Viscosity – Viscosity is defined as the tangential force per unit area necessary to maintain unit relative velocity between two parallel plates in the liquid unit distance apart. This may be presented algebraically as, , where T is the sheer stress between layers,  is the velocity gradient taken perpendicular to the two layers, and  is the dynamic viscosity. Viscosity can be colloquially thought of as the ‘thickness’ of a liquid or its resistance to deformation by sheer or extensional stress. It can also be thought of as a form of fluid friction.  There are a number of different ‘viscosities’ the two most common being the one defined above called the dynamic viscosity and the other commonly used is the kinematic viscosity obtained by dividing the dynamic viscosity by the fluid density. The dynamic viscosity is usually measured by determining the flow rate of the liquid through a capillary tube of appropriate and known dimensions.  The kinematic viscosity is usually determined by timing the fall of spheres through the liquid over a known distance.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links