(682b) Microfluidic Sensing of Soft Micromaterials: Subtleties of Microgel and Capsule Fabrication in Microscale Flows

Fluidic droplet generation technology is increasingly used in the fabrication of soft polymeric materials for encapsulation, both for biomedical research purposes and to develop consumer products from pharmaceuticals to cosmetics. While the size, shape, and frequency of generation of soft materials in flow is readily measured using microscopy, optimization of their mechanical properties can be cumbersome. Traditional assays to measure and then optimize mechanical properties require removal of the material in flow, and are often destructive and time-consuming. We use in-line, on-chip fluidic platforms to investigate both the structural ‘zoology’ and the mechanical response of fluidic-fabricated micromaterials flowing through constrictions. Our case studies include investigations of both microgel particles and capsules. One commonly used polymer for particle formation is the blank slate hydrogel polyethylene glycol diacrylate (PEGDA), which can be polymerized and crosslinked using UV light. Typically, formation of UV-crosslinked PEGDA microgel particles in fluidics is accomplished by over-illumination of the polymer to ensure complete gelation. We find that adjusting UV illumination not only controls the transition from PEGDA-filled droplets to fully gelled particles, but also reveals the emergence of interesting structures and morphologies of fluidic-generated soft materials through the gelation transition. Fluidic techniques reveal similarly interesting behavior in materials that form capsules in flow, whether due to gelation of a polymer shell or the formation or removal of an oxide layer around a liquid metal droplet. We couple our fluidic techniques with macroscopic techniques like pendant drop and bulk rheology to better elucidate the impact of flow on the properties of fluidic-generated materials.