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 σ in the gel. The laser spot size, from 600.3em0exμm0.3em0exto0.3em0ex2000.3em0exμm in radius, was chosen so that either the Rayleigh waves or the capillary waves, selectively excited, associated with G or σ, respectively. The result of G showed a dependence on the agarose concentration that is consistent with the theoretical prediction of the percolation model, while σ has little dependence on the concentration. The surface state of 0.20.3em0exwt.0.2em0ex% agarose solution was controlled with sodium-dodecyl-sulfate (SDS) additives, and σ of the gel and the sol was observed at different SDS concentrations: The result showed (i) σ decreased with increasing SDS concentration up to 39×10^-30.3em0exmol∕l and kept a constant value thereafter, and (ii) the gel and the sol have the same value of σ 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.