On the uniqueness of advancing contact angle – Water drop on smooth wood

Abstract

An advancing contact angle (CA) can be measured by numerous different techniques that monitor different time intervals of a dynamic wetting process. This does raise the question: would the same value of advancing CA be obtained at different time intervals of the wetting process? This study monitored the relaxations of CA and wetting diameter of a water drop on a Diospyros crassiflora substrate (Rq = 0.41 μm) from 0.001 to 0.3 s and from 0.1 to 70 s by utilizing the same sessile drop contact angle analyzer with two different acquisition rates of 6770 and 30 fps, respectively. Subsequent analysis of the data showed that when the relaxations of the CA and wetting diameter were monitored from the moment a water drop came in contact with wood the substrate, two regions having a near-constant CA with a near quasi-equilibrium advance of the triple line were observed, and both these regions matched well with the definition of an advancing CA. For a water drop, the average CA at these two regions were found to be significantly different; ∼66° and ∼50°. The damped oscillatory behavior exhibited by the water drop during these two regions was discussed in detail as well.