Final answer:
There is always excess pressure on the concave side of the meniscus due to surface tension; for a liquid drop, the excess pressure can be described by the Laplace law, ΔP = 2σ/R, where ΔP is the pressure difference, σ is the surface tension, and R is the radius of the drop.
Step-by-step explanation:
The phenomenon of surface tension describes the property of the surface of a liquid that allows it to resist an external force. It is a result of the cohesive forces among liquid molecules. Surface tension is the reason why the meniscus of a liquid in a container is curved, and it leads to excess pressure on the concave side of the meniscus.
To obtain an expression for the excess pressure inside a liquid drop, we can employ the Young-Laplace equation which relates the pressure difference to the surface tension (σ) and the radii of curvature of the surface. For a spherical liquid drop, which has only one radius of curvature (R), the pressure difference (ΔP) inside the drop compared to outside is given by:
ΔP = 2σ/R
This equation is known as the Laplace law. The factor of 2 comes from the fact that a drop has an inner and outer surface, both contributing to the pressure difference. This internal pressure is higher than the ambient atmospheric pressure, a consequence of surface tension working to minimize the surface area of the drop.