Surface Light Scattering

Thermal fluctuations of fluid molecules cause waves to be generated on the surfaces of interfaces. These waves have very small amplitudes; ~10^-10m and are in general a superposition of waves of different wavelengths. The surface tension of the interface exerts a restoring force on these fluctuations while the viscosity of the fluid provides the damping force to the fluctuations.

A brief schematic (not drawn to scale) of our surface scattering apparatus is given below. We use a CW Ar⁺ laser (514.5nm) operating in the TEM₀₀ mode.We use a phase grating to obtain a stable local oscillator and use the set of lens to image the grating onto the surface. The reflected light is detected with the help of photodiodes which is placed in front of the first order diffracted spot.

The surface waves or ripplons can be either overdamped or underdamped depending on the density, surface tension and viscosity of the two phases at the interface. The graph given below is a intensity autocorrelation function for a surface waves at an Acetone/Air interface obtained from our experimental set up. The data is fitted to a decaying cosine curve from which we can calculate the surface tension and viscosity fo the material.

A picture of our experimental setup is shown below. Please check back later for more updates!!!!

This web page is maintained by:
Bivash Dasgupta
9 & 15 Oxford Street
Gordon McKay Laboratory
Cambridge, MA 02138
617 496 9562
bivash@deas.harvard.edu