Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V4: Microgels: Colloidal Properties of Gels |
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Sponsoring Units: DPOLY DCMP Chair: Weitz David, Harvard University Room: 306/307 |
Thursday, March 19, 2009 8:00AM - 8:36AM |
V4.00001: Melting and Frustration in Temperature-Sensitive Colloids Invited Speaker: I will review experiments from my laboratory that employ temperature-sensitive microgel particles to induce novel phase behavior in suspension. This phenomenon offers a fantastic new variable for control of lyotropic suspensions. Recent experiments, for example, have enabled us to learn how three-dimensional crystals first begin to melt [1], to directly observe melting in 2-D wherein intermediate hexatic phases form [2], and to create geometrically frustrated colloidal ``anti-ferromagnets'' [3]. \\[4pt] References: \\[0pt] [1] Alsayed, A.M., Islam, M.F., Zhang, J., Collings, P.J., Yodh, A.G., Science 309, 1207-1210, (2005). \\[0pt] [2] Han Y, Ha NY, Alsayed AM and Yodh AG, Phys. Rev. E, Vol. 77 (2008). \\[0pt] [3] Y. Han, Y. Shokef, A. M. Alsayed, P. Yunker, T. C. Lubensky, and A. G. Yodh, ``Geometric frustration in buckled colloidal monolayers,'' to be published in Nature (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V4.00002: Equilibrium features in the arrested phase separation of PNiPAM microgels Invited Speaker: We investigate the arrested phase separation of poly-N-isopropylacrylamide (PNiPAM) microgels. At large enough concentrations we observe the formation of a macroscopic gel-body that exhibits a peculiar temperature dependence. In a temperature-range, where the volume of the individual particles no longer changes, the final dimension of the macroscopic gel body depends on the depth of the quench into the phase separation regime. Increasing the quench depth results in a decrease of the dimension of the gel-body; this is reminiscent of a thermodynamically driven phenomenon and contrasts with the fact that the formation of the gel-bodies is due to the arrest of a phase separation process. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V4.00003: New materials with microgels Invited Speaker: This talk introduces a flexible and straightforward method for generating responsive microgel materials with new structures by using a microfluidic technique. We demonstrate that this approach enables tight control over the size and monodispersity of droplets as well as the interfacial structures, which is essential for determining release and transport kinetics of encapsulated components. We also show that responsiveness of microgel materials is controllable by tuning their structure, thereby allowing us to overcome the limitation of length scales, since the diffusion of water molecules through the structured gel phase is much faster than through a bulk gel phase of similar dimensions. We have generated a variety of novel gel structures: microgels with complex structures, microgel shells, 3D gel network with a truly fast response, and responsive colloidosomes. The robustness and versatility of this approach are expected to generate more complex systems and create new possibilities to develop novel materials in practical applications, including drug delivery, foods, and cosmetics. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V4.00004: Sublimation dynamics of colloidal microgel crystals Invited Speaker: Polymer microspheres in suspension serve as a powerful model system for probing thermodynamic phase transitions. These particles are large enough to visualize using optical microscopy and the particle trajectories can be obtained with nanometer-scale resolution from the images. Equally important is the ability to tune the interactions among the particles using charge or adsorbed polymers to induce repulsion, or non-adsorbing polymers to induce attraction by the depletion effect. I will focus on short-ranged depletion attraction induced by micelles of molecular surfactants or triblock copolymer (Pluronic). Because the micellar size and concentration depend sensitively on temperature, the magnitude and range of the attraction can be tuned in situ. This approach lets us track individual microspheres as they form crystals following a quench, or as crystals sublimate when superheated. We focus on systems where the particles are attracted to a flat surface by depletion and thus confined to two dimensions. We find that when crystallites are superheated, they first sublimate by thermally-excited bond-breaking at the perimeter. Below a cross-over size, however, the crystallites rapidly become amorphous throughout, then evaporate very fast at an approximately diffusion-limited rate. The cross-over size varies from 20-100, depending on temperature and concentation. A similar two-stage process is followed during crystallization. During nucleation, we measure the free energy as a function of cluster size and thereby obtain interfacial tensions and chemical potentials. The results point to a thermodynamically meta- or unstable fluid phase, which is not found in equilibrium but which plays a key role in phase-separation dynamics according to Ostwald's Rule. Results will be compared to experiments, simulations, and theory of crystallization of globular proteins. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V4.00005: Phase behavior and rheology of ionic microgels Invited Speaker: Our aim is to understand and control the mechanical properties of dense microgel suspensions, where the softness of the constituent particles can have important effects over the macroscopic behavior. In particular, we are using ionic microgel particles based on poly(vinylpyridine), a monomer that ionizes with pH. When de-swollen, the particles are essentially charged hard spheres and crystallize at high enough volume fractions. By contrast, when the microgels are swollen, light and neutron scattering experiments show that the suspension does not crystallize, irrespective of particle density. But even more remarkably, these highly packed systems remain essentially liquid and do not seem to exhibit glassy behavior. This phenomenology is markedly different to that of ordinary colloids and suggests that the properties of the single particle can dramatically affect the phase behavior and mechanical properties of the packed suspension. [Preview Abstract] |
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