Competing Timescales for Surfactant Transport and Flow at Microscale Fluid-Fluid Interfaces

Friday December 3
Chem. Sci. & Engineering Room 211
10:00 a.m.

Presenter: Dr. Shelley L. Anna, Carnegie Mellon UniversityDepartment of Chemical Engineering and Mechanical Engineering

Abstract: The presence of surface-active species dramatically alters interfacial processes such as droplet breakup and coalescence, and coating and film flows. Tipstreaming is an example of a unique surfactant-mediated effect that occurs when strong convection past a stationary droplet drives surfactant to the pole and draws a high curvature thread from the pointed end of the drop.  We use tipstreaming in a microfluidic device to generate micron-scale droplets.  To gain control of this droplet generation method, we recognize that there are several competing processes that give rise to tipstreaming.  These include: bulk diffusion of soluble surfactants to the interface, kinetics of adsorption and desorption, convection, and relaxation of surface concentration gradients.  To properly determine the magnitudes of the timescales involved in tipstreaming, we first need to know the kinetic rate constants for each surfactant-fluid-fluid system of interest.  Kinetic constants are not readily available for oil-water interfaces due to small fluid density differences.  We address this limitation by developing a new method to obtain dynamic surface tension for oil-water interfaces. We then develop a scaling argument showing that kinetics become rate limiting for micron-scale pendant drops, and we use this idea to demonstrate a method for obtaining reliable values for the kinetic rate constants.  Using these measured rate constants combined with tipstreaming experiments, scaling arguments, and numerical simulations, we show that tipstreaming occurs when convection is fast enough to maintain a surface tension gradient, and when adsorption is too slow to quench the gradient.  These conditions are met only when the surface coverage is very small, due to the intimate coupling of fluid velocity and surface concentration.  Finally, we use these observations for three different nonionic surfactants to suggest ways to increase the operating window for tipstreaming and control the production of tiny droplets.  These two problems are examples of ways that we can use competing timescales for surfactant transport and flow to engineer soft materials.

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