Kulim Kht Washburn Method

Washburn Method
This method is chosen when the solid sample to be tested contains a porous architecture which leads to
absorption of the wetting liquid. The solid is brought into contact with the testing liquid and the mass of
liquid absorbed into the solid is measured as a function of time. The amount absorbed is a function of the
viscosity, density and surface tension of the liquid, the material constant of the solid , and the contact angle
of the interaction. If the viscosity, density and surface tension of the liquid are known the material constant
and contact angle can be solved for. KSV instruments produces two instruments capable of finding contact
angles via the Washburn technique, the Sigma 70 and LPR 902. See Application Note #104 for details.

Utilization of Contact Angle Data:
The primary focus of contact angle studies is in assessing the wetting characteristics of solid/liquid
interactions. Contact angle is commonly used as the most direct measure of wetting. Other experimental
parameters may be derived directly from contact angle and surface tension results. Some examples are:
Work of Adhesion: defined as the work required to separate the liquid
and solid phases, or the negative free energy associated with the
adhesion of the solid and liquid phases. Used to express the strength
of the interaction between the two phases. It is given by the Young
-Dupre equation as:
Wa = (1+ cos )
Work of Cohesion: defined as the work required to separate a liquid into
two parts, it is a measure of the strength of molecular interactions
within the liquid. It is given by;
Wc = 2
Work of Spreading: the negative free energy associated with spreading
liquid over solid surface. Also referred to as Spreading Coefficient
it is given as:
Ws = (cos - 1)
Wetting Tension: a measurement of force/length defined as:
= Fw / P = lv cos

Kulim Kht Semi Auto Tensiometer

Kulim Kht Surface Tension & Contact angle

How come contact angle and surface tension are so important ? Molecules inside (bulk) a liquid/solid are in every direction affected by equal attraction forces, whereas the molecules at the surface lack a neighbor towards the air phase and therefore they have larger attraction forces towards the liquid/solid than air (see figure below). This leads to a situation where the interface has excess free energy. This excess free energy is characteristic for any liquid or solid. For liquids a spontaneous contraction of the surface will take place due to this free energy and the Surface Tension of a liquid is a direct measure of it. In the case of solids a contraction is hardly ever seen, but still this free energy is present at the interface of a solid. However, now it is called Surface Free Energy (instead of Surface Tension as for liquids) and can be accessed by measuring the contact angle of a series known liquids placed on the solid surface. The dimension of Surface Tension and Surface Free Energy is mN/m.