Viscosity And Surface Tension

The two phenomena, viscosity and surface tension, are dependent on molecular interactions.

  • However, viscosity results due to collaboration among molecules of the identical molecules located in the same material (in case of fluids).
  • Whereas surface tension is determined by the difference of interactions between the molecules of the material (fluid) with the molecules of the material in contact.

What is Surface Tension?

Surface tension is the amount of energy required to increase the surface of the liquid by unit area. In other words, it is also the property of the liquid surface that resists force. Intuitively, it keeps a barrier between foreign materials and liquid as well as this is the force that holds the liquid molecules bind together.

Surface tension

A paper clip floating on water due to Surface tension.

For example, if we add soap to the water, its surface tension decreases, thereby the liquid with soap easily mixes with dirt on hand and thus cleans it. It is also the property of the liquid surface that resists force.

Surface Tension & Viscosity

Surface Tension & Viscosity

Intuitively, it keeps a barrier between foreign materials and the liquid. This is the force that holds the liquid molecules bonded together.

If we add soap to the water, its surface tension decreases, thereby the liquid with soap easily mixes with dirt on hand and thus cleans it.

Surface tension is sometimes seen in terms of energy. That is more is the surface tension, more is the energy and thus to minimize energy the fluids assume the shape with the smallest surface area. This is the reason behind the water droplets being spherical in shape. A sphere has the minimal surface area, for a given volume.

What is Viscosity?

Viscosity is the resistance of a fluid to flow.

Viscosity

The viscosity of Honey is around 10,000 cP at 21.1°C.

For example, oil has a high viscosity, but when we put it in the car and heat it up, it becomes less viscous and thus flows smoothly through an engine and other parts of the car. Higher the viscosity, slow will be the liquid flow. Viscosity decreases with increase in temperature, with some exceptions.

Viscosity is sometimes also understood as frictional forces that act in between fluid and the surface of contact. The surface can either be a solid surface like a pipe and water can be the fluid. Now the resistance provided by the pipe to the water flow is viscosity. However, viscosity can also arise in between two fluids flowing at different velocities.

The above phenomenon of viscosity is very much particular to liquid, to be precise Newtonian fluids. However, in a case of non-Newtonian fluids, the interactions are quite bizarre and are always hard to model and understand.

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Practise This Question

On heating water, bubbles being formed at the bottom of the vessel detach and rise. Take the bubbles to be spheres of radius R and making a circular contact of radius with the bottom of the vessel. If r << R and the surface tension of water is T, the value of r just before bubbles detach is (density of water is ρw).