RESUMEN
Studying the physical properties of ferrofluids is a challenging task, especially when conventional experimental techniques are adapted to the presence of a magnetic field. To date, there has been no definitive understanding of how the magnetic field affects the surface energy of ferrofluid interfaces. In this study, we perform a direct experimental investigation to assess the effect of magnetic fields on the surface tension of ferrofluids. For this purpose, a modified capillary wave technique was modified for use in the presence of an external magnetic field. A decrease in the wavelength of the capillary wave was observed when the magnetic field was oriented perpendicular to the ferrofluid surface, and an increase was recorded when the magnetic field was parallel. We note that the capillary wave pattern elongates along the magnetic field force lines. The observed effect is attributed to the varying influence of the magnetic field along and across the propagating capillary wave. Analysis of the dispersion relation and evaluation of the impacts of various mechanisms influencing capillary waves revealed, that the changes in the surface tension of ferrofluids in the presence of a magnetic field are responsible for the observed behavior. It is shown that the surface tension of the MK 8-40 ferrofluid gradually increases with the applied magnetic field and reaches a grouth up to 10% in a magnetic field of â¼10 kA/m. Thus, the surface tension is found to be influenced by an external magnetic field.
RESUMEN
Surface diffusion is an important mass transfer mechanism of surfactant molecules within adsorbed layers, which has to be taken into account in many fluid dynamics problems. Although considerable research has been devoted to studying the thermodynamic and rheological properties of surface films, rather less attention has been paid to surface diffusivity measurements. Current measurement methods, which are based on marking part of surfactant molecules in uniform motionless layers with the radiotracer or fluorescence technique, are well suited for use in quite condensed layers, but they do not work in rarefied layers due to increasing contribution of density fluctuations at an interface. In this study, we propose a method for measuring the surface diffusion coefficient in gaseous monolayers of an insoluble surfactant under dynamic conditions, i.e., in the presence of a flow at an interface. Our approach is based on measuring the velocity of thermocapillary flow on the water surface, which contains molecules of an insoluble surfactant. We show that under conditions of the balance between thermo- and solutocapillary tangential stresses the convective motion exists at an interface, which is caused by a blurring of the surface concentration gradient of surfactant molecules due to the surface diffusion mechanism. For calculations of the surface diffusion coefficient, we use the equation proposed earlier in the theoretical study [ Homsy , G. M. ; et al. J. Fluid Mech. 1984 139 , 443 - 459 ]. The surface diffusion coefficient measured by us in gaseous layers is 2-3 orders of magnitude larger than the results for liquid-expanded and liquid-condensed layers obtained by other researchers. Finally, we compare the obtained results with the known measurements of surface diffusion and discuss the limitations of the proposed method.
RESUMEN
We consider the effect of a partially contaminated interface on the steady thermocapillary flow developed in a two-dimensional slot of finite extent. The contamination is due to the presence of an insoluble surfactant which is carried away by the flow and forms a region of stagnant surface. This problem, first studied in the classical theoretical paper by Carpenter and Homsy (1985, J. Fluid Mech. 155, 429), is revisited thanks to new experimental data. We show that there is a qualitative agreement between above theory and our experiments: two different regions simultaneously coexist on the surface, one of which is free from surfactant and subject to vigorous Marangoni flow, while the other is stagnant and subject to creeping flow with the surface velocity smaller about two orders of magnitude. We found, however, significant disagreement between theory predictions for the extent of a stagnant surface region and newly obtained experimental data. In this paper, we provide an explanation for this discrepancy demonstrating that the surface temperature distribution is far from suggested earlier. Another effect, not previously taken into account, is a possible phase transition experienced by the surfactant. We obtain a correct analytic solution for the position of the edge of the stagnation zone and compare it with the experimental data.