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Wetting force at three-phase line was measured by the Wilhelmy technique using fibrous solids/liquid/liquid systems. Advancing and receding contact angles were calculated from the wetting forces du~lg fiber immersion and emersion. The obtained results showed that contact angle hysteresis was due to the heterogeneity of the fiber surfaces. The dispersive and polar components of surface free energies of the fibers were determined from the advancing and receding contact angles, respectively. The Hamaker constants of the fibers were estimated from the dispersive components of their surface free energies.

Wetting force measurements in solid/water/n-alkane systems were carried out using carbon fibers, boron fiber, fibrous quartz, and polymeric fibers by the Wilhelmy technique. Contact angles of water were independent of the velocity of the moving three-phase boundary on repyrolyzed PAN-based carbon fibers which did not show any contact angle hysteresis. Hysteresis in contact angles due to surface heterogeneity began to appear after irradiation with atomic oxygen which oxidized the carbon fibers. On atomic oxygen exposed carbon fibers, advancing contact angles increased and receding ones decreased with increasing velocity, and there was a region where both angles were constant. The range of deviations in contact angles due to surface heterogeneity was determined from local deviations in weight traces at the velocity of three-phase boundary of 0.3 mm min-1 where both angles were independent of the velocity. Deviations in contact angles, ∆θ, were larger in advancing than in receding for hydrophilic solids (e.g. quartz), and were smaller for hydrophobic solids (e.g. polyethylene).

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