Free surface

In <a href="/facts/Physics/a7aH2y08">physics</a>, a free surface is the surface of a fluid that is subject to zero parallel <a href="/facts/Shear_stress/aWWnzIhE">shear stress</a>,
such as the <a href="/facts/Interface_(matter)/HnIAndVA">interface</a> between two homogeneous <a href="/facts/Fluid/tMQ46Tmz">fluids</a>.
An example of two such homogeneous fluids would be a <a href="/facts/Body_of_water/MoDAUuW4">body of water</a> (liquid) and the air in the <a href="/facts/Earth%2527s_atmosphere/FV97Am25">Earth's atmosphere</a> (gas mixture). Unlike <a href="/facts/Liquid/t9GkLWzt">liquids</a>, <a href="/facts/Gas/hVVAoodV">gases</a> cannot form a free surface on their own.
<a href="/facts/Fluidization/jmyvY23g">Fluidized</a>/<a href="/facts/Liquefaction/vFtARMCN">liquified</a> solids, including <a href="/facts/Slurry/erSTjthc">slurries</a>, <a href="/facts/Granular/kBqMRzaj">granular</a> materials, and <a href="/facts/Powder/PGd3HwHE">powders</a> may form a free surface.
A liquid in a <a href="/facts/Gravitational_field/K1uTmJFy">gravitational field</a> will form a free surface if unconfined from above.
Under <a href="/facts/Mechanical_equilibrium/N2XKaK6s">mechanical equilibrium</a> this free surface must be perpendicular to the forces acting on the liquid; if not there would be a force along the surface, and the liquid would flow in that direction. Thus, on the surface of the Earth, all free surfaces of liquids are <a href="/facts/Horizontal_plane/Hhsu9C8x">horizontal</a> unless disturbed (except near solids dipping into them, where <a href="/facts/Surface_tension/EuluYmSM">surface tension</a> distorts the surface in a region called the <a href="/facts/Meniscus_(liquid)/meQwdV3B">meniscus</a>).
In a free liquid that is not affected by outside forces such as a gravitational field, internal attractive forces only play a role (e.g. <a href="/facts/Van_der_Waals_forces/jyGU8Vh9">Van der Waals forces</a>, <a href="/facts/Hydrogen_bonds/85V86IIg">hydrogen bonds</a>). Its free surface will assume the shape with the least surface area for its volume: a perfect <a href="/facts/Sphere/jywYLNM2">sphere</a>. Such behaviour can be expressed in terms of <a href="/facts/Surface_tension/EuluYmSM">surface tension</a>. It can be demonstrated experimentally by observing a large globule of oil placed below the surface of a mixture of water and alcohol having the same <a href="/facts/Density/92p6hh9I">density</a> so the oil has neutral <a href="/facts/Buoyancy/9dIQwlrh">buoyancy</a>.

Free surface open-in-new

Free surface