20 Jun

Viscosity

Equipment for homogenization is chosen based on the viscosity of the sample among other considerations.  But what is viscosity?  The IUPAC Gold book defines Apparent Viscosity as “The ratio of stress to rate of strain” and Dynamic Viscosity as ”the ratio of the shear stress to the velocity gradient perpendicular to the plane of shear”.  Kinematic Viscosity is the Dynamic Viscosity divided by the density of the liquid.  Viscosity can also be defined as resistance to flow or the opposite of fluidity.  The SI unit for dynamic viscosity is the Pascal second - Pa·s.  The cgs unit is the poise – P, usually expressed as cP or centipoise, 100cP = 1P.  The unit for Kinematic viscosity is the Stoke - St, or the centi Stoke -cSt, 100cSt = 1St. Water at 20°C is 1.00 cP and 1 cSt.

Newtonian fluids have a constant viscosity no matter what shear or stress is placed on the fluid.  Water is a Newtonian fluid.  A non-Newtonian fluid changes  viscosity depending on the shear forces or stresses placed upon it.  An example of a non-Newtonian fluid is mayonnaise - a hole is created in the mayonnaise when it is stirred as opposed to the fluid following the spoon as when honey is stirred.

When choosing a homogenizer or overhead stirrer and appropriate stirring elements the viscosity and nature of the fluid must be taken into account.

Robin Prymula