Fundamental principles of emulsion rheology and their applications

Equilibrium and dynamics of adsorption of surfactants at fluid-fluid interfaces

A number of new experiments have been performed on the rise of air bubbles in clean mixtures of distilled water and pure, reagent grade, glycerine covering a range of the relevant parameter, the Morton number, Mo = gv 4 ρ 3 /σ 3 , of 10 13 . Here g is the acceleration due to gravity, v the kinematic viscosity, ρ the density and σ the surface tension of the mixture. In these careful measurements several scaling regimes have been found that have not been discussed before in the extensive literature on the subject. The transitions between these regimes have been delineated and attempts made to discuss the dynamical processes that might be important in each of them.

Experiments on the rise of air bubbles in clean viscous liquids

Axial velocity profiles of single bubbles in water/frother solutions

Surfactants - molecules and particles that preferentially adsorb to fluid interfaces - play a ubiquitous role in the fluids of industry, of nature, and of life. Since most surfactants cannot be seen directly, their behavior must be inferred from their impact on observed flows, like the buoyant rise of a bubble, or the thickness of a coating film. In so doing, however, a difficulty arises: physically distinct surfactant processes can affect measurable flows in qualitatively identical ways, raising the specter of confusion or even misinterpretation. This Perspective describes, in one coherent piece, both the equilibrium properties and dynamic processes of surfactants, to better enable the fluid mechanics community to understand, interpret, and design surfactant/fluid systems. Specifically, §2 treats the equilibrium thermodynamics of surfactants at interfaces, including surface pressure, isotherms of soluble and insoluble surfactants, and surface dilatational moduli (Gibbs and Marangoni). §3 describes surfactant dynamics in fluid systems, including surfactant transport and interfacial stress boundary conditions, the competition between surface diffusion, advection, and adsorption/desorption, Marangoni stresses and flows, and surface excess rheology. §4 discusses paradigmatic problems from fluid mechanics that are impacted by surfactants, including translating drops and bubbles, surfactant adsorption to clean and oscillating interfaces; capillary wave damping, thin film dynamics, foam drainage, and the dynamics of particles and probes at surfactant-laden interfaces. Finally, §5 discusses the additional richness and complexity that frequently arise in 'real' surfactants, including phase transitions, phase coexistence, and polycrystalline phases within surfactant monolayers, and their impact on non-Newtonian surface rheology.

S.K. Agrawal

Papers

The effect of interfacial viscosities on the motion of drops and bubbles