Abstract
The wetted area of a sessile droplet on a practical substrate is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, contact angles of droplets have been studied for more than two hundred years, there are still some unanswered questions. In the last two decades, it was experimentally proven that the advancing and receding contact angles, and the contact angle hysteresis of rough and chemically heterogeneous surfaces, are determined by interactions of the liquid and the solid at the three-phase contact line alone, and the interfacial area within the contact perimeter is irrelevant. However, confusion and misunderstanding still exist in this field regarding the relationship between contact angle and surface roughness and chemical heterogeneity. An extensive review was published on the debate for the dependence of apparent contact angles on drop contact area or the three-phase contact line in 2014. Following this old review, several new articles were published on the same subject. This article presents a review of the novel articles (mostly published after 2014 to present) on the dependency of contact angles on the three-phase contact line, after a short summary is given for this long-lasting debate. Recently, some improvements have been made; for example, a relationship of the apparent contact angle with the properties of the three-phase line was obtained by replacing the solid–vapor interfacial tension term, γSV, with a string tension term containing the edge energy, γSLV, and curvature of the triple contact line, km, terms. In addition, a novel Gibbsian thermodynamics composite system was developed for a liquid drop resting on a heterogeneous multiphase and also on a homogeneous rough solid substrate at equilibrium conditions, and this approach led to the same conclusions given above. Moreover, some publications on the line energy concept along the three-phase contact line, and on the “modified” Cassie equations were also examined in this review.
Highlights
The wettability of a surface can be evaluated by measuring the contact angle (θ) of a liquid droplet on it [1,2,3]
Gao and McCarty published an important paper entitled “How Wenzel and Cassie were wrong” in 2007, where they experimentally proved that contact areas under the drop are unrelated, but only three-phase contact lines are effective on the magnitude of advancing and receding contact angles [34]
These results show that there is no effect of interfacial contact area on contact angles for rough surfaces, and this is a direct experimental proof showing that the Wenzel equation is wrong [34]
Summary
The wettability of a surface can be evaluated by measuring the contact angle (θ) of a liquid droplet on it [1,2,3]. Gao and McCarty published an important paper entitled “How Wenzel and Cassie were wrong” in 2007, where they experimentally proved that contact areas under the drop are unrelated, but only three-phase contact lines are effective on the magnitude of advancing and receding contact angles [34] They fabricated three different hydrophobized surfaces using photolithography, which was applied on silicon wafers—one flat; one rough with a specific pattern design but chemically homogeneous; one flat but chemically heterogeneous in order to study the effect of different topographies and different chemistries on contact angles. When the magnitude of the liquid/solid contact area increased with the increase in the water drop diameter, the value of the water contact angles did not change and were nearly the same These results show that there is no effect of interfacial contact area on contact angles for rough surfaces, and this is a direct experimental proof showing that the Wenzel equation is wrong [34]. Line Energy Concept along the Three-Phase Contact Line and Derivation of Modified
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