Capillary penetration technique


A practicable method to determine the solid surface tension of powders, granules, fiber bundles and other porous solids

Dr. Günter Auernhammer


It is generally agreed that the most practical way to obtain the surface energetics of solids is the measurement of contact angles of well defined liquids on a flat solid surface. However, serious problems arise when contact angles are measured on the surfaces of porous materials, such as membranes or compressed powders, which are of considerable technological interest. Surface roughness, heterogeneity, and the penetration of the liquid drop into the porous material may affect the measured contact angles, causing meaningless angles in terms of Young´s equation and hence in terms of a surface energetic interpretation of the contact angle data.

An alternative way to obtain the solid surface tension (γsv) of rough and porous solids is the capillary penetration technique. Our strategy is based on a modified Washburn equation. However, in contrast to approaches of other authors in the literature, we determine directly the solid surface tension from the capillary penetration experiments. The determination of the penetration velocity of liquids into porous solid systems yields K*γlv*cosθ versus γlv plots, which provide γsv values for these systems; K is an unknown parameter of the constant geometry of the porous solid.

Fig. 1: A plot of K*γlv*cosθ versus γlv for nine liquids penetrating into a PTFE (Teflon 807-N) powder; the maximum indicates the particle surface tension.

In the experiment, the porous solid systems are attached to a microbalance and brought into contact with several liquids. Their penetration velocities are determined by the slope of weight versus time plots. A commercial tensiometer can be used for these gravimetric experiments (Fig. 2). The application of this concept was demonstrated for hydrophobic polymer powders (PTFE, polypropylene, polyethylene, polystyrene), for hydrophilic (cellulose) and hydrophobic (PTFE) membranes, for pigments used in powder coatings, for inorganic fillers used in polymer composites and for textile fiber bundles. The solid surface tension values determined by the capillary penetration technique are in good agreement with the γsv values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials.

Fig. 2: A schematic of the experimental set-up for capillary penetration measurements using a commercially available tensiometer.



Relevant publications

Grundke K, Bogumil T, Gietzelt T, Jacobasch HJ, Kwok DY, Neumann AW:
Wetting measurements on smooth, rough and porous solid surfaces

Progress Colloid and Polymer Science 101 (1996) 58-68

Tröger J, Lunkwitz K, Grundke K, Bürger W:
Determination of the surface tension of microporous membranes using wetting kinetics measurements
Colloids and Surfaces A134 (1998) 299-304

Grundke K, Augsburg A:
On the determination of the surface energetics of porous polymer materials
Journal of Adhesion Science and Technology  14 (2000) 765-775