Free-Standing Temperature-Sensitive Hydrogel-Particle Membranes from Evaporating Drops*

We demonstrate a simple method using evaporating colloidal drops to prepare temperature-sensitive membranes composed of micron-sized colloidal hydrogel particles that are up to a few particle diameters thick. Sessile droplets of hydrogel particle suspension were evaporated on silicon wafers. The radially outward flows that drive the common coffee-ring effect push hydrogel particles towards the drop edge wherein the particles attach to the air-water interface. Most of these microgel particles move radially inward along the interface and coat the drop surface. The particles are then cross-linked, forming a membrane. The resultant thin films exhibit a temperature-responsiveness characteristic of the individual particles, permitting modulation of membrane size, shape, and optical transmission. We understand the optical properties using a Mie scattering model and an assumed membrane structure.