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The modification of the glass transition in confined domains, particularly the length scales associated with cooperative motion, remains a mystery. Hard-sphere suspensions are confined between two surfaces to progressively smaller dimensions to probe the confinement effect on the growth of dynamic heterogeneities via confocal microscopy. The confinement length scale is defined as the critical spacing where deviations from bulk behaviors begin and is observed to occur at progressively larger gap spacings as the volume fraction is increased. However, dynamic length scales extracted from the four-point correlation function are on average smaller than the confinement length scale.

The physical mechanism of colloidal gelation remains inadequately understood, particularly for intermediate to high volume fractions. Experiments to directly probe the complex evolution of structural and viscoelastic properties of gels have been few despite their fundamental importance in elucidating the physical mechanisms responsible for gelation. In this study, we use a home-built micron-gap rheometer combined with confocal microscopy to directly investigate the coupled structural and dynamic properties of colloidal gelation transition by spatial confinement. We observe that confinement-induced gelation proceeds by a spinodal decomposition route where strongly confined colloidal suspensions evolve into "colloid-rich" and "colloid-poor" regions; the propagation of the "colloid-rich" region in three dimensions is responsible for structural arrest and strong viscoelastic enhancement when a critical film thickness approaches 16-25 particle layers.

The contact line in an evaporating drop can stay pinned to form a single ring or can shrink in a discontinuous stepwise manner and generate multiple rings. We demonstrate the latter with DNA solutions and attribute it to a pinning-depinning cycle that generates new contact lines. The new contact line recedes after depinning and is repinned at an internal precipitate ring that determines the location of the next contact line. Each precursor ring is formed when DNAs are trapped by an internal microstagnation flow and precipitation dynamics hence control this unsteady drop motion.

The phenomenon of glass transition, such as the anomalous divergence in viscosity without apparent structural change, remains inadequately understood. We employ spatial confinement to probe length scale dependence on structural relaxation and concomitant glassy dynamics of a hard-sphere poly-(methyl methacrylate) colloidal suspension via confocal microscopy. Remarkable film thickness dependent scaling behavior is observed, where the mobility and relaxation processes of a "fluid" suspension are found to be significantly impeded as film thickness is reduced below 15-20 particle layers.

Using the method of Frantz and Salmeron to cleave mica [Tribol. Lett. 5, 151 (1998)]] we investigate alkane fluids in a surface forces apparatus and confirm several predictions of molecular dynamics (MD) simulation. An oscillatory force-distance profile is observed for the methyl-branched alkane, squalane. Boundary slip is inferred from the frictional sliding of molecularly thin fluids and also from the hydrodynamic flow of thicker films. These findings resolve the paradox that prior experiments disagreed with these aspects of MD predictions, and demonstrate that exceptionally low energy dissipation is possible when fluids move past solid surfaces that are sufficiently smooth.

A controversial point in fluid dynamics is to distinguish the relative importance of surface roughness and fluid-surface intermolecular interactions in determining the boundary condition. Here hydrodynamic forces were compared for flow of Newtonian fluids past surfaces of variable roughness but similar, poorly wetted, surface chemistry. The critical shear stress and shear rate to observe deviations from predictions using the no-slip boundary condition increased nearly exponentially with increasing roughness and diverged at approximately 6 nm rms roughness. We conclude that local intermolecular interactions dominated when the surface was very smooth, but roughness dominated otherwise. This quantifies the limits of both ideas.