Engineering and control of surfactant-laden flows: multi-scale analysis and experiments

Over the past two decades, the use of surfactants as wetting agents has enjoyed considerable attention due to their importance and potential in a variety of industrial and biomedical applications (coating flows, detergency, enhanced-oil-recovery, treatment of respiratory distress syndrome). Despite the research in the area, numerous fundamental open problems remain, as no single approach (experiments, simulations or theory) in isolation is able to probe the formidable complexities of the fluid-solid behaviour of amphiphilic molecules in solution. This project will provide detailed understanding of how surfactants behave at contact lines and adsorb at interfaces, and how this ultimately affects the spreading and wetting of hydrophobic surfaces. In this context, even trivial-sounding problems are not fundamentally understood. For instance, although the equilibrium state of a surfactant-free droplet on a hydrophobic substrate is easily characterised using the Young equation, this is not the case when the droplet is laden with surfactant. How does the surfactant distribution equilibriate and the stresses induced by it balance with the forces at the contact line? How are the latter influenced by surfactant solubility, and the formation of surfactant aggregates at high surfactant concentrations? These questions underlie striking and technologically important, yet poorly eludicated effects, such as superspreading whereby aqueous droplets containing superspreader surfactants (e.g. trisiloxanes) spread rapidly to produce perfect wetting over hydrophobic substrates. The purpose of this proposal is to draw together three world-leading groupings to tackle these fundamental problems in a collaborative, systematic, multi-disciplinary and multi-scale manner. The chemistry and molecular interactions require detailed modelling on the molecular level: EAM brings this expertise. This must then be scaled up to the lengthscale of droplets and the application itself: RVC and OKM have considerable background in modelling surfactant-gradient (Marangoni) driven flows. The problem requires new physicochemical understanding of the phenomena and the developed models must be validated, and indeed informed by, detailed and careful experiments: VMS is a world-leader in both. The deep knowledge of surfactant-laden flows that will be achieved via the proposed, transformative research will be used not only to provide accurate and reliable predictions of these flows but also to rationally design bespoke surfactant molecular architectures for various applications ranging from agrochemicals to enhanced-oil-recovery.

Motivated by a need to understand fundamental physio-chemical processes and deliver unprecedented physical insight into the behaviour of surface active additives near and on interfaces we propose a close-knit combination of experiments, modelling and theory. This will deliver highly reliable, accurate and efficient modelling tools to predict flow behaviour for given molecular architecture with wide applicability and impact; these tools will include the ability to design surfactants to control both interfacial and bulk (wettying/spreading) flows. This requirement is pressing, for instance, the most recent review paper on superspreading , written by a prominent industrial scientist, concludes that " ... the findings [of reseach in this area] should somehow relate to the molecular structure of the surfactants. This could help the synthetic chemist to design surfactants with improved properties. Moreover, the results should enable the practitioner to understand the phenomena observed in the application of trisiloxane surfactants" This is precisely what the unique combination of experiments/MD/continuum modelling described within this proposal will address.