Fluid

In physics, a fluid is a material—such as a liquid, gas, or other substance—that can continuously move and change shape (flow) when a shear stress or external force is applied. Fluids have a zero shear modulus, which means they cannot resist any shear force. While the term usually covers both liquids and gases, its exact meaning differs across scientific fields. The definition of a solid also varies, and some materials can display both fluid and solid traits. For example, non-Newtonian fluids like Silly Putty act like a solid under a sudden force, and very viscous substances like pitch behave as a solid (as seen in the pitch drop experiment). In particle physics, the concept extends to fluidic matters beyond liquids or gases. In medicine or biology, a fluid refers to any liquid part of the body (body fluid), whereas "liquid" is not used that way. Sometimes, liquids given for fluid replacement—by drinking or injection—are also called fluids (e.g., "drink plenty of fluids"). In hydraulics, fluid refers to liquids with certain properties and is broader than just hydraulic oils.

Fluids have two key properties: they offer no resistance to permanent deformation, only resisting the rate of deformation in a dissipative, frictional way; and they can flow, meaning they take the shape of their container. These properties come from their inability to support a shear stress when at rest. In contrast, solids respond to shear with a spring-like restoring force (making deformations reversible) or require a certain initial stress before deforming (plasticity). Solids resist both shear and normal stresses (compressive and tensile), while ideal fluids only resist normal stresses, called pressure. Fluids can handle compressive stress (positive pressure) and tensile stress (negative pressure). Both solids and liquids have tensile strengths; exceeding this in solids causes irreversible deformation and fracture, while in liquids it triggers cavitation. Solids and liquids also have free surfaces, which cost free energy to form. For solids, this energy per unit area is surface energy; for liquids, it is surface tension. Because of surface tension, liquids flow differently from solids, though both tend to minimize surface energy at equilibrium: liquids form rounded droplets, while pure solids form crystals. Gases lack free surfaces and diffuse freely.

In solids, shear stress depends on strain; in fluids, it depends on strain rate. This leads to Pascal’s law, which describes pressure’s role in a fluid’s state. Fluid behavior is modeled by the Navier–Stokes equations—a set of partial differential equations based on continuity (mass conservation), conservation of linear momentum, conservation of angular momentum, and conservation of energy. The study of fluids is fluid mechanics, split into fluid dynamics (motion) and fluid statics (rest).

Fluids are classified by the relationship between shear stress and strain rate. Newtonian fluids have stress directly proportional to strain rate; non-Newtonian fluids do not. Newtonian fluids follow Newton’s law of viscosity and are called viscous fluids. Fluids are also classified by compressibility: compressible fluids change volume or density under pressure or at supersonic speeds; incompressible fluids (like water or oil) do not change volume with pressure or flow velocity (density constant). No real fluid is perfectly Newtonian or incompressible—these are theoretical assumptions. Virtual fluids that ignore viscosity and compressibility are called perfect fluids.