Why does stirring a liquid in a cup create a vortex?

When you disturbance liquid successful a tumbler, you are forcing it to rotation successful circles. For thing to determination successful a circle, it needs an inward pull. In a liquid, this inward propulsion comes from unit differences: the unit is higher adjacent the walls, wherever the liquid moves faster, and little astatine the centre. Gravity inactive pulls the liquid down portion the spinning question pushes it outward. The liquid’s aboveground finds a equilibrium betwixt these forces by forming a curved shape: it rises astatine the edges and dips successful the middle. In fact, the dip is not random but adopts a parabolic shape. The extent astatine the centre deepens successful proportionality to the quadrate of the region from the middle.

Friction besides matters. The liquid adjacent to the tumbler’s sides and bottommost slows down due to the fact that it rubs against the coagulated surface, portion the liquid adjacent the apical moves much freely. This uneven velocity makes the liquid spiral inward on the aboveground and downward astatine the centre. The effect is the funnel-shaped vortex you see. It is the liquid’s mode of balancing rotation, pressure, and gravity each astatine once.