Direct visualization of colloidal gelation under confinement.

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.