Fluid

A fluid is defined as a substance that continually deforms (flows) under an applied plastic solids.

Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between Silly Putty can be considered either a solid or a fluid, depending on the time period over which it is observed.

Fluids display such properties as:

• not resisting deformation, or resisting it only lightly (viscosity), and
  
• the ability to flow (also described as the ability to take on the shape of the container).
  

These properties are typically a function of their inability to support a shear stress in static equilibrium.

Solids can be subjected to shear stresses, and to normal stresses - both shear stress.

In a solid, shear stress is a function of pressure in characterizing a fluid's state.

Depending on the relationship between strain and its derivatives, fluids can be characterized as:

• Newtonian fluids : where stress is directly proportional to rate of strain, and
• Non-Newtonian fluids : where stress is proportional to rate of strain, its higher powers and derivatives.

The behavior of fluids can be described by the Navier-Stokes equations - a set of partial differential equations which are based on:

• continuity (conservation of mass),
• conservation of linear momentum
• conservation of angular momentum
• conservation of energy.

The study of fluids is fluid statics depending on whether the fluid is in motion.

See also

• Shear strain
• Normal strain
• Entropy
• Rheology
• Thermodynamics
• Superfluid
• Perfect fluid

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