Peculiar Solvent Flow along the Gel Network after Gelation at Interfaces

Spatial distributions of solvents in a gel interfacing a solid surface are mapped using confocal Raman microscopy. A binary mixture of completely miscible liquids, p-xylene and octanenitrile, is gelled while in contact with various solid surfaces. Despite its miscibility, p-xylene is found to distribute in excess near the completely wet surfaces soon after the gelation while the gelator distributes uniformly. The concentration excess decreases linearly over the distance from the surface and extends over 200 μm. The excess profile is only kinetically stable and relaxes to a completely mixed state over 10⁴ s. All results can be explained by hydrodynamic theory based on a solvent flow along the network segment, whose property is similar to the spreading of a liquid droplet on a completely wet surface. The strength of the flow is determined by a balance between the spreading p-xylene flux from the adsorbed layer and the outward p-xylene diffusion into the surrounding medium.