Role and use of surface-active species in fragmentation of liquid films and layers – SURFBREAK

Flows wherein liquid films or layers are fragmented occur in many industrial contexts. Enhanced or suppressed fragmentation of liquid ligaments or film rupture often largely determines efficiency, such as in controlled sprays in coating agent/pesticide delivery, combustion, cosmetics, food engineering, or thermally-insulating light foamed materials in the construction industry. Such flows are multiscale systems that involve a huge interfacial area, and their formation/de-stabilization requires large, fast interface deformation. Because surface-active species deeply alter the interfacial behavior, by inducing surface elasticity, Marangoni stresses, intrinsic shear and dilatational viscosities, there are expected to play a key role. Yet, to which extent surfactants actually alter the rupture processes is currently unknown.

Past investigations have demonstrated that interface rheology can have a dramatic impact in a variety of situations – from motion and breakup of bubbles and drops to the instability of parallel two-layer - but have always relied on simplifying assumptions: surface elasticity or interface viscosity is accounted for, but rarely both, creeping flow is assumed, the interface shape is fixed or slightly perturbed. Such restrictions are not tenable to describe the rupture processes, which usually involve drastic change in shape and topology of the surfaces, as well as inertial effects. The goal of this project is to remedy this situation and investigate the role of surfactants on the interface dynamics under large and fast deformations.

Our approach will combine predictive tools and dedicated experiments. We will develop a computational method based on the level-set technique for simulations of two-phase flows that will fully take into account the complex rheology of interface induced by the presence of surfactants, and which can also describe non-hydrodynamic forces such as disjoining pressure. We will use the method to predict the rheology and coalescence of sheared foams, and confront its predictions to experimental data gathered in two situations: sheared droplets and assisted atomization. In the former, a Taylor/Couette setup will allow to probe the role of the interface rheology on the deformation and breakup of single bubbles and droplets. In the latter, we will investigate droplet entrainment from a liquid layer into a fast gas stream, how surfactants alter the cascade of instabilities, and in fine the drop size distribution. Ultimately, our fundamental understanding of interfacial and inertial effects should allow to develop means whereby fragmentation/rupture of liquid films/layers can be controlled through surface-active species.