Direct experimental evidence of slip in hexadecane: solid interfaces

The boundary condition for the flow velocity of a Newtonian fluid near a solid wall has been probed experimentally with a novel setup using total internal reflection-fluorescence recovery after photobleaching leading to a resolution from the wall of the order of 80 nm. For hexadecane flowing on a hydrocarbon/lyophobic smooth surface, we give what we think to be the first direct experimental evidence of noticeable slip at the wall. We show that the surface roughness and the strength of the fluid-surface interactions both act on wall slip, in antagonist ways.

Rotational and translational mobility of small molecules in sucrose plus polysaccharide solutions.

The effect of different polysaccharides on the rotational (D(rot)) and translational diffusion (D(trans)) coefficients of small molecules in concentrated systems (sucrose solutions) was investigated. Dextran (1 or 10% w/w) with different molecular masses (from 10(4) to 2 x 10(6) Da), gum arabic, or pullulan was added to solutions of sucrose (57.5% w/w). Viscosity measurements of the diffusion medium studied (sucrose and sucrose plus polysaccharide) were made using a Rheometric Scientific viscometer in a temperature range from 20 to -10 degrees C. The rotational mobility of nitroxide radicals (Tempol) dispersed in the concentrated systems was measured by electron spin resonance. The translational diffusion coefficient of fluorescein was determined by the fluorescence recovery after photobleaching method. The studied temperature range for the latter two techniques was from 20 to -16 degrees C. For these conditions of concentration and temperature, there was no ice formation in the samples. No effect of the molecular mass of dextran on D(rot) and D(trans) was observed when solutions with the same dry matter content were compared. Only pullulan and gum arabic, at 10%, had a significant effect on D(trans)( )()of fluorescein. Temperature and total dry matter content were observed to be the most important factors controlling D(rot) and D(trans) in these concentrated systems.

Quantitative studies of evanescent wave intensity profiles using optical fluorescence.

The fluorescence of uniformly distributed chromophores in an organic solution has been used to probe the energy density profile of an optical evanescent wave, generated by total internal reflection at the solid-solution interface. The results obtained in the case of an aqueous fluorescein solution in contact with a highly polished silica surface are in good quantitative agreement with the expected exponential decay of the optical energy at the interface. It also justifies the use of the newly developed evanescent wave-induced fluorescence technique to study adsorption and depletion layers of polymer in solution close to solids walls. In such experiments the fluorescence intensity is assumed to be the Laplace transform of the concentration profile of the fluorescently labeled polymer chains. The present data validate the above assumption.