Interactions in fluids

Interactions in fluids are forces and exchanges among fluid particles or between a fluid and its surroundings. They shape motion, pressure, viscosity, and other bulk properties.

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Intermolecular forces: Attractive or repulsive forces between molecules, arising from their charges and electronic structures. They govern cohesion, surface tension, and many fluid properties.
Fluid: A substance that continuously deforms under shear stress and can flow, including liquids and gases. Interactions in fluids occur within this broader class of deformable matter.
Navier–Stokes equations: Partial differential equations expressing conservation of momentum in fluid motion. Their stress terms encode how pressure and viscosity shape flow.
Ideal gas: A gas model whose particles have negligible volume and no intermolecular forces except during elastic collisions. It provides a limiting case where intermolecular forces are deliberately omitted.
Viscosity: A fluid's resistance to deformation or flow under applied stress. Momentum transfer between neighboring fluid layers produces viscous resistance.
Continuum mechanics: The framework that models matter as continuous fields of properties such as density, velocity, and stress. It translates microscopic interactions into macroscopic fluid behavior.
Reynolds number: A dimensionless ratio of inertial to viscous effects in a flow. It predicts when inertial motion overcomes viscous smoothing.
Real gas: A gas whose behavior departs from the ideal-gas model because particles have finite size and interact. Its deviations reveal the effects of attractions and molecular size.
Pressure: Force exerted per unit area, including the isotropic normal stress in a fluid at rest. Molecular collisions transmit momentum in all directions, producing fluid pressure.
Density: Mass per unit volume of a material. It sets how much mass participates in fluid momentum and force exchange.