# Concept of contact angle, cohesive and adhesive forces

I was reading the topic of contact angle from a book and I couldn't exactly understand it. The first photo depicts the general case when a solid surface touches a liquid surface. $Fs$ is the adhesive force and $Fl$ is the cohesive force and $F$ is the resultant of $Fs$ , $Fl$ and $W$ . I have understood this scenario and what all the above terms mean.

Then it shows a case when the solid surface is just dipped in liquid so that it is not projected out and says $Fs$ will not longer be perpendicular to the solid anymore. Can somebody please tell me what this means? Can someone explain the difference in the two diagrams and their respective situations. What does ' If the solid surface is just dipped in liquid so that it is not projected out ' mean and why is $Fs$ not PERPENDICULAR leading the change in contact angle? • A liquid cannot tolerate shear stress. The surface will always be perpendicular to the net force it experiences hence it must change shape – SmarthBansal Mar 29 '18 at 13:39
• I know that. I want to know what is going on in the second figure. Description of the situation is what I want. Difference between the situations is what I want to understand. What does ' If the solid surface is just dipped in liquid so that it is not projected out ' mean?? – Hola Mar 29 '18 at 17:16

The phrasing is not very explicit, but it is referring to the case when the contact line is not with the plane surface of the solid, but with its edge.

See this other situation: (From this paper: https://doi.org/10.1016/j.precisioneng.2014.12.004)

You see that "around the corner" of the solid edge, the angle of free surface with horizontal has changed by $\pi-\alpha$. This change has happened when the contact line was pinned at solid edge: there, all angles with horizontal $\theta_c \leq \theta \leq \pi - \alpha + \theta_c$ are admissible.

(In the example, $\theta$ and $\alpha$ are nearly equal, don't let this confuse you. Redrawing with small $\alpha$ may help you).

• Thanks a lot for answering. So basically, the adhesive force will no longer be perpendicular to the solid if the water layer, it at the tip of the solid surface right? Such that the solid is just immersed so that it does not project out of surface? Also, can you explain again as to why this will change? I didn't completely understand the second part of your answer – Hola Apr 4 '18 at 18:52
• Added a figure and reformulated. – Joce Apr 5 '18 at 21:21