What is Viscosity?
At its plainest, viscosity is the resistance a fluid shows to flow, arising from the internal friction between its own layers. Technically: it is the molecular friction force that appears as one layer of fluid slides over another.
- High viscosity: Honey, pitch, engine oil (thick, flows slowly).
- Low viscosity: Water, ethyl alcohol, gases (thin, flows quickly).
A core concept of fluid dynamics, viscosity is a critical parameter we encounter at every point, from industrial production through to daily life. Viscosity is the main element that sets how fast a liquid will flow, how much friction a machine will be exposed to, or how a drug will be absorbed in the body. In this article we set out for you, in professional language, everything you need to know about viscosity, from the details through to the calculations and formulas.
Types of viscosity and their mathematical formulas
Viscosity represents the internal resistance of the fluid. It rests on Isaac Newton's model of flow between parallel plates.
1. Dynamic (absolute) viscosity
Expresses the internal resistance a fluid shows to flow.
Formula:
τ = μ · (dv/dy)
τ : Shear stress (N/m²)
μ : Dynamic viscosity (Pa·s)
dv/dy : Velocity gradient (s⁻¹)
Units:
SI: Pa·s
CGS: Poise (P), usually centipoise (cP)
1 cP = 0.001 Pa·s
2. Kinematic viscosity
The ratio of dynamic viscosity to fluid density.
Formula:
ν = μ / ρ
ν : Kinematic viscosity (m²/s)
μ : Dynamic viscosity (Pa·s)
ρ : Density (kg/m³)
Units:
SI: m²/s
CGS: Stokes (St), usually centistokes (cSt)
1 cSt = 10⁻⁶ m²/s
Factors that affect viscosity
The character of a fluid changes with the conditions around it. Understanding these changes is essential to managing industrial processes:
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Temperature: * In liquids: as temperature rises the bonds between molecules weaken and viscosity falls.
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In gases: as temperature rises the rate of molecular collision goes up and viscosity rises.
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Pressure: Although liquids are generally taken to be incompressible, under very high pressure viscosity can rise somewhat because the voids are reduced.
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Molecular structure: Long-chain polymers or complex compounds (crude oil, for example) tend toward higher viscosity.
Industrial comparison table (25 °C)
How is viscosity measured?
The main instruments used to measure viscosity in the laboratory are these:
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Rotational viscometers (Brookfield): Measure the resistance a spindle meets as it turns in the liquid. Ideal for thick and non-Newtonian liquids.
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Capillary viscometers: Measure how long a liquid takes to flow through a narrow tube under gravity. Used for kinematic viscosity.
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Falling sphere viscometer: Based on the speed at which a sphere settles through the liquid.
What viscosity depends on
Viscosity, that is "how much resistance a fluid shows to flow", is easiest to grasp through the familiar differences in consistency we meet every day. At the head of the factors behind it stands temperature: heat honey and it flows like water, but put it in the refrigerator and you cannot turn the spoon in it. The reason is that heated molecules let go of one another and break free. Molecular structure also comes into it: if the molecules are long and complex they tangle together and make flow harder. Seen from industry, and especially with lubricating oils or the heavy fluids in industrial facilities, pressure and the additives used also set this "balance of flow" directly. Viscosity, then, is the outcome of that endless contest a substance runs with temperature and with its own internal structure.
Sector applications
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Automotive: Choosing the right oil viscosity is vital if the engine is not to wear.
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Food: The consistency of sauces, of chocolate, and of dairy products sets how the consumer perceives them.
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Pharmaceuticals: How well syrups can be dosed and how freely injections flow depend on viscosity.
Construction: How well concrete settles into the formwork (workability) is controlled through viscosity.
Viscosity is a mirror that reflects both the physical and the chemical identity of a substance. Whether you are making an engineering calculation or optimizing a production process, settling on the right viscosity value with the temperature and pressure variables in view is what will carry you to success.
Author:
Industrial Design Engineer