SURFACE TENSION

SURFACE TENSION

What’s going on at the surface of a liquid?

What’s going on at the surface of a liquid? Let’s take a look!

Particles that make up a liquid are in constant random motion; they are randomly arranged.

You might expect the particles at the surface, at the micro level, to form a random surface, as shown below.

You might expect the particles at the surface, at the micro level, to form a random surface, as shown below.

= intermolecular attractions But how do intermolecular forces influence the surface?

Under the surface, intermolecular attractions pull on individual molecules in all directions = intermolecular attractions

= intermolecular attractions

= intermolecular attractions

At the surface, pull on the molecules is laterally and downward; there is negligible intermolecular attractions above the molecules (from the medium above, such as air). SO, the net force on surface molecules is downward.

The result of this downward force is that surface particles are pulled down until counter-balanced by the compression resistance of the liquid:

Surface molecules are compressed more tightly together, forming a sort of skin on the surface, with less distance between them compared to the molecules below.

Surface molecules also form a much smoother surface than one would expect from randomly moving molecules.

This explains the characteristic rounded shape that liquids form when dropping through the air: The molecules are all being pulled toward the center.

This explains the characteristic rounded shape that liquids form when dropping through the air: The molecules are all being pulled toward the center.

Water in particular has a very high surface tension. What property does water have that would give it such a strong surface tension?

This is the water strider insect. Have you seen one?

How is this possible?