Elasticity of foam bubbles measured by profile analysis tensiometry

Elastic modulus of foam bubbles, stabilized with tetraethylene glycol octyl ether (C8E4) and 1 × 10−5 M NaCl, was determined by cyclic expansion and shrinking of foam bubbles with frequency of 0.1 Hz and volumetric amplitude of 2 mm3. The film tension was monitored by a commercial profile analysis tensiometer (Sinterface Technologies, GmbH). The elastic moduli of foam bubbles were obtained as a function of surfactant concentration in the range of 2 × 10−3–1 × 10−2 M. The theory of Lucassen and van den Tempel [1] for the elastic modulus of a single liquid/air interface at a given frequency was employed. In the theoretical analysis the bulk diffusion coefficient of surfactant molecules was considered as a unknown model parameter which was obtained by matching the theory with the experimental data. Hence, the dependence of the bulk diffusion coefficient of C8E4 molecules upon the C8E4 concentration was obtained. The diffusion coefficient reached a maximum at 5 × 10−3 M C8E4 (D = 8.5 × 10−11 m2/s). In the experimental surfactant concentration range (2 × 10−3–1 × 10−2 M, CMC = 7.5 × 10−3 M) the foam bubbles were relatively dry, with visible interferometric fringes corresponding to thin films stabilized by repulsion of the electrostatic disjoining pressure. Hence, the overall dynamics of periodical expansion and shrinking of the foam bubbles occurred within the thin film state.