Question

A small drop is formed using a dropper over a clean solid surface, then which of the following statement(s) is/are correct?
Where, $S_{la}=$ surface force of liquid and air,
$S_{sa}=$ surface force of solid and air and
$S_{ls}=$ surface force of liquid and solid.

• A. If $S_{ls}>S_{la}$, then angle of contact is less than $90^{\circ }$.
• B. If $S_{ls}>S_{la}$, then angle of contact is equal to $90^{\circ }$.
• C. If $S_{ls}>S_{la}$, then liquid spreads over solid surface.
• D. If $S_{ls}>S_{la}$, then liquid does not spread on solid surface.

Answer 1

Answer: (D)

According to question, the situation can be depicted as below,

From, this we have,
$\cos \theta +S_{ls}=S_{sa}$
It is given that a small drop is formed, so the angle of contact should be greater than $90^{\circ }$, i.e. $\theta >90^{\circ }$ and $\cos \theta$ is negative.
This implies that $S_{ls}>S_{la}$. Hence, liquid does not spread on solid surface.
Thus, the statement given in option is correct, rest are incorrect.