Domain Growth and Wetting in a Low Molecular Weight Binary Fluid System
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DOMAIN GROWTH AND WETTING IN A LOW MOLECULAR WEIGHT BINARY FLUID SYSTEM BILL Q. SHI and ANDREW W. CUMMING University of Florida, Department of Physics, Gainesville, FL 32611
Abstract: We present the results of phase-separation experiments performed on the pseudo-binary fluid system guaiacol-glycerol-water. Elastic light scattering and optical microscopy were used to follow the phase-separation after quenches into the coexistence region of the phase diagram. For critical quenches, we observed the well known bicontinuous infinite cluster morphology normally associated with spinodal decomposition, but with two distinct growth modes. In the bulk, at early times we observed L(t) - t 1/3 consistent with diffusion driven dynamics, crossing over to L(t) - t 1 at later times, as hydrodynamics became important. Near the sample walls, there obtained a novel fast growth with L(t) _ t 3/2, just as in the case of recent studies with polymer blends, inconsistent with either diffusion or interface driven dynamics. We attribute this large exponent to wetting effects, and the observation of the same phenomenon in such disparate systems as polymer blends and polar organic solvents is strong evidence that the phenomenon is generic to binary systems.
Experimental Details: It has been well established theoretically1, 2 and experimentally 3 that in the nonlinear pattern coarsening regime of spinodal decomposition that the pattern length scale grows with time as L(t) - t 1/3 in the case that material transport is by diffusion alone, and L(t) - t in the case that bulk hydrodynamic flows driven by surface tension are allowed. Recently, 4 power-law growth with a new exponent, L(t) - t 3/ 2 ,was observed in a binary polymer system. On the basis of a dynamical scaling argument, this was shown to arise from a twodimensional scattering structure. Mixtures of guaiacol and glycerol with small 5 amounts of water (GGW) form a closed-loop miscibility curve phase diagram. The proportion of water determines the size of the loop in the space of temperature and fraction guaiacol by volume. Figure 1 shows typical phase diagram measurements in the neighborhood of the lower critical solution temperature (LCST). We have performed a series of quench experiments on this system, and observed both bulk modes as well as the new L(t) - t 3/2 fast mode, previously seen only in the polymers. Recently, the role played by wetting in binary systems has received much theoretical and experimental attention, 6 , 7, 8 although a derivation for our observation oft 3/2 behavior has yet eluded theorists. Neslab bath/circulators were set up and allowed to equilibrate at each of the low (one-phase) and high (two-phase) temperatures. Heat exchangers were immersed in each of the baths, and connected to independent pumps and filters. Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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