Stand-Alone Exhibits KL26 Slow bubbles


Do the big bubbles or smaller ones rise faster?

Pump bubbles into the tube  and watch them rise.

Larger bubbles rise faster than smaller bubbles.

The bubbles rise because they are lighter than the silicone oil surrounding them. The bubble is lifted by the raising force of buoyancy. The force of buoyancy is equal to the weight of the liquid displaced by the bubble; in other words, the buoyant force depends on the volume of the bubble. In the silicone oil in the tube there are forces that slow the motion of particles in relation to one another. The property that is proportional to the magnitude of the force opposing the motion (friction) is called the liquid’s internal friction. The factor representing the internal friction of a liquid is called viscosity. The viscosity of silicone oil is high, and the force opposing the lift of the bubble depends on the bubble’s surface area. When the bubble’s diameter increases, its volume increases relatively more than its area, so the buoyant force increases faster than the internal friction of the liquid that is opposing the lift. Thus, larger bubbles rise faster.

When you use straws to blow air into two glasses, one containing water and the other syrup, you notice that the bubbles in the water are large and rise quickly. The bubbles created in the syrup rise extremely slowly, their motion is barely visible. It is also more difficult to blow in the syrup. This is due to the fact that the viscosity of the syrup is much higher than that of water.

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