Fluids are everywhere. They are the water we drink, the air we breathe, and the blood that flows through our bodies. In nature, water striders seem to defy physics, but they simply combine surface tension of the water with the hydrophobicity of their legs to walk on water. Fluids have unique properties, including surface tension, viscosity, and capillary action, that engineers have taken advantage of to create practical applications, such as reverse osmosis, inkjet printing, and DNA sequencing.
The bulk properties of fluids are usually explained by the Navier–Stokes equations, which connect various fluid parameters, including density, pressure, and velocity. The equations describe exceptionally well the behavior of fluids at the macroscale—for example, the airflow around an airplane or the water in a kitchen tap. But the equations break down at the molecular scale.
The Navier–Stokes equations assume that the fluid is in the hydrodynamic limit, in which the continuum...
