Sunday, 30 January 2011
Which bubble will rise the fastest?
Pump bubbles into the tube and watch them rise.
The larger bubbles rise faster than the smaller bubbles.
The pressure in the liquid increases as you go deeper. The difference in the pressures on the upper and lower surface of an object in liquid creates the buoyancy of the object. In this example, the bubble is the object. Buoyancy lifts the bubble to the surface, because air is lighter than liquid.
The liquid resists the movement of the objects within it. This phenomenon is known as fluid resistance. The degree of the fluid resistance depends upon, for example, the speed and cross-sectional area of the object, and the viscosity of the liquid. The viscosity of the silicon oil in the tube is high. Therefore, the bubbles rise slowly. The larger bubbles rise faster than the smaller bubbles, because the buoyancy of the bubble is proportional to the size of the bubble, and the fluid resistance is proportional to its cross-sectional area. When the size of the bubble increases, the volume increases relatively more than the cross-sectional area.
In silicon, the bubbles rapidly reach their terminal velocity. Once this occurs, all bubbles of the same size will rise at an equal rate. If a large bubble catches up to and “swallows” a smaller bubble, the resulting larger bubble will again for a moment accelerate in speed.
The word viscosity derives from “viscum album”, the Latin name for mistletoe. People used to smear tree branches with the thick, gluey resin from mistletoe berries in order to catch birds when they land.