Measuring the equation of state for a 2D colloidal membrane: A microfluidic approach to buffer exchange

Previous work has shown that monodisperse rod-like colloidal particles, such as a filamentous bacteriophage, self assemble into a 2D monolayer smectic in the presence of a non-adsorbing depleting polymer. These structures have the same functional form of bending rigidity and lateral compressibility as conventional lipid bi-layers, so we name the monolayer smectic a colloidal membrane. We have developed a microfluidic device such that the osmotic pressure acting on a colloidal membrane may be controlled via a full in situ buffer exchange. Rod density within individual colloidal membranes was measured as a function of osmotic pressure and a first order phase transition, from 2D fluid to 2D solid, was observed. k$_{\mathrm{on\thinspace }}$and k$_{\mathrm{off}}$ rates of rod to membrane binding were measured by lowering the osmotic pressure until membrane evaporation occurred.