What is viscosity Significance at a microscopic level

The concept of viscosity was first introduced by Newton in the seventeenth century as the proportionality factor between the velocity gradient dr/dx in the direction perpendicular to the flow direction (shear rate) and the force per unit area F/S required to maintain the flow (shear stress). We now call a fluid that obeys such a linear relation a Newtonian fluid. 

In 1851, Stokes (well known for his pioneering work on luminescence see Chapter 1) showed that the relation linking the force exerted by a fluid on a sphere to the viscosity tj of the medium is F = Gitt/rv, where r is the radius of the sphere and v its constant velocity. In this relation, the quantity 6ra/r appears as a friction coefficient, i.e. the ratio of the viscous force to the velocity. 

Einstein s work on the diffusion of particles (1906) led to the well known Stokes-Einstein relation giving the diffusion coefficient D of a sphere  

Technique Measured fluorescence characteristics Phenomenon Comments 

Molecular rotors fluorescence quantum internal torsional very sensitive to free 

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What is viscosity Significance at a microscopic level 227 Tab. 8.1. Main fluorescence techniques for the determination of fluidity (from Valeur, 1993)... 

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