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Viscosity: how fluids resist flow and why it matters

Viscosity quantifies a fluid's resistance to flow. This article explains types, units, measurement methods, temperature effects, common examples (water, oils, lava), and key distinctions.

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Overview

Viscosity is a measurable physical property of fluids that describes their internal resistance to motion or resistance to deformation when subjected to shear or stress. Informally, viscosity distinguishes "thin" liquids that run easily from "thick" ones that flow slowly. The everyday notion of how a liquid moves, its flow, is determined in large part by viscosity.

Common examples make the idea clear: water is low-viscosity and pours readily, while syrup and tar are high-viscosity and move sluggishly. A simple demonstration is timing how fast a substance runs down an inclined plane: low-viscosity fluids reach the bottom quickly and high-viscosity fluids take much longer.

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Viscosities
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Types and units

Scientists distinguish two linked measures. Dynamic viscosity characterizes the internal friction between layers of fluid sliding past each other and is commonly expressed in units related to pascal seconds. Kinematic viscosity divides dynamic viscosity by the fluid density and is therefore expressed in units such as square meters per second (often described in other unit names in practical use). The two measures are related but answer slightly different questions: dynamic viscosity measures resistance to shear, kinematic viscosity measures resistance to flow under gravity.

How viscosity is measured

  • Capillary viscometers, where fluid is timed flowing through a narrow tube;
  • Rotational viscometers and rheometers, which measure torque on a rotating probe immersed in the fluid;
  • Falling-sphere methods, observing how fast a small sphere sinks through the fluid; and
  • Other specialized techniques for complex or non-homogeneous materials.

Causes, temperature dependence and behavior

Viscosity arises from molecular interactions and the ease with which molecules slide past one another. In liquids, stronger intermolecular forces generally raise viscosity; in gases, viscosity behaves differently because momentum transfer between molecules dominates. Temperature strongly affects viscosity: most liquids become less viscous when heated, while gases typically become more viscous as temperature rises. Some materials do not follow a single simple law: a Newtonian fluid keeps a constant viscosity at fixed temperature and pressure, whereas non-Newtonian fluids change apparent viscosity with applied stress or time (examples include ketchup, blood, and many polymers).

Applications, examples and geological importance

Viscosity matters across engineering, biology and Earth science. It determines lubrication performance in engines, affects how paints and inks spread, controls mixing and transport in chemical processes, and is a key parameter in biomedical flows. In geology, the viscosity of molten rock influences eruptive behavior: more viscous lava resists flow and can trap gases, making it viscous and increasing the potential to erupt violently, while low-viscosity lavas tend to produce gentle, effusive flows.

History, terminology and notable facts

The term viscosity and related words derive from historical linguistic roots; for example, the adjective "viscous" traces back to a Latin root meaning sticky. Over time, precise laboratory definitions and standardized units were developed so engineers and scientists worldwide can quantify and compare flow resistance. Understanding viscosity remains essential wherever fluids are handled, from designing pipelines to predicting volcanic hazards.

For further reading on measurement methods and material-specific behavior, consult technical references or specialized resources linked here: fluid properties, rheology basics, and kinematic vs dynamic.

The blue-green liquid on the left is low viscosity like water, the orange one on the right is high viscosity like honey.
The blue-green liquid on the left is low viscosity like water, the orange one on the right is high viscosity like honey.

Definition

Imagine two plates of area arranged in paralleld at a distance A. Between these plates is a fluid that adheres to both plates. In our imagination, the space containing the fluid is said to be divided into layers. Now, if the upper plate is moving with velocity v , the layer in the immediate vicinity is also moving with velocity v due to the adhesion. As the bottom plate is at rest, its neighboring layer is also at rest. The interior fluid layers slide past each other at different velocities. The velocity increases from the plate at rest to the one in motion.

The uppermost layer adhering to the plate exerts a tangential force on the layer below. The latter consequently moves with the velocity v_{1}.This layer in turn acts on the underlying layer, moving it with velocity v_{2}.

In experiment it can be shown that in the ideal case the force Fnecessary to move the upper plate is proportional to the area Athe velocity difference Δ \Delta vand antiproportional to the distance between the plates Δ : \Delta y

F \sim Aand F \sim \Delta vand F \sim \frac{1}{\Delta y}

This gives the equation

{\displaystyle F=\eta A{\frac {\Delta v}{\Delta y}}}

The proportionality constant η \eta is the dynamic viscosity. The change in velocity perpendicular to the direction of motion, i.e. the velocity gradient

{\displaystyle {\dot {\gamma }}={\frac {\Delta v}{\Delta y}}={\frac {\mathrm {d} v}{\mathrm {d} y}}}

also Gdenoted as D or , is called deformation rate, shear rate or shear rate. With the shear stress

\tau=\frac{F}{A}

the connection is

{\displaystyle \tau =\eta \cdot {\dot {\gamma }}}

Figure 2: Definition of viscosity: The fluid (blue) is sheared between the stationary plate (bottom) and the moving plate (top).
Figure 2: Definition of viscosity: The fluid (blue) is sheared between the stationary plate (bottom) and the moving plate (top).

Units

In the SI system of units, the following applies: A substance located between two plates has the dynamic viscosity 1 Ns/m² if, given a size of the plates of 1 m² and a distance between the plates of 1 m, a force of 1 N is required to move the plates against each other at a speed of 1 m/s. The physical unit of the dynamic viscosity is therefore:

{\displaystyle 1\,{\rm {N}}=[\eta ]\cdot \left({\frac {{\rm {m}}^{2}\,{\rm {m}}}{{\rm {m}}\,{\rm {s}}}}\right)\Rightarrow [\eta ]={\frac {{\rm {N}}\cdot {\rm {s}}}{{\rm {m}}^{2}}}=\mathrm {\frac {kg}{m\cdot s}} ={\rm {1\,Pa\cdot s}}}

For the SI unit of kinematic viscosity holds:

{\displaystyle [\nu ]={\frac {\rm {m^{2}}}{\rm {s}}}}

In practice, in addition to the Pa-s (Pascal second), the thousandth part of the SI unit mPa-s (millipascal second) is also used for the dynamic viscosity for media of low viscosity.

In the CGS system, the dynamic viscosity is measured in Poise (P), where 1 Ns/m² = 1 Pa-s = 10 Poise = 1000 Centipoise = 1000 cP = 1 kg/ms, and the kinematic viscosity in Stokes (St), 1 St = 10-4 m2/s.

The Engler degree is an obsolete unit for viscosity. This unit indicates the viscosity compared to water.

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