Fluid Defined in Physics, Biology, and Engineering

In physics, a fluid is defined as a liquid, gas, or other material that continuously moves and deforms under an applied shear stress or external force [1]. This fundamental property means fluids have zero shear modulus, indicating they cannot resist any applied shear force [1].

Fluids are characterized by their inability to support shear stress in static equilibrium and their capacity to flow, taking on the shape of their container [1]. Unlike solids, which respond to shear with spring-like restoring forces, ideal fluids only respond with restoring forces to normal stresses, known as pressure [1]. Both solids and liquids possess tensile strengths, with liquids experiencing cavitation when this strength is exceeded [1].

Classification and Properties

The definition of a fluid can vary across scientific disciplines. While it generally includes both liquid and gas phases, some substances exhibit both fluid and solid properties depending on the context, such as non-Newtonian fluids like Silly Putty, which can behave like a solid under sudden force [1]. In particle physics, the concept extends beyond traditional liquids or gases to include other fluidic matters [1].

Fluids are modeled using the Navier–Stokes equations, which are based on the conservation of mass, linear momentum, angular momentum, and energy [1]. The study of fluids, known as fluid mechanics, is subdivided into fluid dynamics (for fluids in motion) and fluid statics (for fluids at rest) [1].

Fluids can be classified based on their relationship between shear stress and strain rate:

• Newtonian fluids: Stress is directly proportional to the rate of strain [1].
• Non-Newtonian fluids: Stress is not proportional to the rate of strain or its derivatives [1].

They can also be categorized by compressibility:

• Compressible fluid: Volume or density changes with applied pressure or supersonic flow [1].
• Incompressible fluid: Volume does not vary with changes in pressure or flow velocity, such as water or oil [1].

It is important to note that perfectly Newtonian and incompressible fluids are theoretical constructs, while virtual fluids that ignore viscosity and compressibility are termed perfect fluids [1].

Biological and Engineering Contexts

In medicine and biology, "fluid" refers to any liquid constituent of the body, often called body fluid [1]. These include liquids within an organism, such as intracellular fluid (inside cells) and extracellular fluid (outside cells) [2]. For instance, total body water in lean adult men is approximately 60% of total body weight, with about 28 liters inside cells and 14 liters outside [2]. The extracellular fluid further divides into interstitial fluid (about 12 liters) and intravascular volume (about 4 liters) [2].

In hydraulics, "fluid" refers to liquids with specific properties, often broader than just hydraulic oils [1]. This application highlights the practical engineering relevance of fluid mechanics, which extends to fields like aerospace, civil, chemical, and biomedical engineering, as well as geophysics and oceanography [3].

What to watch

• Interdisciplinary definitions: Monitor how the term "fluid" is applied and defined in emerging scientific and engineering contexts, particularly in fields like particle physics or novel material science [1].
• Non-Newtonian applications: Observe developments in the practical application and modeling of non-Newtonian fluids, given their complex behavior under varying stress conditions [1].
• Biological fluid research: Track advancements in the understanding and manipulation of body fluids, which are critical for medical diagnostics and treatments [2].

The diverse definitions and applications of "fluid" across physics, biology, and engineering underscore its fundamental importance as a concept for understanding material behavior and biological systems.