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Poise (prounounced poiz or pwaz) is a basic unit for describing the viscosity of a liquid, which is its resistance to sideways forces or stretching forces. It is sometimes measured in a Rheometer by the amount of liquid that passes through an orifice in a certain amount of time, or by the amount that flows through a capillary tube[1], or even by measuring the force a spinning outer cylinder induces on an inner cylinder through a fluid between them. [2]

Shear stress on a fluid (sideways force) can be illustrated if we imagine a fluid flowing in a flat bottomed channel, the fluid that touches the bottom will be at rest while the fluid farther away from the bottom will be moving at the velocity of the liquid.[3] This change of fluid velocity with height is called a gradient, and the resistance to shear where one level of fluid is moving faster than another level is a way to measure the viscosity.

Extension stress on a fluid is illustrated by terrain which forms crevices when it is stretched. This stretching of the crust is the result of extensional stresses. [4]

Jean Louis Marie Poiseuille

Jean Louis Marie Poiseuille [5] was a French scientist who was interested in the flow of blood through small tubes like veins and capillaries. The unit of viscosity was named after him in order to honor his researches.

A Pascal is a unit of pressure in which one newton of force is spread over a square meter of area. One hundred Pascals (one hectopascal) is equal to 1 millibar of pressure, so that one atmosphere of pressure (14.7 pounds per square inch) is 1013 hectopascals or millibars of pressure. So, one Poise is defined as .1 Pascal seconds.[6]

Wikipedia relates viscosity to the force needed to move one plate past another when a fluid is between them, it notes that "The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), which is identical to kg·m−1·s−1. If a fluid with a viscosity of one Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one pascal, it moves a distance equal to the thickness of the layer between the plates in one second."[7] The Infoplease Dictionary has a similar definition when it says a Poise is, "a centimeter-gram-second unit of viscosity, equal to the viscosity of a fluid in which a stress of one dyne per square centimeter is required to maintain a difference of velocity of one centimeter per second between two parallel planes in the fluid that lie in the direction of flow and are separated by a distance of one centimeter. [8]

Water has a viscosity of about 1 centapoise, milk has a viscosity of 3, and blood about 10. Motor oil has a viscosity of between 140 and 900 centapoise, while honey has a viscosity of 10,000 and ketchup a viscosity of 50,000 centapoise.[9]


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