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Quasi-two-dimensional (quasi-2D) colloidal hard-sphere suspensions confinedin a slit geometry are widely used as two dimensional (2D) model systems inexperiments that probe the glassy relaxation dynamics of 2D systems. However,the question to what extent these quasi-2D systems indeed represent 2D systemsis rarely brought up. Here, we use computer simulations that take into accounthydrodynamic interactions to show that dense quasi-2D colloidal bi-dispersehard-sphere suspensions exhibit much more rapid diffusion and relaxation thantheir 2D counterparts at the same area fraction. This difference is induced bythe additional vertical space in the quasi-2D samples in which the smallcolloids can move out of the 2D plane, therefore allowing overlap betweenparticles in the projected trajectories. Surprisingly, this difference in thedynamics can be accounted for if, instead of using the surface density, onecharacterizes the systems by means of a suitable structural quantity related tothe radial distribution function. This implies that in the two geometries therelevant physics for glass-formation is essentially identical. Our resultsprovide not only practical implications on 2D colloidal experiments but alsointeresting insights into the 3D-to-2D crossover in glass-forming systems.

Are strongly confined colloids good models for two dimensional liquids?