Fluid Physics

When water is splashed out of a glass, it can achieve liftoff, just like an airplane.  The splashing is dependent on the amount of water and the speed it has or momentum when it leaves the glass, but since you won’t get a splash in a vacuum, the surrounding atmosphere has a lot to do with it.  There’s some complex science behind a simple water splash, and the origin of what causes water to splash still remains a big mystery.  Scientists think that they’re coming closer to understanding what happens during a splash at the microscopic level.  The flow of fluids is one of the most complex, beautiful, and amazing things in physics.  Slow motion pictures of drops landing on water or of two fluids mixing can be simply gorgeous.  Even more amazing, the basic physics of fluid flow, or fluid dynamics was worked out way back in the 19th century.  Those equations still hold riches that are being uncovered today.  Fluid flow has many aspects which are usually defined with opposite pairs, like steady and unsteady flow, uniform and non-uniform flow, laminar and turbulent flow, compressible or incompressible, viscous or non-viscous, rotational or irrotational and it often lies between the extremes.

Water that is flung out of a glass should take a round shape due the equal amount of atmospheric pressure exerted by the air from all directions on it, but there are other external factors to consider like gravity, velocity, viscosity, temperature which all come into play.  The momentum of the water droplets will compete with their surface tension, and as the momentum tries to force the drop to spread out at a speed governed by the mass of fluid and the speed at which the drops are traveling, the surface tension will try to pull the droplets back together, resisting the spreading motion.  Hence, droplets will rapidly expand to some radius where the forces are in balance.  Surface tension is a kind of elasticity in water, which makes the surface of a liquid act like a flexible membrane and some people refer to this as a skin, because it holds droplets of water together.  Water will always try to form into the smallest shape possible, which is a sphere.  Water molecules inside of the drops will cling to each other because of cohesion, but the outside water molecules that are in contact with the air will create the surface tension that holds them together.  When water is flung into the air from a glass, the pockets of air will have to move out of the way and moving air will speed up any time it encounters an obstacle, so air pressure and viscosity will also affect the shape of a splash.

Written for Sadje at Keep It Alive What Do You See #140, where the Image credit goes to Avis @ Pexels.

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