The spherical shape of liquid droplets is due to:

Surface tension is the property of a liquid surface that makes it behave like a stretched elastic membrane. It is defined as the force acting per unit length on an imaginary line drawn on the surface of the liquid, perpendicular to the line. Its SI unit is newton per metre (N/m), and its dimensional formula is [M T⁻²].

Surface tension arises because of the cohesive forces between the molecules of a liquid. A molecule inside the liquid is pulled equally in all directions by neighbouring molecules, so the net force on it is zero. But a molecule at the surface has liquid molecules only below and beside it, so it experiences a net inward pull. As a result, the surface tends to contract and has the smallest possible area.

For a given volume, a sphere has the minimum surface area. So, when there are no other strong forces acting, such as in free fall or in space, liquid drops take a spherical shape. This is why raindrops, dew drops and mercury droplets are nearly round. In zero gravity, astronauts see water form floating spheres.

Other effects of surface tension:

• Small insects, such as water striders, can walk on water.
• A greased needle or a razor blade can float on water.
• Hair of a brush cling together when taken out of water.

Soap and detergents reduce the surface tension of water, which helps them spread and wet clothes better and remove dirt. Warm water also has lower surface tension, which is why it cleans better.

Oil spread on the sea surface calms the waves by reducing surface tension.

Surface tension decreases as temperature increases, and becomes zero at the critical temperature.

Related phenomena:

Capillarity: the rise or fall of a liquid in a narrow tube, due to surface tension and adhesion. Water rises in a glass capillary tube, while mercury falls. Capillary action helps water rise in plants and oil rise in a lamp wick.

Viscosity: the internal friction of a fluid, which resists its flow.

Why not the other options?

• ✗Viscosity is the resistance of a fluid to flow.
• ✗Gravity tends to flatten large drops, not make them spherical.
• ✗Capillarity is the rise or fall of liquid in narrow tubes.