The capillary wave method is a well-known classical technique to measure surface tension and surface rheological properties. Despite the large number of theoretical works devoted to capillary waves, this technique has serious difficulties associated with its implementation, and therefore, it is not widely used by researchers. In this paper, we introduce our modifications of the existing method to overcome its drawbacks. First, a capillary wave is excited by pressure fluctuations generated locally at the interface. Being contactless, the proposed method is suitable for any liquid irrespective of its electrical properties. Second, the application of optical interferometry together with the spatial phase shifting method allows to quantify the surface profile with high accuracy. A new data processing algorithm makes it possible to subtract the parasitic deformation of the surface caused by external perturbations avoiding, thereby the thorough vibroisolation procedure. The relative error for surface measurements and surface tension calculations is 0.3%. The results of surface tension measurements of several liquids obtained by the modified method are in good agreement with the data determined by the Wilhelmy plate technique. The main advantage of our method is that is well suited for measurements of low liquid volumes, which makes it of particular interest in biological and chemistry applications. Additionally, our version of the examined method allows one to extend the frequency range to 103–104 Hz, where only the quasi-elastic light scattering technique is currently applicable.

We study, both experimentally and theoretically, the fluid flow driven by a thermocapillary effect applied to a partially contaminated interface in a two-dimensional slot of finite extent. The contamination is due to the presence of an insoluble surfactant which is convected by the flow forming a stagnant zone by the colder edge of the interface. The thermocapillary surface stress is produced by a special optocapillary system, which makes it possible, first, to get an almost linear temperature profile along the interface and, second, to apply a surface pressure large enough to force the surfactant to experience a phase transition to a more condensed state. This enabled us for the first time since the release of the paper by Carpenter & Homsy (J. Fluid Mech., vol. 155, 1985, pp. 429–439) to test experimentally their theoretical predictions and obtain new results for the case when the contamination exists simultaneously in two phase states within the interface. We show that one part of the surface is free of surfactant and subject to vigorous thermocapillary flow, while another part is stagnant and subject to creeping flow with a surface velocity which is approximately two orders of magnitude smaller. We found that the extent of the stagnant zone theoretically predicted earlier does not coincide with the newly obtained experimental data. In this paper, we suggest analytical and numerical solutions for the position of the edge of the stagnation zone, which are in perfect agreement with the experimental data.

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In this work, experimental studies were carried out to investigate the structure of a surface flow and to understand potential mechanisms leading to the formation of this type instability. The surface flow was generated by feeding water through three sources: a lumped source with free upper boundary, a slot gap, and a lumped source for inducing a capillary-driven Marangoni flow. For flow visualization, a traditional light knife technique was used. The application of a method of spatial separation of the water volume into two isolated parts whose common surface remained unchanged and the realization of a reverse situation with a divided surface made it possible to study in detail the surface flow structure and to determine the conditions for the appearance of such hydrodynamic instability. It is shown that the formation of a vortex flow is caused by the interaction between the coordinate of the flow homogeneous along the transverse flow and the layer of a surface-active substance adsorbed at the interface. The obtained experimental results demonstrate the importance of setting different boundary conditions for potential and vortex velocity components of a convective flow in the region near the interface occupied by a surfactant.