Properties of Fluids

Surface Tension

Objects of density less than that of the water floats, while objects denser than water sinks. But there are some objects which floats on water even if their density is far more than the density of water. This property of a fluid is called as surface tension.

Surface tension is the energy required to increase the surface area of a liquid by a given amount. It occurs due to the attractive forces exerted by liquid molecules to each other. The liquid surface behaves like a membrane under tension. The particles at the surface experiences no attraction above them which will balance the attraction from below. Within the interior of the liquid, there is zero net force but the surface molecule is drawn into the interior, making the liquid tend to minimize its surface area.

The force of attraction between the particles and the surface tension are directly proportional to each other. The greater the attractions between the particles, the greater also the surface tension.

A spider and mosquito can walk at the surface of a pond or water due to surface tension. Thin blades and a piece of paper can also float at the surface of water because of surface tension.

Will stains be removed from clothes with water only? No. Dirt molecules cannot penetrate the surface of the water drops. Another explanation is, the water molecules must be forced to enter through the tiny fibers of the clothes to remove the dirt, which is harder to do due to surface tension. Thus, the surface tension of the water must be decreased and to do this, soaps and detergents are used. These are surfactants or compounds that can decrease the surface tension of water.


Viscosity is the resistance of the fluid to flow. Based on the molecular behavior of a liquid, the particles are closer to each other, thus attractive force is greater, making the movement of the fluid becomes slow.

Different factors may affect the viscosity of different kinds of liquids such as temperature, the type of intermolecular forces involved, and the size shape of the molecule.

The temperature of a fluid varies inversely to the viscosity of the fluid. The higher the temperature, the less viscous a fluid is. The lower the temperature, the more viscous the fluid is. When the temperature is increased, the kinetic energy of the fluid molecules also increases making it easier for them to overcome the intermolecular forces that keep them from flowing. Let us consider a butter as an example. Spreading butter on a cold bread is harder than on a hot one. If the bread is hot, the butter will melt right away making it easier to be spread. In a cold weather, cooking oil hardens due to the decrease in temperature making it harder to pour out.

The intermolecular forces of the fluid molecules also have a direct relationship to the fluid’s viscosity. The stronger the intermolecular forces, the more viscous the fluid is. The weaker the intermolecular forces, the lower the viscosity.

The total kinetic energy of a substance is determined by its mass and velocity. In fluids, the shape and size of its particles also affect its viscosity. Comparing two fluids, one with larger molecules than the other, but of the same attractive force, the one with larger molecules would like to have greater viscosity due to its size. Thus, the fluid with larger molecule will move slower than the smaller molecule. Cooking oils and motor oils are substances made up of molecules with long chain. If molecules are long, there is less distance between atoms, meaning the molecules are more compact, thus the attraction between the molecules tend to be greater, making the flow of these substances slower.