![]() ![]() (Values are at 25 ☌ unless otherwise stated) Liquid :Ībove shows Newton’s law of viscosity, however this is merely an approximation which holds true for some materials (Newtonian fluids) but not for others. This viscometer works by drawing the fluid up by suction from the lower bowl (A) through the capillary (C) into reservoir (B), the timing marks are used to measure the time taken to fill the reservoir. The viscometer is usually held in a temperature controlled bath to ensure both the test substance and the viscometer itself do not succumb to any temperature changes which can affect the final test result. Falling-Ball ViscometerĪ capillary viscometer is used to measure the rate at which a substance can pass through a capillary (C). There are several methods used to obtain measurement of viscosity from various substances. One poiseuille is 10 poise or 1000 cP, while 1 cP = 1 mPa It is sometimes referred to as the “poiseuille” (symbol Pl). This is the SI unit of viscosity,Įquivalent to newton-second per square metre (N Then there is the Pascal-second (symbol: Pa The basic unit is the centipoise (cP), the cP is equivalent to 0.01 Poise. There are two commonly used units of measurement for the dynamic viscosity of fluids. Viscosity is represented by the symbol η "eta" and is the ratio of the shearing stress( F/ A) to the velocity gradient (Δ vx/Δ z) in a fluid. The similarity to Newton's second law of motion ( F = ma) should be apparent. Newton's equation, states that the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. ![]() As to what causes this friction nobody can say for sure however the general consensus is a combination of electromagnetic attraction/repulsion between atoms and long chain molecules looping around each other in complex compounds as in polymers. So why are some substances more viscous than others? The flow speed is determined by how much friction exists between the molecules of the liquid as the liquid flows. The viscosity of a substance can be affected by temperature as you are probably aware, for example heated honeys and syrups flow much more easily than those which have been pulled cold straight from the fridge. Stirring your cup of tea is considered easy, this is due to water and milk having low viscosities, while if you were to try and stir a cup of honey you would find this considerably harder as honey is much more viscous. What is viscosity? Put simply viscosity is the measure of how fast liquids flow or the resistance to flow of a fluid and the resistance to the movement of an object through a fluid. ![]()
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