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Faculty > Kathleen J. Stebe Kathleen J. Stebe
Richer and Elizabeth Goodwin Professor of Engineering and Applied Science Email:
Current Focus of Research Complex fluids, self-assembly, surfactants, non-equilibrium interfaces, parallelizable means of creating nanostructured materials Engineering of fluid interfaces using amphiphilic assemblies Amphiphilic organization can be exploited to engineer remarkable properties at fluid interfaces. For example, the reduction in interfacial tension caused by the assembly of amphiphiles into monolayers is the basis for the formation of emulsions and foams, and the control over drop break up in fluid dispersions. While the thermodynamics of self assembly have received considerable attention, the kinetics, which are of over-riding concern in the usual case far from equilibrium, are less understood. The identification of key kinetic constants, and their exploitation to control the mechanical response of fluid interfaces form the core of my research program. Applications range from microfluidics to highly parallelizable means of creating ordered assemblies of nanomaterials. Surfactants on strongly deforming surfaces Surfactants strongly alter drop break up modes. For example, in extensional flows, in the absence of surfactants, drops fail to fragment while the flow is in effect, but rather breakup after flow cessation by end-pinching. In the presence of surfactants, drops break by significantly different modes. For dilute amounts of surfactant, the drops poles assume transient, pointed shapes. Small droplets are shed from the pointed drop tips, while the parent drop remains relatively unchanged in shape. This ‘tip streaming’ phenomenon was first reported by Taylor and has since been reported in experiments on low viscosity drops in fundamental studies of extensional flows and in multiphase flows in microfluidics devices. If more surfactant is added, the drops break by a different mode, ‘tip dropping’, in which drops comparable to the parent drop diameter are shed.. We simulate these breakup modes using boundary integral simulations of surfactant-laden drops in creeping flow to capture the role of surfactants in the break up process, and the importance of surfactant concentration in determining the break up mode. A priori calculations of the arc-length of parent drop that becomes highly deformable are underway. Evaporative assembly of small particles Evaporating drops are used as highly parallelizable means of depositing and organizing small particles and solutes, including the organization of nanoparticles and the deposition of solutes on microarrays for rapid throughput analysis. Two means of influencing this process are explored. First, surfactants at the liquid-gas interface are used to influence the surface stresses. Evaporating drops have flow fields within them. Surfactants change these flow fields dramatically. Submicron particles suspended within the drop are convected and deposit on substrates in patterns dictated by the flow field. Then, surfaces of patterned wettability are used as substrates, with small wet features on a continuous, poorly-wet surface. Evaporating drops with diameters that are large compared to the dimensions of the patterns are studied. As the 3 phase contact line of the drop recedes, spontaneous dewetting of the hydrophobic domains and flow into the hydrophilic domains creates discrete fluid elements with peripheries that mimic the underlying surface topography. Suspended particles are carried with the fluid into the wetted regions, and deposit there in a variety of patterns as the discrete fluid domains evaporate. Dynamic surface tension Dynamic surface tension of a quiescent drop or bubble is determined by the dependence of the surface tension on the surface excess of surfactant, and rate of arrival of surfactant. When a fluid interface is freshly formed in solution, surfactant adsorbs, diffuses from solution, and desorbs to establish its equilibrium surface excess. The transport coefficients that describe these events (the adsorption and desorption kinetic constants, the surfactant diffusivity) are measured by pendant bubble method. Simple scaling expressions for surfactant diffusion time scales are developed based upon the surfactant adsorption characteristics, and the curvature of the adsorbing surface. Characteristic kinetic time scales are also identified. Using these simple scaling arguments, the role of surfactant physicochemistry and surface curvature in determining the controlling mass transfer mechanisms are explained. These coefficients are then exploited in experiments with leading order convection. The insights gained areused to understand the role of surfactant mass transfer in determining the evolution of strongly deforming interfaces, described above. Awards and Honors
Selected Publications
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