Dynamics of liquids spreading on compliant substrates

Lead Research Organisation: Imperial College London
Department Name: Department of Chemical Engineering

Abstract

The proposed work will involve a detailed examination of the interaction between a spreading liquid, laden with surface active additives, and an underlying compliant substrate made of a gel-like material. This three-year investigation will involve careful experimentation, to be carried out by a postgraduate student, and detailed modelling, to be carried out by a two-year postdoctoral research associated. The start of the latter will lag that of the former by a year in order to allow the experimental findings to guide the modelling effort. This work will be a collaboration between a chemical engineer, a chemist and an applied mathematician (from the Dept. of Chemical Engineering and the Dept. of Mathematics at ICL); the student and the postdoc will be located in the Dept. of Chemical Engineering. The experiments will examine the effect of gel strength and thickness, and surfactant type and concentration on the spreading characteristics using optical methods, while the modelling effort will use lubrication and elasticity theory to derive predictive models. The results of this multi-disciplinary work will elucidate the complex interactions between spreading liquids and underlying gel-like materials, which is a problem that lies at the heart of a range of engineering, biological and biomedical settings, such as drug delivery and spreading of gel-like materials (e.g. mucus) in the human body, deposition and spreading of gel layers in the manufacturing of photographic films, and cracking and subsequent removal of gels in oil reservoirs.

Publications

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Beacham DR (2009) Surfactant-enhanced rapid spreading of drops on solid surfaces. in Langmuir : the ACS journal of surfaces and colloids

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Craster R. V. (2009) Dynamics and stability of thin liquid films in REVIEWS OF MODERN PHYSICS

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Matar O (2009) Dynamics of surfactant-assisted spreading in Soft Matter

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Seevaratnam G (2010) Laminar flow deformation of a droplet adhering to a wall in a channel in Chemical Engineering Science

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Spandagos C (2012) Surface tension-induced gel fracture. Part 2. Fracture of gelatin gels. in Langmuir : the ACS journal of surfaces and colloids

 
Description This work involves an investigation of the spreading of liquids on gel layers in the presence of surfactants and an instability that manifests itself via the formation of "starbursts" resembling cracking patterns on the gel surface. A parametric study that involves different types of surfactants on various types of gels aims 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 these patterns. 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. An attempt to quantify the stresses that form the cracks and the rheological characterisation of the gels are also included. In terms of modelling, the mechanisms driving the surfactant-enhanced spreading of droplets on the surface of solid substrates, and particularly those underlying the superspreading behaviour sometimes observed, were investigated theoretically. Lubrication theory for the droplet motion, together with advection-diffusion equations and chemical kinetic fluxes for the surfactant transport, lead to coupled evolution equations for the drop thickness, interfacial concentrations of surfactant monomers and bulk concentrations of monomers and micellar, or other, aggregates. The surfactant can adsorb on the substrate either directly from the bulk monomer concentrations or from the liquid-air interface through the contact line. An important feature of the spreading model developed here is the surfactant behaviour at the contact line; this is modelled using a constitutive relation, which is dependant on the local surfactant concentration. The evolution equations were solved numerically, using the finite element method, and a thorough parametric analysis for cases of both insoluble and soluble surfactants was carried out with concentrations, in the latter case, that can exceed the critical micelle, or aggregate, concentration. The results show that basal adsorption of surfactant plays a crucial role in the spreading process; the continuous removal of surfactant that lies upon the liquid-air interface, due to the adsorption at the solid surface, is capable of inducing high Marangoni stresses, close to the droplet edge, driving very fast spreading. The droplet radius grows at a rate close to the reported experimental values for superspreading. The spreading rates follow a non-monotonic variation with the initial surfactant concentration also in accordance with experimental observations. This work was then extended to examine sessile liquid lenses spreading over a fluid layer, in the presence of Marangoni stresses due to surfactants; these systems show a surprisingly wide range of interesting behaviour ranging from complete spreading of the lens, to spreading followed by retraction, to sustained pulsating oscillations. Models for the spreading process, the that drive these phenomena and the regular oscillatory beating of lenses is shown to occur in specific limits. The next step is to include non-Newtonian rheology in the lower layer and model the details of the cracking process. Unfortunately, this coincided with the end of the project.
Exploitation Route The work undertaken in this project is fundamental in nature and seeks to achieve understanding of interfacial phenomena, which are common to a class of flows involving the interaction of a spreading surfactant-laden liquid with a gel-like material. The results of this multi-disciplinary work has elucidated the complex interactions between spreading liquids and underlying gel-like materials, which is a problem that lies at the heart of a range of engineering, biological and biomedical settings, such as drug delivery over compliant substrates, building scaffolds for tissue engineering, emplacement, treatment and removal of gels in fractured reservoirs in the oil industry, patterning of compliant substrates, and the manufacture of photographic films, which involve the successive deposition of gel layers. This work will also be of considerable interest to academics in physics, biology, chemistry, chemical engineering and applied mathematics, particularly those interested in interfacial fluid dynamics, rheology, pattern formation, nonlinear dynamics and stability theory.
Sectors Agriculture, Food and Drink,Chemicals,Education,Manufacturing, including Industrial Biotechology

 
Description Our findings have been used to elucidate the complex interactions between surfactant solutions and gels. Drops of the former can drive the formation of stress distributions on the surface of the latter giving rise to crack-formation.
 
Description Surfactant effects on vertical gas-liquid flows
Amount £146,000 (GBP)
Organisation Shell Global Solutions International BV 
Department Shell Global Solutions UK
Sector Private
Country Netherlands
Start 09/2014 
End 09/2016