Capillary waves modified technique

Abstract

Capillary waves are frequently used to measure the surface tension of liquids. However, this approach has not found wide application in the manufacture of modern commercial tensiometers because of the limitations imposed by capillary wave excitation techniques and the labor input associated with its practical implementation. In this paper we introduce a modified version of the capillary wave method which allows one to avoid the existing limitations and disadvantages. The distinguishing features of the proposed technique are as follows: acoustic wave generation and application of an interferometry technique for 3D surface profile reconstruction. A dynamic speaker with controlled vibration frequency and amplitude is used to produce acoustic vibrations, which are transferred via waveguides and create the local source of pressure oscillations on the fluid surface. Application of a conventional Fizeau interferometer and the spatial phase shifting method makes it possible to perform surface form measurements with an accuracy of 5 nm. For calculating wavelengths and the damping coefficient, the surface profile is fitted with a decaying cylindrical wave equation. The accuracy of surface tension measurement by the modified capillary wave technique is 0.3 % , and the measured values coincide with the known values of these parameters determined using other measuring instruments. Owing to the non-contact way of wave generation and the small amounts of the examined fluid, the proposed method can be used in biological, magnetic fluids studies and rheological investigations in physical chemistry.