Viscosity Definitions and Terminology

Viscosity is a measure of the resistance of a liquid to shear deformation, i.e., a measure of the ratio between the applied shearing force and the rate of flow of the liquid. If a tangential force difference, F, is applied to two parallel planes of area. A, which are separated by a distance, d, the viscosity, r, is given by the expression  

The original unit for viscosity was based on the cgs system, where the viscosity is given in dyne s cm . This unit, which is termed a Poise and given the symbol P, is used in virtually all literature prior to 1970 and is still used extensively throughout the glass industry. In SI units, which have replaced cgs units in much of the recent literature, viscosity is given in N s m , or, since a Pascal is a N m, the viscosity is reported in Pa s. Since 1 Pa s = 10 P, the conversion of viscosity data from one unit to the other is very straightforward. The viscosity of water at room temperature is 0.01 P, or 0.001 Pa s. 

Fluidity is the reciprocal of the viscosity. A melt with a large fluidity will flow readily, whereas a melt with a large viscosity has a large resistance to flow. While fluidity is often used in dealing with ordinary liquids, virtually all literature dealing with glass forming melts discusses flow behavior in terms of the viscosity. 

The viscosity of a typical melt under conditions where fining and homogeneity can be obtained in a reasonable time is termed as the melting temperature. Melting usually occurs at a viscosity of 10 Pas 

The softening point is more properly termed the Littleton softening point, after the specific test used to define this reference point. The viscosity of 10 Pa s does not represent the deformation temperature for all objects. This particular reference point is defined in terms of a well-specified test involving a fiber -0.7 mm in diameter, with a length of 24 cm. The softening point is defined as the temperature at which this fiber elongates at a rate of 1 mm min when the top 10 cm of the fiber is heated at a rate of 5 K min. In fact, if the density of the fiber is significantly different from that of a typical soda-lime-silica composition, the viscosity will not be exactly 10 Pa s at this temperature. 

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