Tracer diffusion small probes

Phillies, etal. (77) re-examined results of Brown and Zhou(78) and Zhou and Brown (79) on probe diffusion by silica spheres and tracer diffusion of polyisobutylene chains through polyisobutylene chloroform solutions. These comparisons are the most precise available in the literature, in the sense that all measurements were made in the same laboratory using exactly the same matrix polymer samples, and were in part targeted at supporting the comparison made by Phillies, et al.(Jl). Comparisons were made between silica sphere probes and polymer chains having similar Dp and Dt in the absence of polyisobutylene. For each probe sphere and probe chain, the concentration dependence of the single-particle diffusion coefficient is accurately described by a stretched exponential in c. For large probes (160 nm silica spheres, 4.9 MDa polyisobutylene) in solutions of a small (610 kDa) polyisobutylene, Dp c)/Dt(c) remains very nearly independent of c as Dp c) falls 100-fold from its dilute solution limit. 

Diffusion of Water in poly[PFSA] Membranes To describe diffusion of water through the membrane in the presence of a water activity gradient, an appropriate interdiffusion coefficient must be determined. Experimental methods used to study diffusion of water in these polymers, such as radiotracer and pulsed gradient spin-echo NMR techniques, probe intrad-iffusion coefficients, often referred to as tracer or self-diffusion coefficients, determined in the absence of a chemical potential gradient. Intra- and interdiffusion coefficients are related for the case of diffusion of a small molecule in a polymeric matrix as follows [28] ... 

In micro- and nanoscale fluid mechanics, measurements of mass transport and fluid velocity are used to probe fundamental physical phenomena and evaluate the performance of microfluidic devices. Evanescent wave illumination has been combined with several other diagnostic techniques to make such measurements within a few hundred nanometers of fluid—solid interfaces with a resolution as small as several nanometers. Laser Doppler velocimetry has been applied to measure single-point tracer particle velocities in the boundary layer of a fluid within 1 pm of a wall. By seeding fluid with fluorescent dye, total internal reflection fluorescence recovery after photobleaching (FRAP) has been used to measure near-wall diffusion coefficients and velocity (for a summary of early applications, see Zettner and Yoda [2]). 

As part of a study of probe diffusion and tracer chain diffusion in dextran solutions, Furukawa, et al. report the viscosity of 40 and 150 kDa dextrans in aqueous solution(20). Viscosities measured with Ubbelohde viscometers reached 100 cP concentrations extended up to 400 g/1. Their measurements and functional fits appear as Figure 12.4. While t (c) is described reasonably weU by stretched exponentials in c, for solutions of the 150 kDa dextran at small concentrations the measured viscosities clearly lie below the stretched-exponeniial fit. 

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