Surface smoothness of a slowly frozen metal droplet

[timelessmidgen]

TL;DR

Liquid metal looks smooth and polished (highly reflective in visible light). If a liquid metal droplet is left to cool slowly and undisturbed and away from oxygen, will the solidified droplet remain highly polished? Or will the freezing process roughen up the surface a bit?

Would a small (of order 5 microns to 0.5 mm) liquid metal droplet, if cooled slowly away from external perturbations and not in the presence of oxygen, retain its highly smooth and polished surface as it froze? What phenomena would influence the surface roughness?

I assume that simple density changes from liquid to solid could cause roughness from differential shrinking/expanding, but I think these could be minimized by cooling more slowly/uniformly, reducing droplet size, and selecting metals with small or zero (in the case of some alloys) volume expansion coefficients. Is there an additional roughness introduced due to solid grain growth on the molecular scale?

Your thoughts and resources regarding this topic would be much appreciated. My general googling of the issue turns up many resources dealing with homogeneous/heterogeneous nucleation and growth of solid-liquid interfaces, but not generally anything about the external solid-vacuum interface.

 

[phinds]

On "freezing", a liquid metal becomes a crystaline structure and will not have as smooth a surface as the liquid.

 

[timelessmidgen]

Thanks! Is it possible to estimate how rough the surface becomes or how much less reflective it would be than a polished surface?

ETA: The underlying reason I'm curious about this is because I'm thinking about possible optical applications of phase change materials. Ultimately I'm curious about a simple metal droplet temperature-dependent reflector, and how much of a change in reflectivity might be reliably seen over many freeze-melt cycles. An accurate number would, I suspect, require detailed numerical simulation of a particular material. Nonetheless I would be curious to know if there is even an order-of-magnitude estimate (like, "the freshly solidified droplet will be about 20% less reflective than a polished surface" or "the freshly solidified droplet will be about 0.2% less reflective than a polished surface")

 

[Tom.G]

Just as a macro example, common Tin-Lead solder used in electronic asembly, when left to cool undisturbed from molten has a quite smooth shiny surface.

If it is mechanically disturbed when near its solidification temperature it flash-freezes and has a visually obvious crystalline surface.

A non-shiny surface on a solder joint qualifies as an instant Quality Control failure, and rework.

 

[timelessmidgen]

Just as a macro example, common Tin-Lead solder used in electronic asembly, when left to cool undisturbed from molten has a quite smooth shiny surface.

If it is mechanically disturbed when near its solidification temperature it flash-freezes and has a visually obvious crystalline surface.

A non-shiny surface on a solder joint qualifies as an instant Quality Control failure, and rework.

Ok interesting, thanks! An example that the change in reflectivity might be very slight.

 

[phinds]

A non-shiny surface on a solder joint qualifies as an instant Quality Control failure, and rework.

HA ! 50 years ago I would have though of that right away. Thanks for the reminder.