Use this URL to cite or link to this record in EThOS:
Title: Dynamics of surfactant-laden fluids spreading on compliant substrates
Author: Spandagos, Constantinos Spyridonos
ISNI:       0000 0004 0125 1952
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
Availability of Full Text:
Access from EThOS:
Access from Institution:
This work involves an experimental investigation of the spreading of liquids on gel layers in the presence of surfactants. Of primary interest is the instability that accompanies the cracking of gels through the deposition and subsequent spreading of a drop of surfactant solution on their surface. This instability, which has been reported recently in the literature, manifests itself via the shaping of crack-like spreading “arms”, in formations that resemble “starbursts”. The main aim of this study is to elucidate the complex interactions between spreading surfactants and underlying gels and to achieve fundamental understanding of the mechanism behind the observed phenomenon of the cracking pattern formation. This is hoped to be beneficial for improving a wide range of engineering, biological, biomedical and environmental settings where such systems are of central importance. Towards this aim, a parametric experimental work that involves the spreading of different types of surfactants on various types of gels was conducted, in order to explore the ways that system parameters such as the surfactant chemistry and concentration and the gel type and strength can affect the morphology and dynamics of the “starburst” patterns. The surfactants used were SDS (sodium dodecyl sulphate) and Silwet L-77 (a trisiloxane ethoxylate); the former is one of the most common materials used in surfactant-related studies and the latter belongs to a certain class of surfactants, termed “superspreaders” which are known for exhibiting unique spreading behaviour. The different gel substrates were made of agar (a polysaccharide-based gel), and gelatine (a protein-based gel). In terms of the spreading dynamics, the crack propagation was attempted to fit to a power-law by measuring the temporal evolution of the length of the spreading “arms” that form each one of the observed patterns. The values of the exponent of the power law are inside the predicted limits for Marangoni-driven spreading on thick layers. Therefore, Marangoni stresses induced by surface tension gradients between the spreading surfactant and the underlying gel layer are identified to be the main driving force behind these phenomena, while gravitational forces were also found to play an important role. A mechanism that involves the “unzipping” of the gel in a manner perpendicular to the direction of the largest surface tension gradient is suggested. An attempt to quantify the stresses that form the cracks and a detailed rheological characterisation of the gels are also included.
Supervisor: Matar, Omar ; Luckham, Paul Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral