(227i) Microfluidic Platforms for Characterization of Flocculation and Gelation in Colloidal Suspensions

Many complex fluids are multicomponent formulations that include colloids, polymers, surfactants and other additives.   Phase behavior and regions of composition space with drastic changes in mechanical properties, such as gelation and yield stresses, are of particular interest.   Developing tools to allow these behaviors to be mapped out is the goal of this project.  This work focuses on droplet based devices in which hydrodynamics are used to generate separate droplets of sample.   Flow-through devices allow composition space to be characterized rapidly to find regions of flocculation of electrostatically stabilized systems through the addition of salts.  Soft lithography generated devices offer the ability to make array devices for control of environment.  Controlling partitioning of the solvent, in this case water, into the continuous oil phase allows the concentration in the sample drops to be altered.   These devices allow samples at dilute composition to be loaded and suspension behavior to be observed visually as the samples concentrate.  Phase behavior and aggregation are observed as suspensions become unstable.  For suspensions that exhibit yield stresses or gelation, the drops stop decreasing in size as the Laplace pressure balances the mechanical properties of the suspension.   This provides a simple, passive tool to rapidly scan the phase behavior and some of the rheological properties of complex suspensions.   Results for aqueous suspensions of silica dispersions, clay dispersions and polymer-colloid dispersions will be discussed.