A steaming travel mug of black tea. How much particulate matter does that steam carry anyway?

Factual Questions
1

Lengthy background: I use a large travel mug at home, it keeps my tea warm for the time it takes me to drink it. It is metal, and has a nifty ceramic coating inside that after several years is still pristine and unstained from tea, a condition achieved with simple soap and water.

It has a clear plastic cover that fits inside the rim of the mug, and that also has a lip that fits over the top of the mug. There is a hole for drinking out of, but I don’t use that, I only keep the lid on while I’m moving from the kitchen, where I brew my tea, to downstairs, where I usually drink it. So most of the time that cover is not even on the mug. The cover has a plastic slider to cover the drinking hole, that contains a piece of silicone that is the same shape as the drinking hole and fits over it snugly when the slider is closed. But the slider is almost never closed, because with a mug full of hot tea, the steam will do its best to push up the whole lid, and I’m not so sloppy with the mug that I need to close that slider. End background.

The slider, especially the area that contains that piece of silicone, and the piece of silicone itself, somehow get filthy with what is apparently tea residue. It takes a few weeks, and when it gets bad I clean it. There’s no way to just wash it out, I have to dip and soak it in the same stuff I use to clean my stainless steal tea maker. Which is not really a big deal, but I can’t figure out how it gets so filthy, not only near the drinking hole but all the way to the back of the slider. The actual tea never or almost never touches that area, really the only thing that does is the steam, and since the slider is almost always open, I would expect the steam to be going up and out, taking the path of least resistance.

So, science-y people, is that steam really carrying all that particulate matter, or is there some other explanation?

2

I’d suspect sloshing before I would evaporative particulates.

3

Yes, it is. While the steam generally goes up and out, it’s going to go up from all points along the tea’s surface, which means a lot of that steam is going to hit the entire lid of your travel mug, including the slider. As the steam flows towards the exit hole, surface adhesion is going to make a tiny layer of steam “stick” to the entire surface of the lid. You end up with a gradient where the steam touching the lid is completely motionless, and the further away from the lid the steam moves faster until you get to the fastest part of the steam that is moving towards the exit hole.

The steam doesn’t just take the path of least resistance, as you put it. It takes all paths. It’s just that more of the steam goes along the path of least resistance than the amount of steam that takes slower and less direct paths.

You will also have vortices forming around the slider (since it’s an uneven surface) which, again working with surface adhesion, will cause more of the particles to be deposited there.

The same thing will happen near the exit hole (again, lower pressure vortices caused by the uneven surface), but you’ll get much more significant deposits from the actual tea sloshing against the hole.

4

Thank you for that, I think I even understood most of it. As it happens, the slider is entirely on the outside of the cap, so the steam has to go through the exit hole before it is able to wend its way into all the parts of the slider. As for sloshing, I don’t think there is much of that, the mug just isn’t that full. But I suppose it wouldn’t take much.

I just cleaned the thing tonight, along with the tea maker and other parts, so the modest aggravation of it is on my mind. I am considering surgery on the cap to remove the slider, since I don’t use it, and it would make cleaning a lot easier.