Glossary

Washburn Method

This method allows to measure the contact angle of disperse particles such as powders or fibres. A glass tube is filled with the material and closed with a porous lid. The filling procedure is extremely important to obtain reproducible results and depends on the material. The measurement is an automated procedure in the software of the TE 2/3. The principle steps of the experiment are shown in Fig. V.7. First the filled tube is hooked to the balance (1) and then brought into contact with the liquid (2). The liquid starts to wet the powder, hence the liquid meniscus rises inside the tube. After a transition period (3) a constant rising speed is reached, which later levels off and terminates (4). Fig. V.8 shows schematically the result of an experiment.
Scientific Setup

Figure 1: Principle of the Washburn method to determine the contact angle of a powder

Scientific Setup

Figure 1: Schematic of the function force F versus time

The Washburn equation which describes the experiment reads

Formula

The parameters in Eq. (V.17) are: r - density difference between liquid and air, h - viscosity of the liquid, g - surface tension of the liquid. The coefficient K stands for the porosity of the powder in the tube and has to be determined separately. Traditionally it is determined by a separate experiment using a liquid that wets the particles perfectly, for example hexane. Then the porosity coefficient k results to

Formula

as the contact angle can be assumed to be zero. This technique is easy to handle, however, reproducible results require a lot of experience in particular with respect to the preparation of the tube packing, as any changes in the porosity coefficient influences the measured contact angle directly. When experiments with other liquids are performed the advancing contact angle can be obtained from the equation

Formula
Back to Science Glossary