Spinning drop tensiometer
A spinning drop tensiometer is an instrument for determining the interfacial tension. Here, a horizontally arranged capillary filled with a bulk phase and a specifically lighter drop phase is set in rotation. The diameter or curvature of the drop which is elongated by centrifugal force correlates with the interfacial tension.
When a heavy bulk phase and a light drop phase are situated in a horizontal, rotating capillary, the drop radius perpendicular to the axis of rotation depends on the interfacial tension ;σ between the phases, the angular frequency ;ω of the rotation and the density difference ;Δρ. In the classic approach according to Vonnegut's equation, the interfacial tension can be calculated from the measured drop diameter d (=2r) with a given speed of rotation and with known densities of the two phases:
When using the Vonnegut method, the length of the drop along the axis of rotation must be at least four times the diameter of the drop in order to minimize the error due to the curvature of the interface.
In the more recent Young-Laplace approach, the curvature of the drop shape is used instead of the diameter. This extends the measurement range to higher interfacial tension values. The drop diameter or curvature is determined from the video image of the drop by means of drop shape analysis.
Extremely low interfacial tensions can be measured with a spinning drop tensiometer. The spinning drop method is frequently used when the conditions for forming a microemulsion are to be investigated, e.g. with surfactant flooding in enhanced oil recovery (EOR) or in solvent-free degreasing.