What is it that stops liquid flowing out of a straw when then top is covered?
In: Physics
I think the force is suction. The air cannot displace the liquid, so the liquid stays still.
With a bit of air between the liquid and your finger (or whatever is covering the other end) it creates a little vacuum. Theres definitely a limit, but the weight of the liquid falling out is less than how much it would take to change the pressure of the little bubble of air. If your finger isn’t there, there is no vacuum as new air can come in from one end of the straw and replace the air that travels down the straw following the liquid out of the other end.
When you cover 1 end of the straw, atmospheric pressure, or air, can no longer get through the cover-straw interface/gap/crack to push the liquid towards the the other end of the straw. The surface tension and pressure of the liquid inside the half-closed straw also able to overcome gravitation force, so that the liquid stays in the half-closed straw even if you point the open end towards the ground.
On a side note, if you use a needle to introduce air back into the liquid (from the open air) then the air, with lesser density than most liquid, would go to the top of the half-closed straw. Its existence creates pressure inside the straw, and its volume will displace the same volume of the liquid, forcing that volume of liquid out of the half-closed straw.
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