Why does glass get elastic when immersed in HF; like this video???

[Panthera Leo]

Hi,

I was surfing YouTube and came across a very interesting video... Check it out yourself;

http://youtu.be/gzUDjHDj0fQ

Why did the glass become elastic???

I was thinking it might be due to destruction of some bonds on SiO2 but not all causing mobility of the molecules such that they slide over one and other??? But its just a guess, I have no idea.

Thanks in advance.

[Mapes]

The elasticity of the glass didn't change, but the HF smoothed the glass by etching it for a short time. The etching removed surface imperfections that produce high stress concentrations and make glass the brittle material that we're used to.

Incidentally, I can't approve of someone pouring undiluted HF over their head, with little personal safety gear, and outside of a fume hood. This person sets a bad example.

[chemisttree]

The elasticity of the glass didn't change, but the HF smoothed the glass by etching it for a short time. The etching removed surface imperfections that produce high stress concentrations and make glass the brittle material that we're used to.

You could be right but I have a different take on it. I think that HF etches glass and would introduce lots of defects making it more brittle. What I think is going on is that the glass rod isn't annealed and the glass near the surface that already under stress is preferentially etched away in the HF. All the glass rods I have ever used are flame polished and don't have any defects like micropits and so forth. None of the ones that I have ever used are ever annealed after flame polishing so they have significant surface stresses frozen into them. The untreated rod breaks early because the additional stress of the mass added to the inherent stress frozen into the glass rod during manufacture is too much. A rupture occurs near the surface.

[Panthera Leo]

What I think is going on is that the glass rod isn't annealed and the glass near the surface that already under stress is preferentially etched away in the HF.

Thats right according to page 525 of this book:

HANDBOOK OF NANOTECHNOLOGY (http://books.google.com/books?id=MOrlqX_5fakC&pg=PA525&lpg=PA525&dq=preferential+etching+of+glass&source=bl&ots=Co--vaHZ2h&sig=56F5zrBvfXbZ5x8OUBJ3OZo97go&hl=en&ei=R-BWTs7VCILZrQfimOjdCg&sa=X&oi=book_result&ct=result&resnum=2&sqi=2&ved=0CDYQ6AEwAQ#v=onepage&q=preferential%20etching%20of%20glass&f=false)

[Mapes]

That link doesn't discuss strength at all (?!) Here (http://books.google.com/books?id=5PRpsZKwhewC&lpg=PA139&dq=glass%20%22stress%20concentration%22%20hf&pg=PA139#v=onepage&q=glass%20%22stress%20concentration%22%20hf&f=false) is a discussion of how HF can eliminate stress concentrations and increase strength by smoothing a surface.

[chemisttree]

That link doesn't discuss strength at all (?!) Here (http://books.google.com/books?id=5PRpsZKwhewC&lpg=PA139&dq=glass%20%22stress%20concentration%22%20hf&pg=PA139#v=onepage&q=glass%20%22stress%20concentration%22%20hf&f=false) is a discussion of how HF can eliminate stress concentrations and increase strength by smoothing a surface.

I think it's just amazing that there are at least two books crammed full of our correct answers!

[Mapes]

I think it's just amazing that there are at least two books crammed full of our correct answers!

That's what makes it so nice for us to kick back in our easy chairs and speculate. :smile: