Measuring the surface dilatational viscosities of surfactant monolayers at the air-water interface by the shape analysis of deforming pendant drops.

摘要

At rest, a surfactant fluid interface is under a state of isotropic stress that is given by the compositional surface tension as a function of the surfactant surface concentration. When the surfactant fluid interface is set in motion, surface flow can induce surfactant interactions and generate interfacial stresses apart from the compositional surface tension. A surface flow can be surface shear, dilatational, or shear-dilatational. The Boussinesq-Scriven constitutive equation completely characterized by the compositional surface tension and surface shear and dilatational viscosities is the simplest relationship between the surface strain rate and surface stress. Compositional surface tension and shear viscosity measurements are very well established. On the other hand, surface dilatational viscosity measurements are difficult because a flow which changes the surface area also changes the surfactant surface concentration creating changes in the equilibrium interfacial tension that must be also taken into account.; In this research, we investigate how Langmuir surfactant monolayers subjected to a dilatational strain rate behave and focus on developing a new technique for measuring their surface dilatational viscosities. To fulfill our research goals, we built a pendant drop apparatus with a fast motion analyzer to deform and video record a pendant water drop with a surfactant monolayer at the air-water interface, developed a hydrodynamic surfactant model to obtain the surface dilatational viscosity from the recorded images, and measured the surface dilatational viscosities employing constant as well as sinusoidal volumetric flow rates. We also developed a correction that allows us to partially account for bulk hydrodynamic effects for pendant drops oscillating at high frequencies. We measured that the surface dilatational viscosity of the expanded phase of the phospholipid L-α-dipalmitoyl phosphatidylcholine (DPPC) is approximately 1.3 mNs/m from the compression experiments and 2.3 mNs/m at 0.2 Hz and 0.8 mNs/m at 1 Hz. We also measured from the oscillatory experiments that the surface dilatational viscosity of tripalmitin is 1.0 mNs/m at 1 Hz. In addition, we measured the surface viscoelasticities of mixed binary monolayers of DPPC and octadecanol in mole ratios 1:1 and 3:1 and found that the surface viscoelasticities of DPPC are twice with the addition of octadecanol.