M. G. Cabezas

An experimental analysis of the linear vibration of axisymmetric liquid bridges

The linear vibration of axisymmetric liquid bridges of millimetric size was analyzed experimentally. The equilibrium shape and the evolution of the interface position were recorded by a high-speed video camera. To calculate the surface tension, the Theoretical Image Fitting Analysis method was adapted to process images of liquid bridges at equilibrium. An image processing method was developed to obtain both the position of the solid supports delimiting the liquid bridge and the interface deformation as a function of time. The former allows one to accurately measure the inertial force acting on the liquid bridge. The interface deformation was determined at the subpixel level, so that vibrations of very small amplitude could be analyzed. The results are compared with various theoretical approaches in the linear regime.

An analysis of the sensitivity of pendant drops and liquid bridges to measure the interfacial tension

Drop shape techniques, such as axisymmetric drop shape analysis, provide accurate measurements of the interfacial tension from images of pendant drops for a wide variety of experimental conditions. However, these techniques are known to fail when dealing with nearly spherical drop shapes, which may occur, for instance, when working with interfaces between liquids of similar densities and/or under microgravity. We analyzed the advantages of using liquid bridges close to the minimum volume stability limit instead of pendant drops to measure the interfacial tension under different experimental conditions. First, the sensitivity of both configurations to a variation of the interfacial tension is studied numerically as a function of the volume for several Bond numbers B. The results indicate that a liquid bridge close to the minimum volume stability limit is generally more sensitive than a pendant drop of the same volume, especially for small values of the density difference across the interface and/or gravity. This suggests that the use of liquid bridges may extend the range of applicability of drop shape techniques. To explore this possibility, synthetic images of both pendant drops and liquid bridges were generated and then processed by TIFA-AI. The results demonstrated that the use of liquid bridges enhances the range of Bond numbers for which drop shape techniques work satisfactorily. More specifically, similar accuracy is obtained from both configurations for B ~ 10−1, while the use of liquid bridges yields much better results for B ~ 10−2. Finally, experiments were conducted to partially validate the analysis based on synthetic images. Good agreement was found between the values determined from the real and synthetic images.

A new experimental technique for measuring the dynamical free surface deformation in liquid bridges due to thermal convection

The dynamical free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges of 5 cSt silicone oil was studied experimentally using optical imaging. The experiments were conducted under ground-based conditions for a wide range of temperature differences between the solid supports, and for several liquid bridge volumes. Oscillatory motion set in at a critical value of the temperature difference and caused oscillation of the free surface. The amplitude and fundamental frequency characterizing the magnitude and shape of that oscillation were obtained by optical imaging. The procedure allows one to describe the oscillation up to sub-micron order. The temperature oscillations at five points inside the liquid bridge were measured by five thermocouples. Their amplitude and fundamental frequency were compared with the corresponding values of the free surface oscillations. The optical imaging revealed the same dynamics as was obtained from the (more intrusive) temperature measurements. The spatial structure of the free surface oscillations along the liquid bridge axis was also analysed.

Liquid bridge equilibrium contours between non-circular supports

The equilibrium shape of a liquid bridge confined between two parallel solid supports of arbitrary shape is analyzed theoretically. The influence of the shape of the solid supports, the volume of the liquid bridge and the axial, lateral, and centrifugal forces acting on it are considered. The theoretical study is performed using three different techniques: (i) a numerical process for the minimization of the free energy associated with the fluid configuration, (ii) a second-order analytical expansion around the cylindrical solution to the Young-Laplace equation, and (iii) a finite-difference scheme to numerically integrate the Young-Laplace equation. The two numerical methods are in excellent agreement, indicating the degree of accuracy of the calculations, and there is also good agreement between the analytical expansion and the numerical solutions for moderately large values of the perturbation parameters.