Revision: Class 11 >> Mechanical Properties of Fluids: Surface Tension NEET (UG) Mechanical Properties of Fluids: Surface Tension

When a liquid is in contact with a solid, the angle between the tangent drawn to the free surface of the liquid and the surface of solid at the point of contact measured inside the liquid is called the angle of contact.

Surface tension is defined as the force per unit length acting at right angles to an imaginary line drawn on the free surface of the liquid. 

Surface tension is defined as the force acting on a unit length of an imaginary line drawn on the free surface of the liquid, the direction of the force being perpendicular to the line so drawn and acting parallel to the surface.

The angle between tangents drawn at the point of contact to the liquid surface and the solid surface inside the liquid is called the angle of contact for a pair of solid and liquid. It is denoted by θ.

An imaginary sphere drawn round a molecule (taken as centre) with a radius equal to the range of molecular attraction is called the sphere of influence of that molecule.

The work per unit area done by the force that creates a new surface is called surface energy.

OR

The energy required to increase the surface area of a liquid is called surface energy.

The difference of pressure between the two sides of a liquid surface, which arises in equilibrium because the pressure inside a bubble or drop is greater than outside, is called excess pressure.

The potential energy is greater for molecules at the surface film as compared to molecules well inside the liquid. This extra energy of the molecule on the surface layer of a liquid is called the surface energy of the liquid.

A thin film of liquid near its surface having thickness equal to the molecular range of attraction is called surface film.

The property of a liquid due to which its free surface tries to have minimum surface area and behaves as if it were under tension somewhat like a stretched elastic membrane is called surface tension.

OR

The force acting along the surface of a liquid per unit length is called surface tension.

The angle between the surface of the solid and the tangent drawn to the surface of the liquid at the point of contact on the side of liquid is called the angle of contact of that liquid with that solid.

OR

The angle enclosed between the tangents to the liquid surface and the solid surface inside the liquid, both the tangents being drawn at the point of contact of the liquid with the solid, is called the angle of contact.

When adhesive forces are stronger than cohesive forces, the curved liquid surface formed when the liquid is in contact with a solid is called a concave meniscus.

When cohesive forces are stronger than adhesive forces (e.g., mercury in glass), the curved liquid surface formed where cos θ is negative and the liquid level is lower is called a convex meniscus.

The phenomenon where a liquid in a capillary tube either ascends or descends relative to the surrounding liquid when a tube of very narrow bore is dipped in it is called capillarity.

The rise or fall of level of liquid in a capillary tube is called capillary action or capillarity.

A tube with a hole of very small diameter is called a capillary tube or capillary.

The vertical height attained by a liquid in a capillary tube at equilibrium, which is independent of the shape of the capillary provided the radius of meniscus remains the same, is called the capillary rise height (h).

The phenomenon in which a liquid rises in a capillary tube when the angle of contact is acute, or falls when the angle of contact is obtuse, due to the interplay of pressure caused by the liquid column and pressure difference due to surface tension, is called capillary ascent (or descent).

Case 1: θ < 90° (Concave Meniscus)

For two different liquids in the same tube:

\[\frac{h_1}{h_2}=\frac{\rho_2T_1}{\rho_1T_2}\]

For the same liquid in tubes of different radii:

h₁​r₁​ = h₂​r₂​

Principle: Pressure due to liquid column = Pressure difference due to surface tension

h = \[\frac {2T}{Rdg}\] = \[\frac {2T cos θ}{rdg}\]

where r = radius of capillary tube and θ = angle of contact.

T = `F/L`

where F = Force (N), L = Length (m)

= SI unit of T = `N/m`

Surface tension can also be written as

T = `W/A`

where W = Work (J), A = Area (m²)

= SI unit of T = `J/m^2`

We know

1J=1N×1m

So,

`J/m^2 = (N * m)/m^2 = N/m`

Both units are the same

`1N/m equiv 1J/m^2`

• A highly soluble impurity increases surface tension, while a partially soluble impurity (e.g., detergent) decreases it; a waterproofing agent increases it.
• Surface tension decreases with increase in temperature, given by T = T₀(1 − αθ), where T₀​ is surface tension at 0°C and α is the temperature coefficient.
• When a soap bubble is charged (positively or negatively), force acts outward on the surface, increasing its radius — thus electrification always decreases surface tension.

• Surface tension depends only on the nature of liquid and is independent of area of surface or length of line considered.
• Surface tension of a liquid decreases with rise of temperature; it is zero at boiling point and vanishes at critical temperature.
• Due to surface tension, a drop or bubble tends to contract, which increases internal pressure — this difference between inside and outside pressure is called excess pressure.
• For a drop and bubble in liquid: Δp = \[\frac {2T}{R}\]​; for a bubble in air: Δp = \[\frac {4T}{R}\]​(two free surfaces).

• Surface tension causes liquid drops and bubbles to be spherical (minimum surface area for given volume = sphere).
• Air bubble in liquid → 1 liquid-air interface → excess pressure = 2T/r.
• Soap bubble → 2 surfaces → excess pressure = 4T/r.