In physics, a fluid is a substance that continually deforms (flows and cannot resist deformation) under applied shear stress or external force. Fluids include gases, liquids, and plasma.
All fluids are compressible (that is, their density increases under increasing pressure) to some extent, but liquids are much less compressible than gases and are generally considered incompressible. Even gases may be treated as incompressible provided the airflow speeds involved are not high. For subsonic airflow over an airplane below about 150 m/s (492 ft./s or about 336 mph), air may be treated as incompressible, i.e., the density remains the same throughout the flow. At higher speeds, the effects of compressibility must be taken into account.
Superfluid (state of matter)
Superfluidity is a special quantum state of matter in which a substance (called superfluid) flows with zero viscosity (without loss of kinetic energy), by the absence of entropy and by having infinite thermal conductivity. The superfluids, if placed in a closed path, can flow infinitely without friction.
Superfluids have many unusual properties. They behave like typical components of solutions, with all the properties associated with normal fluid and superfluid components. Therefore it is impossible to set a temperature gradient in a superfluid, as it is impossible to set a potential difference in a superconductor.
Superfluidity was discovered by Pëtr Leonidovič Kapica, John F. Allen, and Don Misener in 1937. The study of superfluids is called quantum hydrodynamics.
An important application of superfluids is in dilution coolers. In the field of chemistry, superfluid helium-4 has been successfully used in spectroscopy techniques as a quantum solvent. Called Superfluid Helium Droplet Spectroscopy (SHeDS), it is of enormous interest in the study of gas molecules; since a single molecule solvated in a superfluid medium benefit from freedom of rotation: in this way, the molecule behaves as it would in the gaseous phase.