Mesoscopic probes

The nature of polymer motion in semidilute and concentrated solutions remains a major question of macromolecular science. Extant models describe polymer dynamics very differently 3-11). Many experimental methods have been used to study polymer dynamics (12). One meAod is probe diffusion, in which inferences about polymer dynamics are made by observing the motions of dilute mesoscopic probe particles diffusing in the polymer solution of interest. Probe diffusion can be observed by several experimental techniques, for example, quasi-elastic light scattering spectroscopy (QELSS), fluorescence recovery after photobleaching (FRAP), and forced Rayleigh scattering (FRS). 

K. A. Streletzky and G. D. J. Phillies. Translational diffusion of small and large mesoscopic probes in hydroxypropylcellulose-water in the solutionlike regime. J. Chem. Phys., 108 (1998), 2975-2988. 

The transition from small to large viscosity behavior has repeatedly been found near p 5 cP, but the full significance of this particular value for t] is uncertain. With mesoscopic probes, e.g., polystyrene latex spheres, D (T/t]) extends to much larger viscosities than with small molecule probes. 

The literature examined here includes three major experimental approaches, namely (i) optical probe diffusion studies, largely made with quasi elastic light scattering spectroscopy (QELSS), to observe diffusion of dilute probe particles, (ii) particle tracking studies in which the detailed motions of individual particles are recorded, and (iii) true microrheology measurements of the driven motion of mesoscopic probes. 

In simple fluids, the diffusion coefficient of mesoscopic probes follows the Stokes-Einstein equation with 1 / . If the Stokes-Einstein equation remained... 

Y. Cheng, R. K. Prud homme, and J. L. Thomas. Diffusion of mesoscopic probes in aqueous polymer solutions measured by fluorescence recovery after photobleaching. Macromolecules, 35 (2002), 8111-8121. 

Chapter 5 considers translation and rotation by solvent molecules in small-molecule liquids and polymer solutions. Correlations between solution properties are already more complex than might have been expected. At small rj, the diffusion coefficient and equivalent conductance of small-molecule probes in simple liquids scale as At larger rj, D and A are instead The boundary between small and large t] seen in the literature is uniformly near 5 cP. It is unclear why this particular value of r should not be system-specific. In contrast to smaU-molecule probes, mesoscopic probes such as polystyrene latex spheres in potentially highly viscous mixed solvents such as water glycerol retain D T/ri behavior over three or more orders of magnitude in rj. 

Chapter 9 considers the experimental literature on diffusion of mesoscopic probe particles through polymer solutions. A coherent description was obtained, namely that the probe diffusion coefficient generally depends on polymer concentration as Dp = Dpoe p(—ac ). The parameters a and v both depend strongly on polymer molecular weight, with a M > and y close to 1. Parameter u tends toward 1.0 -with much variation - at small matrix M, but reached v 0.5 at large M. 

Although we have focused on the application of the CSAS method to the particular polymer systems of the artificially synthesized and biosynthesized cellulose, as examples of the application, the method can be applied to various polymer systems that have or may have hierarchically self-assembled stmctures. Typical examples are polymer composites that are built up by nanoparticles,nanosheets,nanofibrils,or nanoholes in the matrices of polymers. The method can also be applied to various systems that involve the chemical reaction-induced self-assembly into various hierarchical stmctures.The CSAS or SAS method combined with microscopic probes such as IR, Raman, NMR, UV-vis, and/or mesoscopic probes such as optical microscopy is... 

MODERN APPROACHES IN POINT-CONTACT SPECTROSCOPY AND THEIR APPLICATION TO PROBE NANOCLUSTERS IN MESOSCOPIC MATERIALS... 

Using theoretical and computational techniques, one can identify the mesoscopic structures leading to a requisite function. Once identified, these structural motifs can be used to guide experimental chemical imaging probes. 

H. Doron-Mor, A. Hatzor, A. Vaskevich, T. van der Boom-Moav, A. Shanzer, I. Rubinstein, and H. Cohen, Controlled Surface Charging as a Depth-Profiling Probe for Mesoscopic Layers, Nature 406, 382-385 (2000). 

The objective of this chapter is to show that particles in the mesoscopic regime have very different properties to the bulk phase and, specifically, to demonstrate how in-situ STM and FTIR spectroscopy have been successfully employed to determine information on the structure of model catalysts based on modification of substrate electrodes with metal particles of mesoscopic dimensions, and the effect of this structure on reactivity. It will be shown that studying these model electrodes helps provide a link between single-crystal electrodes, which have provided a wealth of useful information, and electrodes for real application. FTIR has long been invaluable as a probe for localized particle reaction on surfaces in electrochemical processes, and the present work will show how it can complement STM in providing excellent characterization of mesoscopic properties. 

Abstract We review two experiments in microwave cavity QED where we have demonstrated entanglement between a two-level atom and a mesoscopic field and probed the coherence of the field state superpositions produced in the process. These studies constitute an exploration of the quantum-classical boundary and open the way to studies of non-local mesoscopic state superpositions. 

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