Ever since I was a child I have noticed a strange phenomenon in hot tub/jacuzzi jets, if you partially cover them, the water will still flow but the bubbles stop. No matter what hot tub I'm at, I can put my hand, or any obstacle for that matter, closer and closer to the jet until about at a centimeter away (perhaps less) from the opening, all of a sudden the bubbles stop flowing although there is still plenty of water flow. I'm sure I'm not the only one whose noticed this. Why do the bubbles stop? Why don't they just flow around the obstacle like the water does?
[Physics] Bubbles in Hot Tub Jets
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The bubbles form as air is entrained in the water during the pouring process. The key factor here is how fast the bubbles collapse. This may seem a funny distinction, but bubbles are always thermodynamically unstable compared to the bulk liquid because it always costs energy to create them. The only reason we see long lived bubbles is that there is a kinetic barrier that stops the water films collapsing.
This image shows schematically what the water film in a bubble looks like (this is a soap bubble but the principle applies to all bubbles):
The water contains a chemical (soap in this case) that is surface active i.e. it preferentially adsorbs on the surface. In the case of soap the end that sticks into the water (the red blob in the picture) is negatively charged and the negative charges on the opposite sides of the water film repel each and stop the film from thinning and collapsing.
In your case you say there's no soap suds present, though note that there may be very small amounts of surfactant present, even on apparently clean glass, because surfactant adsorbs on glass and it can be difficult to wash it all off. However lots of other things are surface active including many biopolymers, and your glass may have traces of some polymer. Absolutely clean water in an absolutely clean glass will not form a stable foam, so in your case there must be some contaminent present, either on the glass or in the tap water.
Best Answer
I've never been in a jacuzzi, but I'll try to imagine one. I read elsewhere that the bubbles are added using venturi-effect, or using a blower. Main thing is, they aren't pumped together with the water (the pump wouldn't like that anyway).
The Venturi effect creates low pressure (suction) by increasing the velocity of the water in a narrow section of the pipe (smaller diameter, same volume water, so it has to speed up). increased velocity = lower static pressure (See Bernouilli principle for the details). An air blower is obvious, it blows air in some network of tubes connected to the water jets.
The reason why the bubbles stop is the same in both cases: by partly blocking the water flow you increase the pressure in the pipe. The pressure in the venturi is now higher than the air pressure instead of lower, so no more air is sucked in. If it's an air blower, it won't provide much pressure (would be bad design, since it isn't needed), and all jets are open, a little extra pressure at one jet and the air will go to the other jets instead.
The only case in which this wouldn't work is when the air enters the water before it's split into separate lines for all the jets.