Surface tension

A tensiometer is a device for measuring the surface tension of a liquid and/or the interfacial tension between two liquids. There are various types of tensiometers with different measuring methods for particular measuring ranges and problems:

Bubble pressure tensiometer: The maximum internal pressure of a gas bubble which is formed in a liquid by means of a capillary is measured. Used in measuring dynamic surface tension as a function of surface age. 

Drop volume tensiometer: The volume of a drop of liquid produced at a vertical capillary in another liquid is measured at the moment of its detachment. More rarely, it is also used for measuring the surface tension (drop phase in air). Used in measuring the dynamic interfacial tension as a function of interface age. 

Spinning drop tensiometer: A horizontal capillary filled with a bulk phase and a drop phase is set in rotation. The diameter of the drop which is elongated by centrifugal force correlates with the interfacial tension. Used in measuring the static interfacial tension, particularly suitable for very small interfacial tensions.

Force tensiometer: The tensile force which occurs as a result of wetting an immersed probe is measured with reference to the wetted length. The measuring probe is usually a ring (Du Noüy ring method) or a plate (Wilhelmy plate method). Used in measuring static interfacial tension and surface tension in the medium to upper measuring range (1-100 mN/m). 

The Wilhelmy Plate method is used for measuring the surface tension (SFT) of a liquid, the interfacial tension (IFT) between two liquids and the contact angle between a liquid and a solid. The force acting on a vertically immersed plate is measured. When a vertically suspended plate touches a liquid surface or interface, then a force F (which correlates with the surface tension or interfacial tension σ and with the contact angle θ according to the following equation) acts on this plate:

The wetted length L of the plate is equal to its perimeter. To measure the force (F), the plate is attached to a force sensor of a tensiometer.

Schematic diagram of the Wilhelmy Plate method

Measurement of the surface tension or interfacial tension: Platinum is chosen as the plate material when measuring the SFT or IFT as it is chemically inert and easy to clean, and because it can be optimally wetted on account of its very high surface free energy and therefore generally forms a contact angle θ of 0° (cos θ = 1) with liquids. The required variable σ can be calculated directly from the measured force. Unlike the quasi-static Du Noüy ring method, in which the surface is deformed during the measurement, the plate method is purely static. This makes it possible to record the change in SFT or IFT with respect to time as well as the end values at equilibrium. 

Measurement of the contact angle: To measure the contact angle, the solid sample - which is usually in plate form - is immersed in a liquid with known SFT. The contact angle can be calculated from the measured force by transposing the Wilhelmy equation:

With the Wilhelmy method, a dynamic contact angle is measured by slowly immersing and then withdrawing the solid. The advancing angle is determined during the wetting process and the receding angle during the de-wetting process.

The wetted length (L) of the solid (perimeter at the level of the liquid surface) must be known and must not be dependent on the immersion depth. In order to determine the wetted length, a measurement can be carried out with an optimally wetting liquid with known SFT with which the contact angle is 0° (cos θ = 1). Results for the Wilhelmy contact angle can be used to determine the surface free energy of the solid (www.kruss-scientific.com)

In house:      KRÜSS EasyDyne Tensiometer

The EasyDyne Tensiometer measures surface and interfacial tension of liquids using the Wilhelmy Plate or Du Noüy Ring method. It is also able to measure the density of liquids.