Why does a frozen balloon have this structure?

[musicgold]

Hi,

I forgot a water-filled balloon in my backyard last fall. When my kid brought the frozen balloon in and removed the rubber, we were surprised to see the beautiful structure inside the ball of glass. I remember that glass paperweights used to have similar designs inside them. I used to think that air bubbles were blown into the glass to create the structure. But it looks like it is a natural phenomenon. I think even ice cubes in the fridge also have a similar structure.

Can someone please explain me or point to a resource that explains why this triangle structure, with air bubbles trapped at the center, is formed? Why don't we get uniform transparent ice?

Thanks.

 

[lekh2003]

The air bubbles must have been some air stuck inside the water balloon. I can't say I know about the triangle pattern inside.

 

[A.T.]

Why don't we get uniform transparent ice?

The freezing is not uniform. It probably goes inwards.

 

[OmCheeto]

According to a paper at the NIH; "As freezing progresses, the concentration of dissolved gases surpasses a critical value, the water-containing fluid at the interface becomes supersaturated, and the gas bubbles may nucleate and grow to a visible size either along the interface and/or be trapped within the advancing solid." [ref]

So following the bubble tracks tells us that the outer surface froze first, and progressed towards the middle.

My guess as to why there is a triangle shape, is that the ground was warmer than the surrounding air during the process.