Why a heavy liquid does not stay on top
Put a denser liquid above a lighter one, and the arrangement is gravitationally unstable: even a small downward nudge lets the heavy fluid move lower while the light fluid moves higher. The boundary between them may begin smooth, but it cannot remain a neat dividing line. A dense patch pushes downward into the lighter liquid, and the displaced light fluid must flow around it and upward.
How a smooth boundary becomes a tangle
Those first motions amplify one another. A slight bend in the boundary creates a sinking finger of dense liquid and a rising finger of light liquid. As the fingers lengthen, they drag neighboring fluid along, curl, and split into new folds. This growing pattern is called a Rayleigh–Taylor instability—a name for the overturning that occurs when a heavier fluid is supported by a lighter one under gravity.
The motion is not simply two blocks sliding past each other. Each plume stretches and wrinkles the interface, bringing the liquids into increasingly intricate contact. Viscosity—the internal friction that resists flow—and mixing can soften the motion, but they do not erase the basic drive: dense material tends to descend, and light material tends to rise.
Why the layers trade places unevenly
The result depends on the fluids and the scale of the disturbance. A viscous liquid may form broad, slow fingers; a less resistant one can develop finer curls and more complicated motion. Either way, the original boundary is replaced by interleaving plumes. The visible chaos is gravity turning a tiny imbalance into a growing pattern of rising and sinking streams.
