Surface tension and elasticity of gel studied with laser-induced surface-deformation spectroscopy

The laser manipulation technique was effectively used for agarose solutions and the frequency specstrum of the surface response to the periodical laser irradiation yielded shear elasticity G and surface tension sigma in the gel. The laser spot size, from 60 mum to 200 mum in radius, was chosen so that either the Rayleigh waves or the capillary waves, selectively excited, associated with G or sigma , respectively. The result of G showed a dependence on the agarose concentration that is consistent with the theoretical prediction of the percolation model, while sigma has little dependence on the concentration. The surface state of 0.2 wt.% agarose solution was controlled with sodium-dodecyl-sulfate (SDS) additives, and sigma of the gel and the sol was observed at different SDS concentrations: The result showed (i) sigma decreased with increasing SDS concentration up to 39 x 10-3 mol/l and kept a constant value thereafter, and (ii) the gel and the sol have the same value of sigma and the same dependence on the concentration. These results were considered from a viewpoint of surface pressure and a partially quantitative discussion was made on the surface adsorbed with SDS and agarose molecules.