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Over the past few weeks, when making tea, I've noticed that after filling the mug with water, the teabag that has been placed inside sometimes floats to the top, and sometimes sinks/stays at the bottom of the mug. I don't have any proper statistics, but I'd say this occurs on a roughly 50/50 basis. I should add that the teabags are all from the same box.

I'm unsure as to why this happens - I'd expect the result to be consistent instead of varying. Does this happen because of the way the boiling water is poured over the tea bag? Does the temperature of the water matter (sometimes I leave the kettle standing for longer than other times)? Perhaps the convection current is stronger the hotter the water is which would lead to the bag rising for hotter water temperatures?

My friend also suggested that perhaps the oxygen content of the leaves plays a role. Say some tea bags (in the same pack) are on average "fresher" than others, meaning that the leaves have more oxygen in them. Perhaps a higher oxygen content of the leaves means a higher chance that the tea bag will float? Perhaps some tea bags are just more aerated than others?

Many thanks for any answers.

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  • $\begingroup$ All of the things you mention could play a role. What is needed is some research (ahem) - either your own systematic experiments or googling your title for the results of others' experiments. eg sciencewithtoys.wikispaces.com/Floating+tea+bags $\endgroup$ – sammy gerbil Jun 18 '18 at 14:20
  • $\begingroup$ Probably surface tension has something to do with this. Are the bags all equally wetted in each case? $\endgroup$ – Chet Miller Jun 18 '18 at 14:29
  • $\begingroup$ @ChesterMiller Yes, during the pouring process I suspect that the bags get completely covered by water. $\endgroup$ – mathphys Jun 18 '18 at 14:47
  • $\begingroup$ What is different about the two scenarios? That is what you need to identify. It is IMO pointless to speculate on which factor is responsible if we do not know how the conditions vary between different trials. $\endgroup$ – sammy gerbil Jun 18 '18 at 15:57
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The material from which the teabag is made is porous and allows air to pass through it. So if you gently lower a teabag into liquid the rising liquid expels the air through the (dry) upper part of the teabag and the teabag sinks.

However, if you wet the material of the teabag then the water spreads out over the material to form a liquid film that seals all the pores in the material, so the wet material will not allow air to pass through it. If you managed to uniformly wet the material of teabag before dropping it into the tea then you have in effect created a bubble of air and the teabag will float.

These two scenarios are extremes, and in practice the wetting is rather more chaotic. Whether any air is trapped in the teabag will depend on how exactly the water pours over it, and I'm not sure I care to make any predictions without trying the experiment (and since I'm a coffee drinker not a tea drinker I have no tea bags to hand).

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    $\begingroup$ My favorite case is that of ginger tea. When placed smoothly into the water it immediately sinks, then after about a minute it floats, then after another two minutes it sinks to the bottom again. I'm pretty sure the delayed rise is due to changing density as the hot water forces expansion, but I'm not entirely clear on why the bag sinks again. $\endgroup$ – zxq9 Jan 9 at 15:53

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