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I want to make a thin, smooth film of pure lithium in a glovebox.

In general, I think that I can use a phase separation approach. I will heat a bath of mineral oil above Li's melting point, then pour liquid Li inside and let it cool slowly. The Li should cool on top and the only curvature should be due to miniscus effect.

I would think that if I used a third, lighter liquid than the Li, I could put a large column of it on top of the Li. However, this requires having a lighter liquid. Li, at melting point, has a density around $\pu{500 kg m-3}$.

Can anyone suggest a suitable lighter liquid?

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    $\begingroup$ I can't think of any. Then again, what's the point? The meniscus would still be there. $\endgroup$
    – Ivan Neretin
    Commented Aug 25, 2017 at 13:57
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    $\begingroup$ Top liquid would supply more pressure and reduce meniscus. At least thats my thinking in theory, doesnt mean itll work in practice $\endgroup$
    – User2341
    Commented Aug 25, 2017 at 14:02
  • $\begingroup$ I don't think there's any liquid that could be used here, pressurised gas may work. $\endgroup$
    – Mithoron
    Commented Aug 25, 2017 at 14:07
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    $\begingroup$ I have a suspicion it won't work. Molten metals usually have strong surface tension, so a thin film might fragment into rather large drops. $\endgroup$
    – permeakra
    Commented Aug 25, 2017 at 14:55
  • $\begingroup$ Do you have any better suggestions? I'm just trying to make an Li foil/film with a very, very smooth surface. $\endgroup$
    – User2341
    Commented Aug 26, 2017 at 19:19

1 Answer 1

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You are thinking in the wrong direction. Meniscus is not about pressure at all.

You seem to be imagining external hydrostatic pressure as a huge invisible flat lid that we put on top of your molten lithium and kinda force it to stay flat. This is very much not so. The pressure is the same all around, thanks to the Pascal's law. It does not affect the shape. It presses on the curved sides of the meniscus with the same force as it does on top. The very notion of "flat" is simply not there.

If anything, it is gravity force that reduces the meniscus. In effect, adding a liquid on top will partially negate the gravity and thus make things worse. Keep in mind that such liquid, if it is found (I still can't think of any, and I'm not a novice in chemistry, mind you) can't be much less dense than lithium, which almost brings us to the conditions of Plateau's oil drop experiment, with lithium playing the role of oil. Not quite the outcome you wanted, is it?

It is the surface tension that matters, and adding liquid might change that in an unpredictable way. Then again, you can achieve the same result without any extra liquid. Just use different material for the container. I guess using metal instead of glass would make the meniscus bend in the opposite direction. With some tweaking, it might be possible to find the material with wetting angle of $90^\circ$ that would make lithium surface perfectly flat, but I don't think it is worth the effort.

So it goes.

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    $\begingroup$ Solid answer. I learn a little every day. Thanks. $\endgroup$
    – User2341
    Commented Aug 25, 2017 at 20:35

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