Viscosity vs. Density

Difference Between Viscosity and Density

Viscosity and density are two properties of liquids and gases (or referred as fluids). They are physical quantities when it comes to describing statics and dynamics of these substances. Viscosity and density alone can describe more than half the properties of a fluid.


Viscosity is defined as a measure of fluidresistance while it is deformed by shear or tensile stress. In more ordinary words, viscosity is the “internal friction” of a fluid. It’s also referred as the thickness of a fluid. Viscosity is simply the friction between two layers of a fluid when they move relative to each other. Sir Isaac Newton was a pioneer in fluid mechanics. He proposed that, for a Newtonian fluid, the shear stress among layers is proportional to the velocity gradient in the direction perpendicular to the layers. The proportional constant or the proportionality factor is the viscosity of the fluid. The viscosity is typically denoted by the Greek letter “µ”. Fluid viscosity can be measured using Viscometers and Rheometers. The units of viscosity are Pascal-seconds (or Nm-2s). The cgs system uses the unit “poise”, named after Jean Louis Marie Poiseuille, to measure viscosity. Viscosity of a fluid can also be measured by several experiments. The viscosity of a fluid depends on the temperature. The viscosity decreases as the temperature is increased.

Viscosity equations and models are very complex for non-Newtonian fluids.


Density is defined as mass per unit volume. Density plays an elementary role in fluid mechanics. Events such as up thrust are reliant on the density. Density is what we usually refer to as “weight” of a fluid. Density is an idea with which we are really recognizable. It is calculated using the simple equation density=mass/volume. Its units are Kgm-3.

What is the difference between Viscosity and Density?

Whilst most of the people think that viscosity and density is both the same thing articulated in different forms, they are two truly dissimilar concepts. The density is a dimension of the molecular weight of the composition. In simpler words, density = number of molecules x molecular weight/volume occupied, while the viscosity is a measurement of the inter-molecular forces and molecule shapes. Viscosity tells you the “friction” amid two layers of the given fluid, while density varies slightly with temperature, viscosity changes rapidly. Both density and viscosity decreases with temperature, but viscosity mostly has an exponential relationship with temperature. Density holds a linear relationship. This temperature viscosity relationship is the base of the auto lubricant technology.

Viscosity and density are two different physical phenomena depending on entirely dissimilar features. The ordinary misconception of “heavier fluids are more viscos” is to be absent.


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