Wetting on heterogeneous surfaces.

摘要

Dynamic wetting and absorption of water droplets on heterogeneous surfaces, including paper, was studied. The objective was to elucidate the role of surface heterogeneities on wetting and absorption properties of paper. To better understand the phenomena, wetting on glass slides with a controlled level of heterogeneity was investigated. Also, partially hydrophobized glass capillaries were used to simulate capillary penetration into the pores on sized paper.;Dynamic wetting on paper followed a power law model with a lower rate than wetting on a smooth surface. The chemical composition of the paper surface did not affect the wetting dynamics, which was mainly affected by surface roughness in a micron scale. The super-hydrophobic properties of the sized papers were due to air entrapment in the micron-scale roughness on the surface.;Wetting and absorption of water droplets on sized paper occurred in different time scales. A pseudo-equilibrium contact angle was reached at the end of wetting just before absorption of water droplets. Increasing the surface coverage of the hydrophobic domains on paper by sizing increased the pseudo-equilibrium contact angle and delayed absorption into paper. This delay was related to partial dissolution of the surface sizing polymers in the water droplets on the surface.;The equilibrium contact angle of water droplets on partially hydrophobized glass slides was a linear function of a characteristic dimension of the hydrophobic domains and the length of the three phase contact line.;The dynamic rise of water in partially hydrophobized vertical capillaries followed two mechanisms. First, capillary rise was a function of the dynamic contact angle, changing with the velocity of the contact line. Second, local changes of the advancing contact angle due to the heterogeneities on the capillary walls lowered the capillary rise velocity. The stick (pause) and jump of the contact line was another effect of the hydrophobic domains. Capillary rise dynamics was a function of the advancing contact angle of water droplets measured on a flat glass slide with the same coverage of hydrophobic domains.