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 mu m to 200 mu m 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 39x10(-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.