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

Abstract

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 $\ensuremath{\sigma}$ in the gel. The laser spot size, from $60\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}200\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in radius, was chosen so that either the Rayleigh waves or the capillary waves, selectively excited, associated with $G$ or $\ensuremath{\sigma}$, respectively. The result of $G$ showed a dependence on the agarose concentration that is consistent with the theoretical prediction of the percolation model, while $\ensuremath{\sigma}$ has little dependence on the concentration. The surface state of $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{wt.}\phantom{\rule{0.2em}{0ex}}%$ agarose solution was controlled with sodium-dodecyl-sulfate (SDS) additives, and $\ensuremath{\sigma}$ of the gel and the sol was observed at different SDS concentrations: The result showed (i) $\ensuremath{\sigma}$ decreased with increasing SDS concentration up to $39\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\mathrm{mol}∕\mathrm{l}$ and kept a constant value thereafter, and (ii) the gel and the sol have the same value of $\ensuremath{\sigma}$ 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.