Techniques: Tensiometry


Although many different techniques have been developed for the determination of the interfacial tension between two phases, they can all be grouped into four categories dependent upon the physical principles they exploit.

Drop Shape Analysis

The principal method of tensiometry we use is Drop Shape Analysis, using either pendant drops or rising bubbles. The basic setup of the experiment is shown below.

The droplet is constrained to a needle and is illuminated by a light source which projects the image of the drop onto a camera. The camera is connected to a computer allowing easy analysis of the image, via iterative fitting of the shape of the drop which is affected by the gravitational/buoyancy and interfacial tension forces acting on the drop.

In my group, dynamic interfacial tension measurements have been used to determine equations of state and adsorption isotherms for a range of surfactants through the use of the Gibbs equation, which allows the conversion of surface tension data to surface excess concentrations of surfactant at the interface.

Using equilibrium interfacial tension measurements as a function of temperature, surface freezing at the CTAB–tetradecane interface was studied. As shown below, the transitions are identified from kinks in the interfacial tension plots. The surface freezing temperature was found to vary linearly with the surface excess of CTAB.