Photogelling colloidal dispersions based on light-activated assembly of nanoparticles.

Photorheological (PR) fluids, i.e., fluids whose rheology can be tuned by light, have been a recent focus for our laboratory. We are interested in low-cost approaches to PR fluids using molecules or materials that are readily available. Toward this end, we report a new concept for such fluids based on light-activated assembly of nanoparticles into a physical network (gel). Our system consists of disk-like nanoparticles of laponite along with a surfactant stabilizer (Pluronic F127) and the photoacid generator (PAG), diphenyliodonium-2-carboxylate monohydrate. Initially, the nanoparticles are sterically stabilized by the surfactant, and the result is a stable, low-viscosity dispersion. Upon UV irradiation, the PAG gets photolyzed, lowering the pH by approximately 3 units. In turn, the stabilizing surfactant is displaced from the negatively charged faces of the nanoparticle disks while the edges of the disks become positively charged. The particles are thereby induced to assemble into a three-dimensional "house-of-cards" network that extends through the sample volume. The net result is a light-induced sol to gel transition, i.e., from a low, water-like viscosity to an infinite viscosity and yield stress. The yield stress of the photogel is sufficiently high to support the weight of small objects. The gel can be converted back to a sol by increasing either the pH or the surfactant content.