Microfluidics and Soft Matter

We develop and use simple experimental tools - often based on microfluidic devices - for studying the physical mechanisms that govern the properties and behavior of soft materials. 

Insight into even the most complex materials or problems can often come from surprisingly simple experiments

Our starting point is how soft objects deform and flow: using simple experiments, often based on microfluidic devices, we measure the mechanical properties of hydrogels, microgels, and biological cells, probing their viscoelastic response, fluid transport through porous materials, and swelling and deswelling.

We follow how small particles respond to forces, in liquid and in air. In liquids, our experiments have uncovered the origins of exclusion-zone (EZ) formation, a previously unexplained effect where particles exhibit long-range repulsion from a solid surface. We have shown that this effect is due to a phoretic force, in this case diffusiophoresis, the transport of colloidal particles along a gradient of (salt) concentration in the background liquid. In liquids, we employ such phoretic forces for applications; we have used electrophoresis and electro-osmosis to control particle transport in nonpolar liquids, which can be used to create faster switching in electronic-paper displays. In air, we use both inertial and electrostatic forces to transport and concentrate aerosol particles in a detection area. We did this earlier for pollen particles, and we now work on monitoring ultrafine particles (UFPs), where we employ nanophotonic fiber-tip optical sensors to detect individual UFPs.

To both study and control how soft materials organize, we build simple microfluidic tools: A recent example is an oil-free platform that enables the controlled formation of two-phase systems (water-in-water droplets) and other phase-separating materials in situ, where tuning the chemistry and channel geometry lets us steer how they exchange, compartmentalize, and reorganize over time. We have already demonstrated this on model PEG-dextran mixtures and on coacervates, and we continue to use the platform to better understand complex phase separation in both synthetic and biological systems.