Shockwave

[Beatsy]

Or, more accurately, the after effects of a shock wave on aluminium foil. The shock was created by smartly clacking a pair of 50mm ball bearings together with the foil trapped between them.

Ordinary paper works too but is less interesting. It instantly sublimes into the smell of burning paper (at the impact point) and leaves nothing but a small round hole (sometimes charred).

My curiosity was piqued by this video from "Action Lab" which explains everything. The size of the balls is critical, also demonstrated in the video. My mild curiosity is satisfied now, but it does look like there are a lot of variables to explore. Thought I'd post here in case anyone was inclined to "look further". Do share if you do - thanks

Video ---> https://www.youtube.com/watch?v=gTh5aABIwoY

MP-E65 @ f/3.5, ISO 100, stack of 80 images. This is the matt side of the foil. The shiny side looks the same, only shinier. FoV ~7mm

[rjlittlefield]

Fascinating stuff!

I wonder what the exact mechanism is, for the rippling.

--Rik

[Beatsy]

Ta Rik. My theory is that it's literally the beginnings of a splash, albeit created at hypersonic speeds. The aluminium is compressed at the centre first and melts due to the shock. This squeezes out a blast wave that propogates from the centre, thinning and buckling the molten foil which is then frozen in time when it cools. The fact that there are ripples in a previously flat piece of foil proves that the material must have been thinned appreciably to cover the (subsequently) larger surface area. This can't all be accounted for by simple mechanical flattening, that appears to happen only to the central mirror-smoothed bit. I guess the buckled-but-matt parts are not melted, but are pushed into "hills" by the rapidly expanding (and thinning) plane of molten metal in the centre.

[Joaquim F.]

Hi, is a really interesting experiment and intriguing image!
Not sure if can be related to a mechanical wave phenomenon called hertzian fracture in some literature.
https://en.wikipedia.org/wiki/Hertzian_cone
Seems very similar to the "crater" and "wallner lines" described in this pdf: http://www.dowdresearch.org/Hertzian-Glossary.html
May be in the case of this practically bidimensional materials other physical phenomena are involved...

Best regards