Viscosity is a term used in rheology. It can be defined as resistance to flow. When a shearing stress (SS) is applied on a given fluid then it flows with some rate of shear (RS) . There is a direct relationship between shearing stress and rate of shear (velocity gradient). Therefore, applying higher shearing stress leads to higher shearing rate. The following equations explain this relationship (Newton's law of flow):
Where η is viscosity (absolute viscosity), SF is shearing force, A is affected area, v is fluid velocity, and r is distance between fluid layers. The first equation can be rewritten to express viscosity as the following:
Units of kinematic viscosity are units of viscosity divided by units of density. The units can be expressed as stoke ( poise/ (g/ cm3) ) or centistoke ( 0.01 × stoke).
Related post:
Effect of temperature on viscosity of liquids.
Where η is viscosity (absolute viscosity), SF is shearing force, A is affected area, v is fluid velocity, and r is distance between fluid layers. The first equation can be rewritten to express viscosity as the following:
η = SS/ RS
Units of viscosity:
According to the above equations, viscosity is expressed in units of stress divided by units of rate of shear. Therefore, it can be expressed as (dyne × second)/ (cm2), g/ (cm × second), poise, or centipoise (cp). In pharmaceutics, the term cp (equal to 0.01 × poise) is commonly used. For example, viscosity of water at 20 °C is 1 cp.
Relative viscosity:
This term is used to compare viscosity of two fluids. For example, olive oil has a viscosity of 100 cp at 20 °C. Therefore, the relative viscosity of olive oil compared to water at 20 °C is 20/ 1. This shows that viscosity of olive oil is 20 times that of water at 20 °C. The equation is described as the following:
Where η1 is the viscosity of the first fluid and η2 is the viscosity of the second fluid. Relative viscosity is unit-less.
Kinematic viscosity:
This term accounts for viscosity of the fluid normalized by its density as shown in the following equation:
Relative viscosity:
This term is used to compare viscosity of two fluids. For example, olive oil has a viscosity of 100 cp at 20 °C. Therefore, the relative viscosity of olive oil compared to water at 20 °C is 20/ 1. This shows that viscosity of olive oil is 20 times that of water at 20 °C. The equation is described as the following:
Relative viscosity = η1 / η2
Where η1 is the viscosity of the first fluid and η2 is the viscosity of the second fluid. Relative viscosity is unit-less.
Kinematic viscosity:
This term accounts for viscosity of the fluid normalized by its density as shown in the following equation:
Kinematic viscosity = η/ density
Units of kinematic viscosity are units of viscosity divided by units of density. The units can be expressed as stoke ( poise/ (g/ cm3) ) or centistoke ( 0.01 × stoke).
Effect of temperature on viscosity of liquids.
No comments:
Post a Comment
Thank you for your comment.