Viscosity is the measurement of a fluid’s internal resistance to flow. Centipoise (cps) and Poise are the common units of measurement, but viscosity can also be expressed as follows:
|MPa s (Millipascal Second)||(= 1 centipoise)|
|Pa s (Pascal Second)||(= 1 poise)|
|Centistoke||(centistoke x density = centipoise)|
Thixotropic. The tendency for the viscosity of a liquid to decrease when subjected to shear. Mayonnaise is a good example of a thixotropic fluid. At rest, it will not pour, but will flow readily when submitted to shear.
Thixotropic Index (aka TI). The ratio of two viscometer readings. The higher the difference in the two readings, the more thixotropic the material is, and easier to move. Typically, one may be taken at 10 rpm, the second at 1rpm. Example:
Brookfield Model RVT, Spindle #5, @77º F.
|1 RPM 25||X||4000||=||100,000 CPS|
|10 RPM 62.5||X||400||=||25,000 CPS|
Thixotropic Index = 100,000 / 25,000 = 4.0
Newtonian Fluid. Newtonian fluids exhibit neither thixotropy or dilatancy and are referred to as true liquids. Their viscosity remains the same whether at rest or in motion. Typical examples are water and oils.
Dilatant Fluid. The opposite of thixotropic, namely, the tendency of a fluid to increase in viscosity when subjected to shear, the resistance to flow increasing with the amount of force applied. These types of fluids are not generally well suited to use with meter/mix systems.
Paste. This is a term loosely applied to a wide range of materials and can include those that will self-level as well as those which do not. If the specification for the material includes a thixotropic index, one can tell how easily or how hard it will be to move. Low TI materials generally require the use of pressure supplied by ram pumps to move them into a metering system.
Rheology. Loosely defined as “flow properties”.
Approximate viscosities of common liquid and semi-solid materials at 70º F:
Material Viscosity in Centipoise
|Corn oil (Mazola)||72|
|Methyl ethyl ketone||.4|
|SAE 140 (transmission grade)||2,200|
|Peanut butter (Skippy)||250,000*|
|Salad dressing (Miracle Whip)||250,000*|
|Syrup (Karo brand-dark)||3,200|
|Syrup (Karo brand-light)||2,500|
|Syrup (Log Cabin)||144|
|Vaseline petroleum jelly||64,000|
* The viscosity given is indicative of the viscosity RANGE of the material in a static state. The material is extremely thixotropic and the viscosity will decrease rapidly when the material is moved, agitated or worked in any manner