title: Capillary meniscus dynamometry—method for determining the surface tension of drops and bubbles with isotropic and anisotropic surface stress distributions.
abstract: The stresses acting in interfacial adsorption layers with surface shear elasticity are, in general, anisotropic and non-uniform. If a pendant drop or buoyant bubble is covered with such elastic layer, the components of surface tension acting along the "meridians" and "parallels", σ(s) and σ(φ), can be different and, then, the conventional drop shape analysis (DSA) is inapplicable. Here, a method for determining σ(s) and σ(φ) is developed for axisymmetric menisci. This method, called 'capillary meniscus dynamometry' (CMD), is based on processing data for the digitized drop/bubble profile and capillary pressure. The principle of the CMD procedure for data processing is essentially different from that of DSA. Applying the tangential and normal surface stress balance equations, σ(s) and σ(φ) are determined in each interfacial point without using any rheological model. The computational procedure is fast and could be used in real time, during a given process. The method is applied to determine σ(s) and σ(φ) for bubbles and drops formed on the tip of a capillary immersed in solutions of the protein HFBII hydrophobin. Upon a surface compression, meridional wrinkles appear on the bubble surface below the bubble "equator", where the azimuthal tension σ(φ) takes negative values. The CMD method allows one to determine the local tensions acting in anisotropic interfacial layers (films, membranes), like those formed from proteins, polymers, asphaltenes and phospholipids. The CMD is applicable also to fluid interfaces (e.g. surfactant solutions), for which it gives the same surface tension as the conventional methods.
authors.affiliation: Department of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria.
authors.forename: Krassimir D
authors.initials: KD
authors.lastname: Danov
authors.affiliation: Department of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria.
authors.forename: Rumyana D
authors.initials: RD
authors.lastname: Stanimirova
authors.affiliation: Department of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria. Electronic address: pk@lcpe.uni-sofia.bg.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Kralchevsky
authors.affiliation: Department of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria.
authors.forename: Krastanka G
authors.initials: KG
authors.lastname: Marinova
authors.affiliation: Department of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Sofia 1164, Bulgaria.
authors.forename: Nikola A
authors.initials: NA
authors.lastname: Alexandrov
authors.affiliation: Unilever Research & Development Vlaardingen, 3133AT Vlaardingen, The Netherlands; Laboratory of Physical Chemistry and Colloid Science, Wageningen University, 6703 HB Wageningen, The Netherlands; Department of Mechanical Engineering, University College London, WC1E 7JE, UK.
authors.forename: Simeon D
authors.initials: SD
authors.lastname: Stoyanov
authors.affiliation: Unilever Research & Development Vlaardingen, 3133AT Vlaardingen, The Netherlands.
authors.forename: Theodorus B J
authors.initials: TB
authors.lastname: Blijdenstein
authors.affiliation: Unilever Research & Development Vlaardingen, 3133AT Vlaardingen, The Netherlands.
authors.forename: Eddie G
authors.initials: EG
authors.lastname: Pelan
mesh_terms: D008422:Materials Testing; D013500:Surface Tension
keywords: Anisotropic interfacial layers; Drop shape analysis; Non-uniform surface tension; Pendant drops and buoyant bubbles; Protein adsorption layers; Surface stress balances