Ashis Mukhopadhyay, Wayne State University, Detroit, MI. Conformation of a Polymer Chain near the Critical Point of a Binary Liquid Mixture. Ashis Mukhopadhyay, Wayne State University, Detroit, MI. Twenty-five years ago, Brochard and de Gennes made the prediction that a linear polymer chain in a critical binary liquid mixture will collapse and subsequently re-swell on approaching the critical temperature (Tc) of the mixture. This had later been confirmed by various theoretical calculations and simulations. On the experimental side, however, only the contraction of the polymer chain has been observed. By using a novel single-molecule sensitive fluorescence correlation spectroscopy (FCS) technique we confirm, for the first time to the best of our knowledge, the complete de Gennes theory. The results will improve our understanding of many biological and other soft materials systems, in which there is complex coupling between two or more macroscopic length scales or interplay of competing type of interactions. Fig. 1. Autocorrelation function (ACF) of dye-labeled polyacrylic acid (PAA) of Mw=134,000 g/mol plotted against logarithmic time lag,  at three different temperatures. The critical mixture was 2, 6 lutidine + water. From the analysis of ACF, the center-of-mass diffusion coefficient (D) of the fluorescing species is obtained: T = Tc-T =2.0 K (squares, D= 2.9 m2/s ), T=0.1 K (circles, D=4.8 m2/s) and T=0.025 K (triangles, D=1.7 m2/s). Histograms of measured diffusion coefficients are shown for two temperatures close to Tc. The hydrodynamic radius (Rh) of the chain is determined from the measured D by using Stokes-Einstein relation. Fig. 2. Ratio (Rh,rel) of the hydrodynamic radius at temperatures close to Tc to that at far away from Tc, plotted against T for PAA. These data indicates that polymer molecules shrink and then re-expand on approaching Tc.