Abstract: Microfluidic techniques enable exquisite control over the physical and chemical properties of experimental systems. We will describe new techniques we have developed to sculpt chemical environments in space and time, and interferometric methods to visualize these concentration fields as they evolve. We will illustrate with direct, dynamic measurements of water sorping into ionic liquids, reagent depletion during interfacial polymerization reactions, and solvent diffusion through hydrogels.

We then use this solution-sculpting to study the diffusiophoretic migration of colloids under imposed chemical gradients. Such phenomena occur quite generally in response to non-equilibrium chemical fluxes, and therefore arises in a wide variety of real-world systems. Nonetheless, diffusiophoresis has remained stubbornly difficult to observe or characterize directly. We will discuss the physico-chemical phenomena that underlie diffusiophoresis, and therefore how to intuitively design systems with desired diffusiophoretic properties.

Finally, we will build on this foundation to introduce conceptually new colloidal interactions that are non-equilibrium but long-lived, much longer-ranged than is possible in aqueous suspensions, and are chemically-specific. We envision a broad role for such interactions in destabilizing suspensions, breaking emulsions, and extracting compounds.

Todd Squires 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 conceptual design of formulated, structured fluids.  Honors include the NSF CAREER award, the Beckman Young Investigator, the Camille and Henry Dreyfus Teacher-Scholar award, the inaugural GSOFT Early Career Award, and Fellowship in the APS.