Surface smoothness of a slowly frozen metal droplet

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Discussion Overview

The discussion revolves around the surface smoothness of small liquid metal droplets as they freeze, particularly focusing on the conditions that influence surface roughness and reflectivity. Participants explore the implications of slow cooling, external perturbations, and the effects of solid grain growth on the surface characteristics of the frozen metal.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a slowly cooled liquid metal droplet would retain a smooth surface upon freezing, suggesting that density changes during the phase transition could introduce roughness, but this might be minimized by careful cooling and material selection.
  • Another participant asserts that the crystalline structure formed during freezing will inherently be less smooth than the liquid state.
  • A participant expresses interest in estimating the roughness and reflectivity changes of the solidified droplet compared to a polished surface, indicating a potential application in optical materials.
  • Examples from the use of Tin-Lead solder in electronics are provided, illustrating that undisturbed cooling can yield a smooth surface, while mechanical disturbances can lead to a visibly rough crystalline surface.
  • One participant reflects on the importance of surface quality in solder joints, noting that a non-shiny surface is considered a quality failure.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the extent of surface roughness or reflectivity changes upon freezing, with differing views on the impact of cooling conditions and mechanical disturbances.

Contextual Notes

The discussion highlights the complexity of factors influencing surface characteristics, including the effects of solid grain growth and the specific cooling conditions employed, which remain unresolved.

Who May Find This Useful

This discussion may be of interest to those involved in materials science, optical engineering, and quality control in manufacturing processes, particularly regarding phase change materials and their applications.

timelessmidgen
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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.
 
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On "freezing", a liquid metal becomes a crystaline structure and will not have as smooth a surface as the liquid.
 
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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")
 
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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.
 
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Tom.G said:
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.
 
Tom.G said:
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.
 

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