Comparison of the static and dynamic contact angle hysteresis at low velocities of the three-phase contact line

Abstract

The forced spreading and retraction of a drop with variable volume onto a poly(ethylene terephthalate) surface has been studied. Water and an aqueous glycerol solution were used as liquid phases. The critical static contact angles of advancement θ A cr and retraction θ R cr at which the contact line motion starts were determined and compared with the values obtained by extrapolation of the velocity dependences of the dynamic advancing, θ a, and receding, θ r, contact angles to V=0. The dependences of the dynamic advancing and receding contact angles on the contact line velocity were studied in the range 0–3 · 10 −3 cm s −1. With the more viscous liquid, θ a and θ r depended on V up to 1 · 10 −3 cm s −1. These parts of the data are well represented by the Blake and Haynes molecular description of the wetting kinetics with equal molecular—kinetic parameters for advancement and retraction. At higher velocities plateaux in both θ a( V) and θ r( V) kinetic relationships have been observed. The receding dynamic angles of the plateaux differ substantially from the critical static receding angles in both systems studied. In this region the dynamic contact angle hysteresis, although being velocity independent, differs from the static contact angle hysteresis. The value of the dynamic hysteresis tends to the static one at velocities lower than 1 · 10 −3 cm s −1, but the two quantities coincide only at V = 0. From the equality of the molecular kinetic parameters at advancement and retraction, it follows that one can determine the Young angle θ 0 from a simultaneous measurement of the advancing and receding dynamic contact angles at sufficiently low velocities.