What Are Liquids?

Liquids are one of the four fundamental states of matter, characterized by a definite volume but no fixed shape. Unlike solids, the molecules in a liquid are not bound to fixed positions; unlike gases, they remain in close enough proximity to maintain a cohesive volume under ambient conditions. This intermediate state gives liquids a distinctive combination of properties: they flow, they transmit pressure nearly incompressibly, and they exhibit surface tension and viscosity that depend strongly on temperature and molecular composition.

The scientific and engineering study of liquids spans thermodynamics, physical chemistry, and fluid mechanics. Foundational contributions from figures such as Daniel Bernoulli and Leonhard Euler in the eighteenth century established the equations governing liquid flow, and the Navier-Stokes equations, formulated in the nineteenth century, remain the standard mathematical model for viscous liquid motion across almost all engineering applications.

Fluid Properties and Mechanics

The behavior of a liquid in any engineering context depends on a small set of measurable properties. Density determines buoyancy and hydraulic pressure. Viscosity, the internal resistance to flow, governs energy losses in pipe networks and the lubrication of mechanical components. Surface tension arises from the cohesive forces between molecules at a liquid-gas interface and is responsible for capillary action, droplet formation, and wetting phenomena on solid surfaces. Together these properties determine whether a flow is laminar or turbulent, quantified by the Reynolds number, which is the ratio of inertial to viscous forces.

Engineers apply this understanding across a wide range of problems, from designing pipelines and pumps to modeling blood flow in cardiovascular devices and predicting the behavior of lubricants in rotating machinery.

Electrohydraulics

Electrohydraulics refers to the use of electric fields to control, pump, or otherwise manipulate liquid flows without mechanical moving parts. In electrohydrodynamic (EHD) systems, an applied electric field exerts a body force on a liquid that has free charge carriers, inducing bulk flow. This principle is applied in EHD micropumps used for electronics cooling, in inkjet printing nozzles, and in electrospray ionization systems used for mass spectrometry. A 2014 review in IEEE Transactions on Industry Applications traces the development of EHD from early dielectric liquid experiments in the 1960s through contemporary microfluidic implementations.

A related technique, electroosmosis, drives liquid flow through fine channels by applying a voltage across an electrolyte. This is foundational to lab-on-a-chip devices, where precise, programmable control of nanoliter fluid volumes enables rapid chemical analysis and medical diagnostics without pumps or valves.

Phase Behavior and Thermodynamic Properties

The transition between liquid and other states of matter, freezing at the lower end and boiling at the upper, defines a liquid's practical operating window. For pure substances, these transitions occur at well-defined temperatures and pressures described by phase diagrams. For mixtures, the liquid phase can coexist with vapor across a range of compositions, a behavior central to distillation, refrigeration cycles, and chemical reactor design.

The NIST Chemistry WebBook provides experimentally validated thermodynamic and transport property data for hundreds of pure liquids and many mixtures, and is the standard reference for engineers selecting process fluids or validating computational models.

Applications

Liquids have applications across a wide range of engineering and scientific disciplines, including:

• Hydraulic actuation in aerospace, construction, and manufacturing equipment
• Heat transfer fluids in power generation and electronics thermal management
• Electrolytes in batteries, fuel cells, and electrochemical sensors
• Aerosol generation for drug delivery, combustion, and atmospheric research
• Lubrication of bearings, gears, and turbine components