Bubble pressure tensiometer

A bubble pressure tensiometer is an instrument for determining the dynamic surface tension. The maximum internal pressure of a gas bubble which is formed in a liquid by means of a capillary is measured.


According to the Young-Laplace equation, the internal pressure p of a spherical gas bubble (Laplace pressure) depends on the radius of curvature r and the surface tension σ.

When a gas bubble is produced in a liquid at the tip of a capillary, the curvature initially increases and then decreases, resulting in the occurrence of a pressure maximum. The greatest curvature and therefore the greatest pressure occurs when the radius of curvature is equal to the radius of the capillary.

Pressure characteristic for the bubble pressure measurement, position of pressure maximum

If the radius of the capillary is known, the surface tension can be calculated from the pressure maximum pmax. As the capillary immerses in the liquid, the hydrostatic pressure p0 resulting from the immersion depth and the density of the liquid must be subtracted from the measured pressure. This results in the following formula for the bubble pressure measurement:

The measured value corresponds to the surface tension at a certain surface age, the time from the start of the bubble formation to the occurrence of the pressure maximum. The dependence of surface tension on surface age can be measured by varying the speed at which bubbles are produced.

Chart surface tension vs. surface age

Dependency of the surface tension on the surface age

This dependency plays an important part in the use of surfactants, as in many processes the equilibrium value of the interfacial tension is never reached due to the sometimes low surfactant diffusion and adsorption rates.
Bubble pressure measurements enable surfactant rate parameters such as the diffusion coefficient and the adsorption coefficient to be calculated.