In this method, also the contact angle and surface free energy can be defined according to the Washburn theory. The powder wettability measurement is commonly utilized in many application segments for research and development, but also for quality control. Typical applications include pigment development for ink and paint industries, and powder formulation for pharmaceuticals and material development.

The powder wettability method enables to study liquid sorption into powders, pigments and other porous solids by recording the change of mass over time. The resulting curve provides information about sorption quantity and kinetics. The wetting of powders and pigments involves contact angle phenomena, and it can be defined by the Washburn theory.

Contact angles indicate the degree of wetting when a solid and liquid interact. The lower the contact angle the greater the wetting. Contact angles below 90° indicate that the liquid wets the solid spontaneously (in a thermodynamic rather than kinetic sense). Contact angles above 90° indicate that the liquid does not wet the solid. OneAttension software includes also surface free energy calculations from the measured contact angles.

The Sigma 700/701 tensiometer can measure sorption and the actual contact angles of powders, pigments and porous solids. According to the Washburn theory, when a porous solid is brought into contact with a liquid the rise of the liquid into the pores of the solid will obey the following relationship:

T = [η ⁄ C · ρ^2 · γ · cos θ] · M^2

*T*— time after contactThe terms are defined as follows:*η*— viscosity of liquid- C — material constant characteristic of solid sample
*ρ*— density of liquid*γ*— surface tension of liquid*θ*— contact angle*M*— mass of liquid adsorbed on solid;

An experiment is performed where the mass of the adsorbed liquid is measured against time. A time/mass graph should give a straight line whose slope is:

η ⁄ C · ρ^2 · γ · cos θ

If viscosity, density and surface tension are known then there are only two unknowns left in this term, contact angle and C, the material constant for the solid. To resolve this perform an experiment in which the contact angle is assumed to be zero with a liquid with very low surface tension such as N-hexane. When the experiment is performed, the material constant for the solid may be solved for:

slope = η ⁄ C · ρ^2 · γ

It is then possible to perform the same experiment with any test liquid and calculate the contact angle for that system. Washburn experiments test solids with a completely wetting liquid and assume that *θ* = 0 to allow C to be solved. Then, assuming C is constant, the same solid can be tested against various liquids to find the contact angles.

This method only holds for situations where the material constant, C, can be assumed to remain constant. C, is a term that reflects porosity or the packing of particles in powders. Because this is an assumed constant in the Washburn method, any actual variation will generate error. It is therefore critical to attempt to ensure that C actually is constant.

The material constant, C, reflects the number and orientation of pores present in the solid sample. When testing porous solids that retain a definite shape (i.e. paper, fabric, etc.) ensure that each sample has the same size, shape and orientation.

For testing powders the material constant will depend on the porosity of individual particles and the packing of particles. If all the powder samples are from the same source, porosity can be assumed to be constant. Major variations in C values are usually due to variations in packing. Various methods are used to give consistent packing. Most are based on using a reproducible force to pack a known mass of powder into a sleeve with a porous bottom (e.g. sintered glass or a screen lined with filter paper).

Powder wettability can be studied also by using optical tensiometer and its sessile drop contact angle method. Attension® Theta Optical tensiometer is recommended for the capture of high-speed change in contact angles indicating absorption kinetics. In this method the powder needs to be in a compressed tablet form. Please see more information from a following standards:

ASTM D7490-08 | Standard test method for surface tension of solid coatings, substrates and pigments using contact angle measurements. |

ASTM D7334-08 | Standard practice for surface wettability of coatings, substrates, pigments by advancing contact angle measurement. |