Tailored self-assembly of micro- and nano-particles for technologies ranging from photonic materials to micro/nano-fluidics often entails the application of confinement, which could modify the structural and mechanical properties of the self-assembled aggregates. It is found that the compression rate and film thickness can significantly affect the phase behavior as well as the rheological properties of confined colloidal suspensions, such as whether they are “jammed” or crystalline in nature. However, in situ studies which directly probe the packing configuration and dynamics of confined colloidal suspensions have been rare. In this work, we use a home-built microrheometer interfaced with an inverted confocal microscope to directly visualize the packing configuration and mobility of confined colloidal particles between two solid surfaces, which are subject to oscillatory shear excitation at varied frequencies and amplitudes. We contrast the behavior of two colloidal systems under confinement: 1) a model “hard-sphere” poly-(methyl methacrylate) (PMMA) suspension in a mixture of non-polar solvents, and 2) a poly-(N-isopropylacrylamide) (PNIPAM) suspension in a polar medium.

We observe a strong dependence of film thickness on packing configuration, mobility and thin-film viscoelastic properties of confined suspensions, which deviate sharply from their bulk properties. For the confined PNIPAM suspension, we observe the formation of tenuous, fractal PNIPAM aggregates which grow rapidly as film thickness decreases; the onset of an apparent gel transition is observed at film thickness, H≈25 μm, equivalent to 20 particle layers, where the sizes of gel clusters diverge. Accordingly, the measured viscoelasticity of PNIPAM colloidal thin films is several orders of magnitude higher that that of the bulk. In contrast, we observe a confinement-induced glass transition with confined PMMA hard-sphere suspensions; as film thickness approaches 10-15 particle layers thick, PMMA particles appear to be arrested in cages formed by their nearest neighbors.

To further probe dynamic heterogeneity and hopping events, we employ ultra-fast fluorescence correlation spectroscopy to examine the single-particle dynamics of fluorescent tracer particles of radii ranging from 100-500 nm in unlabeled super-cooled colloidal suspensions. The cage sizes and relaxation processes dramatically increase as the colloidal glass transition of φ=0.58 is approached.