  In the presence of a non-adsorbing polymer, monodisperse rod-like colloids
assemble into one-rod-length thick liquid-like monolayers, called colloidal
membranes. The density of the rods within a colloidal membrane is determined by
a balance between the osmotic pressure exerted by the enveloping polymer
suspension and the repulsion between the colloidal rods. We developed a
microfluidic device for continuously observing an isolated membrane while
dynamically controlling the osmotic pressure of the polymer suspension. Using
this technology we measured the membrane rod density over a range of osmotic
pressures than is wider that what is accessible in equilibrium samples. With
increasing density we observed a first-order phase transition, in which the
in-plane membrane order transforms from a 2D fluid into a 2D solid. In the
limit of low osmotic pressures, we measured the rate at which individual rods
evaporate from the membrane. The developed microfluidic technique could have
wide applicability for in situ investigation of various soft materials and how
their properties depend on the solvent composition.
