Turbulent foams are encountered in processing equipment such as distillation columns, fermenters and boilers. The interfacial forces that stabilise such foams are also important in coating processes, and the design and manufacture of dispersed phase products like ice cream and detergents. Our objective was to measure bubble characteristics by high speed camera with image analysis, and by hot-film anemometry, and to predict interfacial forces and foam stability on the basis of calculation, using results from fundamental studies of flowing liquid surfaces.
We used an overflowing cylinder (OFC) to study non-equilibrium surfaces under steady-state conditions. Liquid containing surfactant is pumped vertically upward through a cylinder and accelerates radially to flow over its horizontal rim. Surface expansion rates are in the range 1-10 s-1. We measured surface concentration with ellipsometry, in which a laser beam is reflected by the surface and the change in its polarisation is measured. The OFC data gave a much better precision than that from static solutions at equilibrium, where trace impurities and natural convection lead to drift. For calibration, we made noninvasive measurements by neutron reflection (at ISIS) and by surface light scattering – neither technique had previously been applied successfully to a flowing liquid surface. Liquid velocity in the OFC, determined with laser Doppler scattering, was in excellent agreement with theoretical predictions. Finally, we proved the feasibility of infrared spectroscopic reflectometry for studying the expanding surface of the OFC – another first.
In the OFC, the surface expansion rate passes through a steep maximum as the bulk concentration increases, for micelle-forming surfactants. Similarly it is observed that foam heights are greatest at intermediate concentrations of such surfactants. This behaviour is typical of mass transport controlled adsorption at the surface, and we can predict the surface forces caused by a given rate of surface expansion, both in the OFC and in a foam if we know the strength of the surfactant and its diffusivity. Weak surfactants require a high rate of surface expansion to cause a significant effect.
A commercial gas treating agent, the tertiary amine MDEA is known to foam, but it caused no surface tension induced flows in the OFC, nor were any gradients in its surface properties detectable by ellipsometry. At certain concentrations, foams were observed on a sieve tray, probably because the surface expansion rate encountered on the tray (~ 10-100 s-1) are greater than those in the OFC (~ 1 s-1 under gravity driven flow). To probe faster time scales, we developed another technique based on ellipsometry of a liquid jet, which allows measurements down to 1 ms exposure.
The surface expansion rate thus emerges as a key parameter determining the surface forces in a surfactant solution. More work is needed to relate the energy dissipation and surface expansion rate in froths and foams to different hydrodynamic conditions.
An EPSRC grant GR/K79611, with additional funding from Unilever Research
and Albright and Wilson supported this collaboration at the University
of Oxford between the chemical engineering group in Engineering Science
(Dr RC Darton), and the surface chemistry group in the Physical and Theoretical
Chemistry Laboratory (Dr CD Bain).