Jan A. van Heiningen and Reghan J. Hill
By measuring the changing electrophoretic mobility of single optically trapped silica microspheres (radius a ≈ 0.4 μm) during poly(ethylene oxide) homopolymer adsorption and desorption, we study polymer-layer kinetics at various polymer solution flow rates, concentrations, molecular weights, and polydispersities. At polymer concentrations c
5 ppm (mg L–1), Péclet numbers Pe 20, and Reynolds numbers Re 
1, the adsorbing layer growth is mass-transport-limited, with time scales 
10 s that are resolved on the uniquely small, micrometer length scale of optical tweezers electrophoresis (OTE) experiments. However, during adsorption, layer growth becomes limited by surface diffusion, reconformation, and exchange processes. Two characteristic relaxation times are revealed by the OTE time series. The faster time scale increases with polymer concentration and plateaus to 3 s when c 10 ppm. This reflects layer development kinetics limited by surface diffusion and reconformation. The slower time scale is 100 s and reflects polymer exchange, which thermodynamically favors large adsorbed coils when solutions are polydisperse. Desorption is even slower but occurs faster than expected by local-equilibrium theory, possibly because of high shear rates 100 s–1. The dynamic states probed by OTE are often sufficiently far from equilibrium that they cannot be adequately described by theories for equilibrium polymer adsorption, mass-transport-limited kinetics, or kinetics based on local equilibrium.
DOI
By measuring the changing electrophoretic mobility of single optically trapped silica microspheres (radius a ≈ 0.4 μm) during poly(ethylene oxide) homopolymer adsorption and desorption, we study polymer-layer kinetics at various polymer solution flow rates, concentrations, molecular weights, and polydispersities. At polymer concentrations c
5 ppm (mg L–1), Péclet numbers Pe 20, and Reynolds numbers Re 1, the adsorbing layer growth is mass-transport-limited, with time scales 10 s that are resolved on the uniquely small, micrometer length scale of optical tweezers electrophoresis (OTE) experiments. However, during adsorption, layer growth becomes limited by surface diffusion, reconformation, and exchange processes. Two characteristic relaxation times are revealed by the OTE time series. The faster time scale increases with polymer concentration and plateaus to 3 s when c 10 ppm. This reflects layer development kinetics limited by surface diffusion and reconformation. The slower time scale is 100 s and reflects polymer exchange, which thermodynamically favors large adsorbed coils when solutions are polydisperse. Desorption is even slower but occurs faster than expected by local-equilibrium theory, possibly because of high shear rates 100 s–1. The dynamic states probed by OTE are often sufficiently far from equilibrium that they cannot be adequately described by theories for equilibrium polymer adsorption, mass-transport-limited kinetics, or kinetics based on local equilibrium.DOI
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