(325d) Concentrated Dispersion Behavior in Aqueous Particle/Polymer Systems Observed in Microfluidic Devices

Nanoliter droplets formed using microfluidic techniques have proven useful as components in tunable emulsions and as uniform microreactors. When observed for extended periods, there is significant transport between the droplet phases and the continuous fluid and material of the microfluidic device, allowing for steadily increasing solute concentration in droplets. In this work, dilute colloid-polymer mixtures are loaded on-chip and concentrated under continuous observation to detect system behavior. A model system consisting of 20nm silica particles and both 7500 g/mol and 100k g/mol PEO polymer is used to investigate the effect of polymer-particle mass ratio on final particle concentration. Results show a significant decrease in final particle concentration at low polymer-particle mass ratios, suggesting a transition in behavior from bridging flocculation to steric stabilization. The effect of polymer molecular weight and presence of salt are investigated. A mechanism based on a shift from polymer bridging to depletion is postulated.