(5w) Colloidal Suspensions: Fundamental Physics and Engineering Applications
Colloidal suspensions are ubiquitous in industrial and technological applications, and serve as excellent model systems for a variety of complex fluids. Engineering functional materials from colloids requires a thorough knowledge of the relationship between the microscopic properties of the suspension and the bulk physical properties.
My graduate work at Harvard University focused on the phase behavior, structure, and rheology of model nonequilibrium colloidal suspensions, including both glasses and gels.
1.1. Colloidal microstructure and dynamics near the colloidal glass transition (with F. W. Starr, P. P. Dhillon, E. R. Weeks, D. R. Reichman, and D. A. Weitz): We use confocal microscopy and simulation to investigate the structure and dynamics of model colloidal suspensions near the glass transition. First, we investigate the relationship of local static structure to dynamics in suspensions of nearly-hard-sphere poly-(methyl methacrylate) colloids. We calculate the Voronoi volume for all particles and show that this quantity is only weakly correlated to local particle displacements; we obtain qualitatively similar results in a simulation of a polymer melt. These results suggest that static local structure alone does not predict dynamical behavior in experimental colloidal suspensions. Second, we investigate the behavior of the slowly-relaxing particles. These particles form spatially correlated clusters that evolve over time; furthermore, the structure of these clusters is correlated to the macroscopic properties of the suspension. In supercooled fluids, the largest cluster spans the system on short time scales but breaks up on longer time scales; in glasses, a percolating cluster exists on all accessible time scales. Using molecular dynamics simulation, we show that these clusters make the dominant contribution to the bulk elasticity of the sample.
1.2. Phase behavior, structure, and rheology of weakly attractive colloidal suspensions (with S. Manley, H. M. Wyss, P. J. Lu, K. Miyazaki, V. Trappe, L. J. Kaufman, A. B. Schofield, D. R. Reichman, and D. A. Weitz): We investigate the phase behavior, structure, dynamics, and rheology of colloidal suspensions with weak attractions. We induce a depletion weak attraction between poly-(methyl methacrylate) colloids by adding linear polystyrene. We investigate the phase behavior of these suspensions as a function of colloid volume fraction and polymer radius of gyration and concentration, and show that we can obtain interconnected gels, stable fluids of clusters, and colloidal fluids. We investigate in detail one region of the phase space, with intermediate volume fraction (0.15-0.35) and a fixed range of the interparticle attraction. In this region colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich and colloid-poor regions. Gelation results when the interconnected colloid-rich regions solidify; we show that this occurs when these regions undergo a local attractive glass transition. This arrest scenario bridges the gap between colloidal gelation and the colloidal glass transition; the resulting structures display hallmarks of both glasses and gels. The resultant macroscopic rheology of these structures is strikingly different than that observed for either colloidal glasses or gels; while the relaxation time depends strongly on colloidal volume fraction, the storage modulus is relatively insensitive to changes in volume fraction.
My postdoctoral work at the University of Illinois focused on studying colloidal suspensions under flow. We investigate the structural and dynamic properties of flowing suspensions, and exploit the flow properties to assemble novel colloidal granules and to pattern colloidal films.
2.1 Flow of attractive colloidal suspensions in microchannels (with J. A. Lewis): We use confocal microscopy to investigate the behavior of suspensions of attractive colloids flowing in microchannels. We flow suspensions of attractive silica colloids through microchannels and directly image both their structure and dynamics during flow. We study two different model systems: polyelectrolyte-flocculated gels, and hydrophobic gels. We investigate the flow properties as a function of applied pressure, microchannel geometry, and colloid volume fraction.
2.2 Microfluidic assembly of homogeneous and Janus colloid-filled hydrogel granules and shells (with R. F. Shepherd, A. Cote, S. M. Menke, S. K. Rhodes, D. R. Link, M. Marquez, D. A. Weitz, and J. A. Lewis): We use microfluidic techniques to assemble colloid-hydrogel granules and shells. We suspend silica microspheres in an aqueous acrylamide suspension and use microfluidic devices to emulsify this suspension in a continuous oil phase. By varying the microchannel geometry, we can form spheres, discoids, and shells; by varying the input streams, we can form both homogeneous and chemically heterogeneous (Janus) surfaces. This approach offers a facile route for assembling granules and shells of controlled size, shape, and composition.
2.3 Patterning colloidal films via evaporative lithography (with D. J. Harris, H. Hu, and J. A. Lewis): We investigate evaporative lithography as a route for patterning aqueous colloidal films. Films are dried beneath a mask that induces periodic variations between regions of free and hindered evaporation; fluid and entrained particles migrate towards the regions of highest evaporative flux, leading to lateral segregation of particles within the film. By tuning the colloidal suspension composition, separation distance between the mask and underlying film, and mask geometry, we can control the pattern formation within the dried films.