The effects of viscosity are not only limited to liquids only but apply to gases also. Just like in the case of friction between moving solids, viscosity determines the energy required to make a fluid flow. At a molecular level, viscosity is a result of the interactions between the different molecules in a fluid. All real fluids (except superfluids) have some resistance to stress, but a fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. However, since the viscosity of most fluids is below 1 Pa-s, the millipascal-second (mPa-s) is often used instead. The SI unit for viscosity η is the Pascal-second (Pa-s), which corresponds to the force (N) per unit area ( ). Therefore, more viscous liquids have a lower rate of flow. The negative sign signifies that the viscous force is directed against the velocity gradient. Here, F denotes the viscous force acting on area A and du/dy is the velocity gradient along the positive y-direction. Mathematically, for a small velocity gradient, we can write: It is quantitatively expressed in terms of the coefficient of viscosity, η, which is defined as the tangential for a unit velocity gradient (the difference in velocity between adjacent layers of the fluid) that exists in the direction perpendicular to the direction of the motion. In technical terms, viscosity is a measure of the resistance to the flow that a liquid offers when it is subjected to shear stress. For instance, honey is thicker than water because it is more viscous. In other words, viscosity corresponds to the “thickness” of a liquid.
It is defined as the property of a liquid by virtue of which an opposite force (internal friction) comes into play between different layers whenever there is a relative motion between these layers of the liquid. Such behavior of a liquid in the flow is described by an intrinsic property called viscosity. Do you remember the first time you used a honey bottle, perhaps for green tea, or something else? You must have noticed when the honey bottle is nearly empty, and it takes very long for honey to reach the mouth of the bottle.
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