Solvent and solute fluxes arise in a wide variety of materials and systems that have not yet equilibrated — e.g. during heterogeneous reactions, during absorption, adsorption, or leaching operations, in the processing and operation of formulated, complex fluid products, and so on. This transport is typically described by the diffusion equation, which is typically the first PDE that any of us has ever seen. Despite the ubiquity and importance of mass transport, solvent and solute concentration profiles cannot generally be visualized directly (unlike fluid velocity profiles), unless the species are e.g. fluorescently tagged. We will describe new techniques we have developed to sculpt chemical micro-environments in space and time, and interferometric methods to visualize these concentration fields as they evolve. This technique has enabled us to probe the kinetics of interfacial polymerization reactions (used for water treatment membranes), the hindered diffusion of solutes that associate with hydrogels, and the dynamic physical and reactive absorption of vapor-phase solutes into ionic liquids.
We then describe a set of non-equilibrium phenomena in which we ’sculpt’ solute/solvent fluxes in order to direct and manipulate colloidal particles and droplets over significantly longer ranges than equilibrium interaction allow. Examples will build upon somewhat classic pictures of diffusiophoresis or solvophoresis, wherein species fluxes drive fluid flows and particle migration. We will lay out a conceptual, intuitive framework to design and manipulate these chemical fluxes — and illustrate with systems that drive particles into or out of dead-end pores; particles that can ‘find’ targets hidden within a maze, and structures that collect specific suspended particles from millimeters away.
Todd Squires is a professor of chemical engineering at uc Santa Barbara. He earned dual B.S./B.A. degrees in Physics and Russian Literature at UCLA, then spent a year as a Churchill Scholar at Cambridge University. He earned his Ph.D. in Physics from Harvard in 2005, spent three years as a Postdoctoral Fellow at Caltech, and joined UCSB’s Chemical Engineering Department in 2005. His research group studies small-scale fluid mechanics and soft materials, both experimentally and theoretically, focusing on microfluidic systems, surfactant function and dysfunction in the lungs and in the field, the manipulation of charges and particles in fluid environments, and the formulation of complex fluid products. Honors include the NSF CAREER award, the Beckman Young Investigator, the Camille and Henry Dreyfus Teacher-Scholar award, the inaugural GSOFT Early Career Award in soft matter, and fellowship in the American Physical Society.