Wetting hysteresis

Wash Load Wet Oil Wetting Film Wetting Hysteresis... 

To sum up, the effects on static contact angles of the departures from ideality of solid surfaces are qualitatively well understood and some of these effects are used in practice to improve or reduce wettability. Moreover, for simple geometries, a semi-quantitative agreement is obtained between experimental results and theoretical predictions. For surfaces with random roughness, predictions of wetting hysteresis present a great difficulty because the relevant size of defects is not yet well-established. 

Wetting hysteresis (or contact angle hysteresis) is defined as the ability of a liquid to form on a solid surface several stable (or metastable) contact... 

N.B. The above descriptions assume thermodynamic equilibrium to be attainable. In practice, it is common to observe the phenomenon of wetting hysteresis. Metastable rather than stable equilibrium is reached, as shown experimentally by a range of contact angles instead of a unique value. The causes are multiple heterogeneity of the solid surface, local adsorption. Roughness of surfaces, and so on. 

C. Priest, R. Sedev, and J. Ralston, A quantitative experimental study of wetting hysteresis on discrete and continuous chemical heterogeneities. Colloid Polym. ScL, 291, 271-277, (2013). 

In order to evaluate the sizes of the defects responsible of the wetting hysteresis, we considered a topographical defect consisting of a bump with lateral size a and a maximum slope Despite the fact that the value of tp is unknown, we estimated that the points reported on Fig. 9.13 correspond to defects sizes ranging from tens of nanometers down to 2 nm, which is compatible with electron microscopy observation of the nanoneedles." We also determined a minimal defect size necessary to obtain hysteresis, by comparing the dissipated energy on the defect to kT. This led to a value comparable with the size of liquid molecules, compatible with previous results, which explains why it is so challenging to obtain hysteresis-free surfaces. 

X. Pepin, D. Rossetti, S. M. Iveson, and S. J. R. Simons, Modeling the evolution and rupture of pendular liquid bridges in the presence of large wetting hysteresis,/ Colloid Interface ScL, 232, 289-297 (2000). 

FIG. 24 Variation of (1) and of the wetting hysteresis H for water (2) vs. input energy W (LDPE surface was washed after treatment). (Adapted from Ref. 150.)... 

Priest C, Albrecht TWJ, Sedev R, Ralston J (2009) Asymmetric wetting hysteresis on hydrophobic microstnictured surfaces. Langmuir 25 5655 5660... 

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