Dynamic Light Scattering from Colloidal Monolayers

Quasi-elastic light scattering has been used to investigate the viscoelastic properties of colloidal monolayers on the surface of water. The frequency shifts, linewidths and intensities of the scattered light demonstrate that it arises from thermally excited capillary waves on the liquid surface. The photon correlation functions are consistent with predictions for a surface monolayer, and have been analysed in terms of the tension and dilational modulus of the surface. The measured values of the associated surface viscosities were compatible with zero. The pi -A variation was in accord with literature data from Wilhelmy plate measurements. However, at small A the light scattering values of the dilational modulus systematically exceeded those derived from the pi -A isotherm. This is as expected for viscoelastic relaxation in the monolayer on time scales much slower than the inverse wave frequencies.