A liquid is matter in the liquid aggregate state . According to a macroscopic definition, it is a substance that offers almost no resistance to a change in shape , but quite a great deal of resistance to a change in volume (the substance is almost incompressible ). According to a microscopic definition, a liquid is a substance whose particles move continuously, non-periodically, are not subject to a long-range order , but are subject to a short-range order and whose mean free path is in the order of magnitude of the particle diameter.
Liquids are therefore volume-stable, dimensionally unstable and are subject to a constant Brownian movement . The liquid state is not only substance-specific, but also depends on external factors such as temperature and pressure . If such a liquid changes its physical state, one speaks of a phase transition , the term phase itself being an umbrella term for the physical state.
Macroscopic description and properties
The temperature-dependent volume expansion of a liquid is quantified by its volume expansion coefficient. The compression modulus is a measure of the adiabatic volume elasticity, that is to say for the “compressibility” of a liquid. In weightlessness or in the absence of external forces , liquids take on a spherical shape due to their surface tension , as this shape minimizes the surface. Liquids exert hydrostatic pressure , for example water pressure , on the wall of the vessel in which they are located . In physical terms, liquids at rest are mainly characterized by this pressure. If you exert pressure on liquids from the outside, the pressure is evenly distributed throughout the liquid. The deeper you immerse a body in a liquid, the greater the hydrostatic pressure on the body. However, this depends not only on the depth, but also on the density of the liquid. In flowing liquids there are additional quantities that are described by fluid dynamics , a sub-area of continuum mechanics . The resistance to change in shape, more precisely the viscosity , can, however, be of any size. In this respect, there is no clear boundary between liquid and solid.
Microscopic description and properties
Due to the lack of translational periodicity in comparison to the solid and the constant movement of particles, liquids must be described using statistical mechanics (e.g. classical density functional theory ). The atomic distribution functions are important here . Many properties of the volume phase of liquids can be calculated using molecular dynamics or Monte Carlo simulation .
- JP Hansen, IR Mcdonald: Theory of simple liquids . Elsevier Academic Press, 2006, ISBN 978-0-12-370535-8
- MP Allen, DJ Tildesly: Computer Simulation of Liquids . Oxford University Press, 1989, ISBN 0-19-855645-4