharmaceutical drugs (e.g. propofol), topical creams (e.g. facial creams) and vegetable oil fuel emulsions. These emulsions are all derived from some form of lipid (fatty acid, triglyceride) that may be converted directly in a surfactant via a hydrolysis or saponification chemical reaction. Converting a percentage of the fatty acids directly into a surfactant would provide an easy one step process for stabilizing certain biologically relevant emulsion. It is well known that surfactants, when adsorbing to an interface from the bulk-dispersed phase, will lead to decelerated motion of a droplet. The decelerated motion is caused by a Marangoni stress generated by concentration gradients due to the accumulation of surfactant at the rear of the droplet that oppose droplet sedimentation. The researchers will study a process to generate desorption dominated motion which would cause a reversal in the Marangoni stresses and therefore produce accelerated droplet motion when compared with sedimentation of a droplet with a clean interface. To generate stresses that cause droplet acceleration one must produce more surfactant at the interface than the long elapsed time, equilibrium surface coverage value. The researchers will demonstrate that this stress reversal may be produced by generating surfactant at the interface through a fast chemical reaction. A key objective of this proposal is a study of both accelerated and decelerated steady low Reynolds number droplet sedimentation in a viscous liquid. The unique feature in this project is droplet sedimentation in the presence of dilute surfactant species generated by an interfacial chemical reaction. The study includes a combination of experiments and mathematical/computational analyses. The goal is to experimentally measure the translational velocity of a single droplet as a function of the input parameters which are the reactant types (vegetable oils and dispersed-aqueous sodium hydroxide), concentration of reactants, and droplet volume. Parallel to the experiments, a mathematical analysis is performed using a power series expansion of Legendre and Gegenbauer polynomials to satisfy the steady interfacial species conservation laws containing thermodynamic relationships for adsorption/desorption and chemical reaction kinetics combined with momentum transport equations in the Stokes flow limit.
This project will explore novel physical phenomenon associated with the sedimentation speed of a single droplet. The research should provide additional insight for current and future processes that utilize the long term storage of dilute emulsions since these processes depend on the velocity of a single sedimenting droplet. Further understanding of long term storage processes will be vital for emulsions produced from biological material since decomposition limits their shelf life. An undergraduate student from an HBCU will work in the PI's lab during the summer to participate in the project. The research results also will be disseminated though departmental websites and at conference meetings.
