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The support from the ACS-PRF grant has resulted in five publications so far. These include papers in Physical Review Letters, Applied Physical Letters, and a book chapter. Another manuscript has been submitted to Physical Review Letters recently. My first student, Chris Grabowski has completed his Ph. D in March, 2009 and currently a post doctoral research associate at the Liquid Crystal Institute, Kent State University. Another student, Gilbert Ayuk completed M.S. during the PRF grant period. I have also supervised several undergraduate students. In recent years, I have published two research articles (Applied Physics Letters, 2009 and The Journal of Physical Chemistry B, 2009) with two of my undergraduate students as co-authors, who were supported by the PRF grant.

We continued our experiments in measuring the critical Casimir force of a binary liquid mixture using a home-built atomic force microscope (AFM). We still need to solve several technical problems to obtain reliable data. The temperature stability for the set-up at present is 50 mK/hr and needs to be improved by at least a factor of ten to make the proposed experiments possible. There is also an issue of the detachment of the microsphere from the cantilever. The sphere needs to be fixed in place for at least few hours, but the organic mixtures used for the experiments dissolve the glue, which is holding the sphere. Currently, we are searching for better glues which can overcome this problem.

Critical Casmir force is expected to play role in the reversible colloidal flocculation near the phase transition temperature of a binary mixture. As the overlap of the order parameter profile on the particles is essential for the occurrence of this force, we performed experiments, which elucidate the effect of surface curvature on critical adsorption. The method we used was fluorescence correlation spectroscopy (FCS). The temperature dependence of the adsorbed film thickness and excess adsorption were determined by measuring the enlarged effective hydrodynamic radius of spherical nanoparticles immersed in a critical binary solvent of 2,6 lutidine + water. Our results indicated that the adsorbed film thickness is of the order of correlation length and the excess adsorption per unit area increases following a power law in reduced temperature with an exponent of -1. This has been confirmed with silica particles of two different radii, 10 nm and 25 nm. The results were also compared with theoretical mean field scaling function (Fig 1). We submitted the results of this work to Physical Review Letters.

In other research, we have used gold nanoparticles as a luminescent contrast agent to study size-dependent dynamics in polymer matrix. The experiments measured the diffusion coefficient of particles in poly(butyl methacrylate) melt by tracking their motion within a diffraction-limited focus of a laser with 150 fsec pulses at 800 nm. Our results indicate that for unentangled polymers, when the particle radius (R) is greater than the gyration radius (R_g) of the chain, the Stokes-Einstein relation can accurately predict particle dynamics. For longer chains, if the entanglement mesh length is larger than R, the particle diffuses ~250 times faster than predicted by the Stokes-Einstein relation (Fig. 2). The article that we published in the Applied Physics Letters has been selected for the February 2009 issue of Virtual Journal of Ultrafast Science. 

We have also studied the diffusion of gold nanoparticles in semidilute and entangled solutions of polystyrene (PS) in toluene using FCS. The polymer concentration was varied from ~ 6c^* to 20c^*, where c^* is the overlap concentration. In our experiments the particle radius (R » 2.5 nm) was much smaller compared to the radius of gyration (R_g » 18 nm) of the chain but comparable to the average mesh size (x) of the fluctuating polymer network. The diffusion coefficient (D) of the particles decreased monotonically with polymer concentration and it can be fitted with a stretched exponential function, D =D₀ exp (-mc^ν), with the value of the scaling parameter, ν » 0.9. At high concentration of the polymer, a clear subdiffusive motion of the particles was observed (Fig. 2). The results were compared with the diffusion of free dyes (coumarin 480), which showed normal diffusive behavior for all concentrations.