Attractive and Repulsive Colloids at High Volume Fractions

As you have seen in other pages describing the research in the Weitz lab, high volume fraction colloidal systems exhibit interesting behavior. By imaging such systems we can examine the behavior of each particle of interest in real time and space. One goal of such experiments is to compare the behavior of glassy systems made up of colloids (~ 1 micron in radius) to that of glassy systems made up of molecules (~ 1 Angstrom in radius), which can not be viewed on a particle-by-particle basis in real time. Is the microscopic behavior of the colloidal systems consistent with the macroscopic behavior of molecular systems? If so, can our microscopic study of glassy colloidal systems inform our understanding of the puzzling aspects of glassy molecular systems? In both colloidal and molecular systems, we wish to expand our understanding of how, when, and why the liquid to glass transition occurs.

Studying colloidal systems gives us significant control over the potential the particles feel. The simplest case is that of hard spheres. We use PMMA (polymethylmethacrylate) colloids made by Andrew Schofield . These colloids have short hydrocarbon chains grafted onto them for steric stabilization. These hydrocarbon "hairs" make the colloids a very close approximation to hard spheres: a colloidal particle only feels the presence of another colloidal particle if it is touching it.





However, if we add non-adsorbing linear polymer to this system, an attractive well develops in the potential. When the colloidal particles approach each other, the polymer coils will not remain between them because this restricts their movement and thus decreases the entropy of the system. The preference of the polymer coils to remain outside of a "depletion zone" around the colloids causes a decrease of osmotic pressure between the colloids. The polymers thus induce an attraction between colloids that are close together. The depth of the attractive portion of the potential is determined by the concentration of the polymer and the width of the attractive well is determined by the relative radii of the polymer coil (radius of gyration) and that of the colloid.

One very interesting property of high volume fraction colloidal systems is "reentrance". It has been predicted in theoretical work (for example, Dawson, K., et al., Phys. Rev. E., 63 , 011401 (2001)) and measured in bulk experiments (for example, Pham,K., Egelhaaf, S., Pusey, P., and Poon, W., Phys. Rev. E, 68, 011503 (2004)) that as polymer is added to the otherwise hard sphere system, a "reentrant glass transition" occurs. This means that if we prepare a system at a volume fraction just above that at which the hard sphere glass transition occurs (~ .58) and we start adding polymer, our system will remain a glass. But, if we add more polymer, the system will start acting like a liquid. If we add yet more polymer it will look like a glass again.



This is a phase diagram that has been mapped out by K.N. Pham, S.U. Egelhaaf, P.N. Pusey, and W.C.K. Poon (Phys. Rev. E 68, 011503 (2004)). The black, solid circles represent what the authors term a "repulsive glass," a nonergodic, noncrystalline state that is governed by the repulsive part of the potential. The black, solid squres represent an "attractive glass," a nonergodic, noncrystalline state presumably governed by the attractive part of the potential. These authors use dynamic light scattering experiments to ascertain whether their colloid/polymer systems are liquids (ergodic) or glasses (nonergodic and disordered).

We would like to examine the region in the phase diagram between the red and blue squares, ie. around the glass transition with varying amounts of polymer added. We want to study these systems in microscopic detail. Do the repulsive glassy states behave the same microscopically as the attractive glassy states? If not, where do the differences lie?

To address these questions we use CARS microscopy to collect images of the particles as they rearrange (or fail to) in three dimensions over many hours. CARS microscopy is a third-order nonlinear microscopy in which the contrast is due to the particular vibrational structure of the entitity being probed. You can find much more information about CARS microscopy here . We analyze our images by tracking all of the colloidal particles using procedures developed by David Grier, John Crocker, and Eric Weeks.

For more information send email to Laura Kaufman . After August 1, 2004, please inquire with Jaci Conrad or find me here.