A versatile interface light-scattering spectrometer

Abstract

A spectrometer for the detection of laser light scattered by thermally excited capillary waves at fluid interfaces is described. Its optical system makes possible precise observations at capillaly mode wave numbers high enough to avoid significant effects of instrumental resolution and permits the beam to be incident upon the fluid interface through either of the two adjacent fluid phases. Its performance was tested on the following three model systems at 20.C: the free surfaces of water and 2-butano1, representing the oscillatory and critically damped capillary wave regimes respectively, and the interface between mutually saturated phases of these two liquids, representing the non-oscillatory regime. Accessible wave numbers for which effects of instrumental resolution were insignificant ranged between approximately 1 x lo5 and 5 x lo5 m-'. Values obtained for surface and interfacial tensions and viscosity agreed well with those obtained using a high-accuracy Wilhelmy plate tensiometer and a capillary viscometer.

Direct measurement of the dispersion relation of capillary waves by laser interferometry

“…Unfortunately, apart from the limitations of the technique, the equipment for surface light scattering is complex and expensive. [11][12][13][14] Several recent attempts have been made to devise scaled down versions of the research instrumentation for use by undergraduates, 15,16 but the inherent limitations of the light scattering technique have limited widespread adoption of the method.…”

Measurement of Flow Properties and Interfacial Tensions for Gas Condensate Systems

“…The light scattering method takes advantage of that the interface between two fluids is never perfectly flat. [11][12][13][14][15] Thermal motion of the molecules creates a little roughness. Although the mean displacement from the flat equilibrium position is very small, the roughness will result in a detectable amount of light being diffusely scattered around the regular specular reflection when a laser beam is sent to and then reflected at such a fluid surface, as shown in Fig.…”

Measurement of the damping of liquid surface wave by diffraction method

“…In addition, the corresponding data from light scattering is greatly affected by the reduction of surface tension at the short wavelengths associated with thermally excited waves. Apart from the limitation of the technique, the equipment for the surface light scattering method is also complex and expensive [25][26][27][28].…”