Measuring the Viscosity of a Dense Two-Dimensional Colloid*

The particles of a colloid are readily imaged using light microscopy, while imaging the dynamics of single molecules is more difficult, as a result, colloidal systems are frequently used to study the glass transition. Experiments intended to study the transition in two-dimensions often employ quasi-two-dimensional samples, where the colloidal particles are several layers thick and the particles are not constrained to move in-plane. We present a true two-dimensional colloidal system in which we measure the viscosity of the colloid as its density increases towards the glass transition. Our experiment consists of a supported lipid bilayer with small, approximately 0.1 micron, lipid vesicles adhered to the surface. Using fluorescence microscopy to image the Brownian motion of the colloidal particles at different particle densities, and using the Stokes-Einstein relation, we determine the viscosity curve of our colloidal system.