Test methods for surface and interfacial tension measurements

4 Test methods for surface and interfacial tension measurements 

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

For the Wilhelmy plate method, a thin plate with a perimeter of about 4 cm is lowered to the surface of a liquid and the downward force directed on the plate is measured. Surface tension is the force divided by the perimeter of the plate. For this method to be valid, the liquid should completely wet the plate before the measurement, which means that the contact angle between the plate and the liquid is zero. Furthermore, the position of the plate should be correct, which means that the lower end of the plate is exactly on the same level as the surface of the liquid. Otherwise the buoyancy effect must be calculated separately. 

The pendant drop technique measures the shape of a liquid drop suspended from the tip of a capillary needle. The drop is optically observed and the surface tension is calculated from the shape of the drop. This method is not as precise as the force measurement method because it depends on the eye of the operator or the sophistication of detection hardware and analysis software. 

Dynamic surface tension is the time trajectory of surface tension before equilibrium is reached. Dynamic surface tension tracks the changes during surface formation when surfactants are added. The bubble pressure method is the one most commonly used for the determination of dynamic surface tension. The details of this method are described in ASTM D3825-90 (2000) [ 19]. In this method a capillary tube is immersed in a sample liquid and a constant flow of gas is maintained through the tube forming bubbles in the sample liquids. The surface tension of the sample is calculated from the pressure difference inside and outside the bubble and the radius of the bubble. 

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