Shear response of Langmuir monolayers of heneicosanoic (C21) acid studied using a torsion pendulum.

Abstract

We have studied the shear relaxation of heneicosanoic (${\mathrm{C}}_{21}$) acid monolayers on the surface of water at different temperatures and pressures, in the CS, ${\mathit{L}}_{2}^{\mathrm{\ensuremath{''}}}$, ${\mathit{L}}_{2}^{\ensuremath{'}}$, and S phases. A torsion pendulum was displaced from its equilibrium position by an angle \ensuremath{\theta}(0) and the relaxation \ensuremath{\theta}(t) was measured. Only the CS phase sustains a static shear (\ensuremath{\sim}160 dyn/cm) at the time scales studied (\ensuremath{\approxeq}${10}^{3}$ s). The data fit well to a stretched exponential form \ensuremath{\theta}(t)=\ensuremath{\theta}(\ensuremath{\infty})+{\ensuremath{\theta}(0)-\ensuremath{\theta}(\ensuremath{\infty})}exp{-(t/\ensuremath{\tau}${)}^{\mathit{n}}$}. We find that k${\mathrm{\ensuremath{\tau}}}^{\mathit{n}}$/n is independent of the torsion constant k; this implies that the viscosity is time dependent. k${\mathrm{\ensuremath{\tau}}}^{\mathit{n}}$/n is a strong function of pressure and temperature. At phase boundaries there is a discontinuity in either k${\mathrm{\ensuremath{\tau}}}^{\mathit{n}}$/n or its slope. \textcopyright{} 1996 The American Physical Society.