Measuring powder contact angles with exact reproducibility

Our new Washburn sorption cell and its preparation set reduce measurement uncertainty and enable standardized sample preparation

The wetting of a powder determines its ability to be distributed in dispersions – examples include pigments in paints or creams and also fillers in polymers. The Washburn sorption method for determining contact angle and surface free energy is used to analyze the behavior of powders with regard to liquids.

Measuring wettability using the Washburn method

With this method, a powder-filled measuring tube fitted with a paper filter base is suspended from the force sensor of a tensiometer. When the tube is immersed in a liquid, the liquid rises inside the powder. The rate of rise measured on the basis of the increase in force depends on the contact angle, which can be calculated in this way. With our new Washburn sorption cell SH0820 and the preparation set SH0824, we negate two sensitive points in the measurement which have a great effect on its reproducibility.

Dished filter base prevents interference due to air bubbles

A problem systematically associated with the Washburn method is air bubbles which, in the case of a flat paper filter base, adhere below the tube after immersion and falsify the measurement. Our new preparation set therefore has a fitted shape for the tube with a slightly dished base. After filling, the powder is pushed down with the help of a stainless steel plunger. In doing so, the filter base is pressed into the dish and curves in a reproducible manner so that air bubbles do not adhere on immersion.

Special plunger ensures uniform powder packing density

The design of our plunger overcomes a second challenge with the Washburn measurement, namely the maintenance of a constant packing density of the same sample. This is essential for obtaining comparable results. To guarantee this, the plunger compresses the powder without bodily force and only as a result of its inherent weight. The mass can be flexibly matched to the particular sample characteristics, as with a dumbbell, by fitting up to four discs, each weighing 500 g.

The new Washburn sorption cell SH0820 for our Force Tensiometer – K100 and the preparation set SH0824 will be available from January 2017.