Precursor film: a key driver to determine wetting behavior in the vicinity of surface heterogeneity

Experimental findings in the recent past have clearly emphasized the role of the three-phase contact line in determining the wetting behavior of liquids. However, at a microscopic level when the precursor film precedes the contact line, the role of thin film in wetting behavior becomes important. The current study presents a series of simple experiments to establish the role of precursor films in controlling the wetting behavior in the context of rising liquids in vertical planes. The experiments have been performed on glass capillaries and cover slips which have been rendered partially hydrophobic by silanisation of some portion. Thus, two regions of significantly different wetting behavior have been created. Experiments have been performed to capture the changes in wetting behavior as manifested by variation in capillary rise height and Wilhelmy force values, where hydrophilic and hydrophobic regions meet each other. The deviations in expected wetting behavior as the liquid approaches the boundary between the two regions from the hydrophilic end have demonstrated the role of precursor films in wetting behavior.