Friction vs Viscosity
Friction and viscosity are two properties of matter, which are vital in understanding the behavior of matter. It is necessary to have a good understanding of viscosity and density in order to describe most of the events occurring in fluid dynamics, fluid statics, solid statics, solid dynamics, and almost every engineering application. These phenomena are seen in day to day lives, and are really easy to understand, given that, the right approach is taken. In this article, we are going to discuss what friction and viscosity are, their definitions, similarities, what causes friction and viscosity, and finally their differences.
Viscosity is defined as a measure of the resistance of a fluid, which is being deformed by either shear stress or tensile stress. In more common 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 the two layers move relative to each other. Sir Isaac Newton was a pioneer in fluid mechanics. He postulated that, for a Newtonian fluid, the shear stress between layers is proportional to the velocity gradient in the direction perpendicular to the layers. The proportional constant (proportionality factor) used here is the viscosity of the fluid. The viscosity is usually denoted by the Greek letter “µ”. Viscosity of a fluid 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.
τ = μ (∂u / ∂y)
Viscosity equations and models are very complex for non-Newtonian fluids. It can be clearly seen that viscosity always acts in a direction, to oppose the flow of the liquid. Viscous forces are distributed throughout the volume of the liquid in a given dynamic condition.
Friction is probably the most common resistive force we experience every day. Friction is caused by the contact of two rough surfaces. Friction has five modes; dry friction that occurs between two solid bodies, fluid friction, which is also known as viscosity, lubricated friction, where two solids are separated by a liquid layer, skin friction, which opposes a moving solid in a liquid, and internal friction that causes the internal components of a solid to make friction. However, the term “friction” is most commonly used in place of dry friction. This is caused by the rough microscopic cavities on each of the surfaces fitting each other and refusing to move. The dry friction between two surfaces depends on the friction coefficient and the reactive force normal to the plane acting on the object. The maximum static friction between two surfaces is just a bit higher than the dynamic friction.
What is the difference between Friction and Viscosity?
• Viscosity is, in fact, a sub category of friction, however, dry friction only occurs between two solid surfaces, while viscosity occurs in fluids between two layers of liquid.
• Dynamic and static conditions are defined separately for dry friction. For viscosity, there is no static condition because liquid molecules are always mobile.