EXPERIMENTAL-STUDY OF WETTING HYSTERESIS ON SURFACES WITH CONTROLLED GEOMETRICAL AND/OR CHEMICAL DEFECTS

The maximum wetting hysteresis is measured using the tensiometric 'immersion-emersion' method and a new optical technique: the 'sandwich drop' method. Experiments are performed on model-systems: liquid Sn at 1173 K, either on stepped SiO2 surfaces or on two-phased Si-SiO2 surfaces, the geometric shape of the defects (of micrometric size) of which are strictly controlled. The three main results are: (1) the hysteresis exists only if the length of the defect overcomes the amplitude of the thermal wandering of the triple line; (2) the wetting hysteresis depends on the linear density of the defects as the chemical pinning of the triple line occurs in only one (advancing or receding) spreading direction; (3) for a same shape and distribution of the defects, the width of the hysteresis changes when the chemistry of the substrate and the defect are exchanged.