Our paper entitled “A Criterion to Assess the Impact of Confined Volumes on Surfactant Transport to Liquid-Fluid Interfaces,” by N.J. Alvarez, L.M. Walker, and S.L. Anna, has been accepted for publication in Soft Matter. UPDATE: The article has been published online as of July 23, 2012.
Abstract: When dissolved surfactant adsorbs at an interface, the bulk concentration decreases. If the initial concentration is low or the interfacial area large, the concentration decrease can be significant, and the solution depleted. Although depletion is not a new phenomenon, properly accounting for it requires a global mass conservation constraint in addition to a mass transport model. The emergence of new applications involving adsorption in finite volumes and with large surface areas, including micro- and nanoscale droplet formation, has introduced new scenarios in which depletion can be significant but complex to analyze. The purpose of this paper is to develop simple criteria to allow practitioners in these applications to rapidly and easily assess the potential impact of depletion. We use a global mass balance to show that two dimensionless parameters fully describe the role of depletion in both equilibrium surface properties and timescales to reach equilibrium. The dimensionless parameters represent the potential mass lost to the interface, denoted ζ, and the surface activity of the surfactant, denoted f. Characteristic transport timescales are shown to be a function of the finite geometry. A scaling analysis is developed for the case of surfactant dissolved inside a spherical drop, and compared with that of a finite spherical shell. The analyses developed here lead to simple criteria that are useful even when the surfactant properties are not well characterized or a full transport analysis is difficult. The criteria can be generalized to adsorption at solid surfaces.
A large group of students and faculty from the Center for Complex Fluids Engineering attended the ACS Colloid and Surface Science Symposium at Johns Hopkins this past week. From the Anna Lab, talks were given by Anthony Kotula, Todd Moyle, Ying Zhang, and Shelley Anna. Shelley co-chaired the Microfluidics sessions with German Drazer. Anthony Kotula was selected to talk in the Langmuir Student Awards session.
Our paper entitled “Interaction of Toroidal Focal Conic Defects with Shear Flow,” by S. Chatterjee and S.L. Anna, has been published in Soft Matter, 8 (2012) 6698 – 6705.
Abstract: Toroidal focal conic defects form in smectic liquid crystals due to antagonistic surface anchoring conditions. The resulting layer structure, defect size, and the formation of ordered patterns of defects have been studied in detail. Here, we investigate the effect of shear flow on toroidal focal conic defects. The defects are formed and subjected to a steady linear Couette flow in a microscale shear cell. In situ visualization of the evolution of the polarization texture of individual defects reveals the impact of three distinct flow regimes on the layer structure. At low Ericksen numbers, individual defects exhibit an early time elastic regime in which the optical intensity of the layer structure does not change in the presence of the applied shear flow. At increasing applied strains, the defect layer structure deforms, resulting in a monotonically increasing optical intensity within its original footprint. At even larger applied strains, satellite defects are emitted from the anchored defect. Cessation of shear flow at moderate strains, prior to the formation of satellite defects, leads to an exponential decay of the polarization intensity of the defect, suggesting that relaxation of the layer structure occurs with a time constant that is consistent with that predicted by a permeation flow mechanism. This study establishes the relevant length and time scales involved in the interaction of shear stresses and elastic stresses in smectic liquid crystalline samples whose structure deviates significantly from an idealized aligned lamellar configuration.