# Drop volume tensiometer

A drop volume tensiometer is an instrument for determining the dynamic interfacial tension. Drops of a liquid are produced in a vertical capillary in a surrounding second liquid. The volume at which the drops detach from the tip of the capillary is measured. The dynamic surface tension can also be measured if measurements are made in air as the bulk phase.

## Background

In the drop volume method, a liquid is introduced into a bulk phase through a capillary. A drop, which tries to move upwards due to buoyancy, forms at the tip of the capillary.

Measuring principle of a drop volume tensiometer

The reverse arrangement, in which drops of the heavy phase drop from the tip of the capillary, is also possible.
As a result of the interfacial tension γ the drop tries to keep the interface with the bulk phase as small as possible. As a new interface comes into being when the drop detaches from the capillary outlet, it is necessary to overcome the corresponding interfacial tension. The drop does not detach until the lifting force or weight compensates for the force resulting from the interfacial tension on the wetted length of the capillary, the circumference. The formula for this relationship is:

g = Acceleration due to gravity, V = Drop volume, d = Inside diameter of capillary, Δp = Difference in density between the phases

## Tensiometer types

Conventionally, the measurement consists in determining the number of drops into which a given total volume divides, or in measuring the mass of the collected drops (stalagmometer). As a rule, it is not possible to specify the flow rate. This results from the viscosity and height of the liquid column of the stored volume, as a result of which it changes during the measurement.
With the DVT50 drop volume tensiometer, the time between two detaching drops at a set flow rate is measured by means of a light barrier. This enables the interface age (time from the start of drop formation to the moment of detachment) to be predetermined by varying the flow rate. The interfacial tension is thus measured as a function of the interface age.

Dependency of the interfacial tension on the interface 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.