Fluid (substance that flows and adapts shape)

Key physical characteristics

Several macroscopic properties are used to describe fluid behaviour. Density quantifies mass per unit volume; pressure represents force per unit area transmitted within the medium; and viscosity measures resistance to relative motion between adjacent layers. Surface tension and capillarity arise at interfaces between a fluid and another phase. Fluids may be compressible (gases) or nearly incompressible (many liquids), and their flows can be steady or unsteady, laminar or turbulent. The scientific study of these behaviors is generally called fluid mechanics.

Types and examples

Conventional categories include:

• Liquids — nearly incompressible fluids such as water, oil, and mercury that maintain a free surface under gravity.
• Gases — compressible fluids like air and steam that expand to fill their containers.
• Plasmas — ionized gases found in stars and some industrial devices, which conduct electricity and respond to magnetic fields.

Non‑ideal fluids and related materials

Not all flowing materials obey the simplest linear relation between stress and strain rate. Non-Newtonian fluids change viscosity with applied shear: examples include shear-thinning paints and shear-thickening suspensions. Viscoelastic substances combine fluid-like flow with elastic memory; a familiar toy that exhibits such behavior is Silly Putty. Collections of many small solid particles, such as powders, can behave like fluids in some contexts but remain granular materials with distinct contact mechanics.

History, theory and applications

Observations of flowing water and air date back to antiquity, but mathematical descriptions developed progressively through work by Euler, Navier and Stokes and many others. Fluid mechanics underpins meteorology, oceanography, aeronautical and civil engineering, biomechanics, and industrial processes such as hydraulics and chemical mixing. Practical devices—pumps, turbines, pipes and aircraft wings—are designed using fluid principles.

Important distinctions and phenomena

Fluids are notable for buoyancy (Archimedes' principle), boundary-layer effects, and instabilities that produce turbulence. Phase changes such as freezing, melting, condensation and vaporization connect fluids to solid and gaseous states. In engineering and science, distinguishing true continuum fluid behavior from particulate or elastic effects is important for correct modeling and design.

For further reading about specific subclasses and mathematical descriptions, see resources on liquids, gases, plasmas, and fluid mechanics.