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, $S_{l a}=$ surface force of liquid and air,
$S_{s a}=$ surface force of solid and air and
$S_{l s}=$ surface force of liquid and solid.

• A. If $S_{l s} >S_{l a}$, then angle of contact is less than $90^{\circ}$.
• B. If $S_{l s} >S_{l a}$, then angle of contact is equal to $90^{\circ}$.
• C. If $S_{l s} >S_{l a}$, then liquid spreads over solid surface.
• D. If $S_{l s} >S_{l a}$, 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,
$\cos \theta+S_{l s}=S_{s a}$
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 $S_{l s}>S_{l a}$. Hence, liquid does not spread on solid surface.
Thus, the statement given in option is correct, rest are incorrect.