Difference Between Surface Tension and Viscosity

Surface Tension vs Viscosity

Viscosity and surface tension are two very important phenomena regarding the mechanics and statics of fluids. Fields such as hydrodynamics, aerodynamics and even aviation are affected by the consequences of these phenomena. It is vital to have a sound knowledge in these phenomena to excel in such fields. This article will compare viscosity and surface tension and present the differences between the two.

What is Surface Tension?

Consider a homogeneous liquid. Every molecule in central parts of the liquid will have exactly same amount of force pulling it to every side. The surrounding molecules are pulling the central molecule uniformly on every direction. Now consider a surface molecule. It has only forces acting upon it toward the liquid. The air – liquid adhesive forces are not even nearly as strong as the liquid – liquid cohesive forces. Therefore, the surface molecules get attracted towards the center of the liquid, creating a packed layer of molecules. This surface layer of molecules acts as a thin film on the liquid. If we take the real life example of the water strider, it uses this thin film to place itself on the surface of the water. It slides on this layer. If it is not for this layer, it would be drown immediately. Surface tension is defined as the force parallel to the surface perpendicular to a unit length line drawn on the surface. The units of surface tension are Nm^-1. Surface tension is also defined as energy per unit area. This also gives surface tension a new unit Jm^-2. Surface tension, which occurs between two immiscible fluids, is known as the interfacial tension.

What is Viscosity?

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 the 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 increases.

τ=μ ∂u/∂y

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