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We report direct measurement of the center-of-mass diffusion coefficient, D, of uncharged flexible linear chains adsorbed at the solid-liquid interface at dilute surface coverage. We find D ∼ N-3/2 (N is degree of polymerization) when N was varied by more than an order of magnitude (N = 48, 113, 244, 456, and 693) and the scatter of the data was low. The experimental system was poly(ethylene glycol), PEG, adsorbed from dilute aqueous solution onto a self-assembled hydrophobic monolayer, condensed octadecyltriethoxysilane. The method of measurement was fluorescence correlation spectroscopy of a rhodamine green derivative dye that was end-attached to one sole end of the adsorbed PEG chains. The observed scaling implies the diffusion time τ ∼ N3 if Rg ∼ N3/4 as expected for a chain in good solvent in two dimensions (Rg is the radius of gyration), but a variety of other theoretical approaches to describe the dynamical scaling are also plausible. The multiplicity of plausible dynamical transport scenarios is compounded by the fact that polymer diffusion is sensitive to chain conformation on the surface which is not directly observable. Various theoretical scenarios are explored, and the need for new experiments, theory, and computer simulation studies to allow definitive interpretation of this observation of simple and clean fractional power law scaling is emphasized.

Dilation (expansion of film thickness) by ∼0.1 Å, which is less than one-tenth of the width of confined fluid molecules, was observed when confined films crossed from the resting state ("static friction") to sliding ("kinetic friction"). These measurements were based on using piezoelectric bimorph sensors possessing extremely high resolution for detecting position changes, during the course of sliding molecularly thin films of squalane, a model lubricant fluid, between atomically smooth single crystals of mica. Detailed inspection of energy balance shows that the dilation data and the friction forces satisfied energy conservation of identifiable energies at the slip point, from static to kinetic friction. This shows experimentally, for the first time to the best of our knowledge, a direct coupling between friction forces and decrease in the mean density of the intervening molecularly thin fluid. [ABSTRACT FROM AUTHOR]

Polymer integral equation theory is employed to calculate structural properties of undecane and short polymer liquids confined between parallel plates. A strong dependence is found on the strength of fluid–surface interactions which is consistent with solvation force measurements using both strongly attractive mica and weakly repulsive hydrocarbon surfaces. [ABSTRACT FROM AUTHOR]