Interfacial and Colloidal Phenomena
Research Group 
Illinois Institute of Technology 

IIT FILM TENSIOMETER

Interfacial rheology is an important factor in determining the dynamic properties (flow characteristics) and dynamic stability of  foams and emulsions. The structural and dynamic properties of foams and emulsions are influenced by the liquid film formed between the gas bubbles, as in the case of foams, and between the dispersed droplets, as in the case of emulsions. The flow characteristics (rheology) of foams and emulsions are related to the rheology of the essential element of the liquid film. Over the years, various methods have been developed to study the shear and dilatational elasticity, and the viscosity of the liquid/gas surface and liquid/liquid interface. However, all previously developed methods measured the rheology at a single interface or surface. Based on the results for the rheology of a single surface or interface, scientists tried to predict the rheology of foams and emulsions. However, in most of the cases, no correlation between the interfacial rheology and rheology of the foam/ emulsion has been found. A review of the interfacial rheology methods and the correlation with the rheology of foams and emulsions is presented in the book, "Interfacial Transport Processes and Rheology", (D. Edwards and H.Brenner, D.Wasan) Butterworth-Heinemann- publisher, Boston, MA (1991).

The Foam Film Tensiometer (FFT) is designed to measure the rheology of the curved foam lamella, particularly it's film dilatational elasticity, E, the foam lamella equilibrium, and dynamic tension. The curved foam lamella represents the actual polyhedra structure of the foam texture and the interactions between bubbles of different sizes (Figures 1). The lamella of a polyhedral foam/emulsion drains continuously and expands due to the liquid driven out by forces, such as capillary and gravitational forces. To model this process, a dynamic film tension measurement needs to be carried out

                             Figure 1. Curved Foam Lamella , Polyhedral Structure and Film Thinning

The FFT methods were developed based on a simple physical concept. A curved foam lamella is formed at the tip of the vertically-oriented cylindrical capillary (Figure 2). After reaching the equilibrium (in the given time, the foam lamella radius and the capillary pressure become constant), the foam lamella is stretched (expanded) to a new radius. The foam lamella capillary pressure and it's curvature (the foam lamella area), were simultaneously electronically monitored. The capillary pressure was monitored by the sensitive piezo pressure transducer with a fast response time, and the film curvature was measured by analyzing the image. When the film surface area was stretched (expanded), the new film surfaces were covered with less surfactant molecules (while the film expansion rate was faster than the surfactant diffusion rate ). As a result, the foam lamella (film) tension will increase ( dg = g - g_eq ), where g is the value of the film tension after expansion (dynamic film tension), and g_eq is the initial (equilibrium) value of the film tension. This results in a surface tension gradient, and the surfactant molecules tend to diffuse from the bulk (film meniscus) to the expanded surfaces of the film.

Figure 2: IIT Schematic of Film Tensiometer