Relationship between Surface Mechanical Properties and Foam Stability

Pang, Yuan

Abstract

This thesis is concerned with foam stability and surface mechanical properties (including surface tension and surface rheological properties) of liquid-air interfaces of foams. In particular, the question as to which surface mechanical property (or properties) primarily governs (or govern) foam stability, the optimal hydrophobicity range of particle to stabilise foam, and the reason why the particle with that hydrophobicity are capable of stabilising foam in terms of the modification of surface mechanical properties of liquid-air interfaces of particle-stabilised foams are investigated systematically in this thesis. In respect of relating foam stability to surface mechanical properties, the oscillating pendent droplet method of obtaining surface rheological properties, the repeatability of dynamic surface tension measurements of ionic surfactant solution using the static pendant droplet method, and the measurement of foam stability using a modified Bikerman test of Li et al. (2010) that controls the relative humidity at the top of column have been studied as a matter of priority. The dependency of surface rheological properties on oscillation frequency and amplitude has been described and this dependency indicates that the measurement of surface rheological properties of different surfactant solutions should be performed at a fixed oscillation frequency and amplitude in order to obtain convincing observations of the relationship between surface mechanical properties and foam stability. Furthermore, it has been demonstrated that the observable differences of dynamic surface tension results between different trials are due to the presence of trace amounts of highly surface-active impurities and it is necessary to perform large numbers of independent trials of dynamic surface tension measurement and then calculate the arithmetic mean of all dynamic surface tension results to obtain the dynamic surface tension value of ionic surfactant solutions with highly surface-active impurity. Moreover, the observation of the dependency of foam stability upon environmental relative humidity has been confirmed and extended. It is precisely to facilitate the investigation of the relationship between foam stability and surface mechanical properties that the environmental relative humidity has to be controlled at a constant value. Foam stability is correlated to surface rheological properties including surface elasticity, surface dilatational viscosity and Gibbs elasticity under the condition that the effect of surface tension on foam stability is kept approximately constant and therefore eliminated. It has been shown that both surface tension and surface rheological properties affect the stability of surfactant-stabilised foams, however, surface tension appears to play more important role, with respect to the stability of surfactant-stabilised foams. The stability of foams stabilised by the mixtures of particles and surfactants, as well as the relationship between surface mechanical properties and stability of particle-stabilised foams, is also studied in this thesis. It has been experimentally demonstrated that only intermediately hydrophobic particles can enhance foam stability by the addition of particles to surfactant solutions, since these particles affect the surface mechanical properties by increasing surface elasticity and Gibbs elasticity whereas decreasing equilibrium surface tension and surface dilatational viscosity, which has a synergistically positive effect on the stability of particle-stabilised foams.

Citation

Pang, Y. (2019). Relationship between Surface Mechanical Properties and Foam Stability. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4912429

• Thesis Type: Thesis
• Deposit Date: Nov 12, 2024
• Publicly Available Date: Nov 12, 2024
• Keywords: Engineering
• Public URL: https://hull-repository.worktribe.com/output/4912429
• Additional Information: Department of Engineering; University of Hull
• Award Date: May 1, 2019