Dense protein adsorption layers at air/water and oil/water interfaces, and phospholipid membranes possess surface shear elasticity. Because of that, the stresses in such interfacial layers are, in general, non-uniform and non-isotropic. With them, the method of axisymmetric drop/bubble shape analysis, based on the Laplace equation with isotropic surface tension, fails. To determine the meridional and azimuthal stresses in each point of the surface of a pendant drop/bubble covered with an elastic layer, we developed a new method, capillary meniscus dynamometry. It is based on measurements of (i) the shape of a bubble/drop formed on the tip of a capillary and (ii) of the capillary pressure of this bubble/drop [1], followed by nontrivial data processing based on the interfacial-stress-balance equations, without making any rheological-model assumptions. After pressing the bubble/drop against a solid plate (substrate), it forms a capillary bridge between the substrate and the capillary tube (Fig. 1). The bridge profile is subjected to the same procedure of data processing, which gives not only the acting local interfacial stresses at each moment of time, but also the force of bubble/drop adhesion to the substrate (capillary bridge dynamometry). The method is tested with bubbles and drops covered by surfactant and protein adsorption layers. It could be applied to quantify the effect of different factors and additives on the surface elasticity and adhesion to substrates.
[1] S.C. Russev, N. Alexandrov, K.G. Marinova, K.D. Danov, et al. Rev. Scientific Instruments 79 (2008) 104102.
pk@lcpe.uni-sofia.bg
