Probing liquid surface waves, liquid properties and liquid films with light diffraction

Abstract

Surface waves on liquids act as a dynamical phase grating for incident light. In this article, we revisit the classical method of probing such waves (wavelengths of the order of mm) as well as inherent properties of liquids and liquid films on liquids, using optical diffraction. A combination of simulation and experiment is proposed to trace out the surface wave profiles in various situations (eg. for one or more vertical, slightly immersed, electrically driven exciters). Subsequently, the surface tension and the spatial damping coefficient (related to viscosity) of a variety of liquids are measured carefully in order to gauge the efficiency of measuring liquid properties using this optical probe. The final set of results deal with liquid films where dispersion relations, surface and interface modes, interfacial tension and related issues are investigated in some detail, both theoretically and experimentally. On the whole, our observations and analysis seem to support the claim that this simple, low-cost apparatus is capable of providing a wealth of information on liquids and liquid surface waves in a non-destructive way.

Determination of surface tension coefficient of liquids by diffraction of light on capillary waves

“…Viscosity causes the attenuation of waves and by monitoring the rate of attenuation we can, by a non-contact method, determine the coefficient of fluid viscosity. It was Stokes who first proposed this more than a century ago [1,16] and a practical difficulty is that we need to measure the amplitude of capillary waves accurately and based on that determine the attenuation coefficient, the value of which is affected directly by the viscosity of liquids. This also represents a challenge for further work on the setting up of this type of experiment that could be performed in every school or faculty laboratory for general physics courses.…”

“…The angle of diffraction δ was determined from the approximate formula for small angles δ = arctan x/L [4, 10, 13, 14] by measuring the distances x and L. In order to measure the distance x, a series of photographs of diffraction patterns was recorded by a CCD camera, and for analysis of these images and measurement of x, a computer program written in Delphi was used. The motive for using the program for measuring the distance x was to obtain the value of this distance with absolute error ( h) = 0.11 mm, less than the measurement error of a meter or ruler [1,11]. Photographs made by a CCD camera were imported onto the computer and analyzed.…”

“…Methods for determining the surface tension of liquids based on light diffraction on capillary waves are very significant because they are nondestructive procedures [1,2]. 3 This method is particularly important in situations where we have very small samples or where the perturbation of the system by the devices used in conventional methods for determining the coefficient of surface tension is unacceptable.…”

Spreading of macromolecules at the air/water interface

“…115 The use of light-diffraction from the capillary waves of liquid interfaces has also been reported. 117,118 In this thesis work, however, only the Wilhelmy method is used, in which the exerted force of a liquid wetted plate is measured. A schematic representation of a wetted plate is given in figure 8.…”