# Why do ice crystals form from the top to the bottom of a bottle filled with supercooled water?

If I bang a bottle filled with supercooled water against a hard surface, the ice crystals form from the top to the bottom:
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YouTube has videos showing this effect: 1; 2; 3; 4; 5.

Question: Why don't the ice crystals begin from the bottom when the force is applied to the bottom?

• Is there any air in the bottle? Commented Jan 29, 2019 at 5:10
• yes I think. Like the distilled water bought from supermarket . Commented Jan 29, 2019 at 8:03
• Hi and welcome to Physics SE! Does this also happen when the bottle is completely filled? If so, I would blame the shape of the bottle. Basically, the energy coming from the shock could be "concentrated" towards the top of the bottle because of the bottle's geometry. If not, I blame the water/air interface: probably it is energetically less expensive for an ice crystal to start to grow close to the water/air interface, or density fluctuations are larger there. Commented Jan 29, 2019 at 9:17
• This article ascribes the freezing to "heterogeneous nucleation mechanisms", then describes how they were able to keep supercooled water from freezing over by sealing the water/air interface with immiscible fluids.
– Nat
Commented Jan 29, 2019 at 13:39
• I'd have thought the obvious comment this deserved was heck, colder water sinks, warmer water rises, so it ought to be crystallising from the bottom up, because the coldest water should have sunk to the bottom. But on reflection, I suppose the lower density of cold water, below 4 degrees C, means cold water actually rises! You don't say what 'supercooled' means here, but it wasn't freezing up until you banged it. So I'd say there's a fair chance that the pre-bang(!) temperature gradient had reversed from what might be expected, so that it was coldest at the top, hence froze from the top. Commented Jan 29, 2019 at 13:54

The water is supercooled,that is below 0°C . So density of water is probably not the issue. The hydrostatic pressure is also unlikely to be significant for a small bottle. If there were nucleation sites, supercooling would not have been possible.

The key issue is the necessity to disturb the water by giving it a "bang" to freeze it. This disturbance would be significant at the surface where there is a open surface. That is the most likely reason

• This is a decent hypothesis. It would be interesting to see if eliminating the air from the bottle changes the way the ice forms. One thing is that removing the air would mean there's less space for expansion, if that's even a factor. At some temperature below -20C or so, it seems the density of supercooled water matches that of ice. Commented Jan 29, 2019 at 17:00
• @Nat The article referred to in your comment on the question appears to bolster this answer: "...by eliminating the primary ice nucleation site on the water/air interface. The supercooled water can withstand vibrational and thermal disturbances with all sealing agents, and even ultrasonic disturbance if it is sealed by alcohols." Commented Jan 29, 2019 at 18:17

Three possibilities:

1. The surface is free to flex, to ripple, and that can promote crystal nucleation. A ripple reflecting at the container surface is doubled in amplitude (by the reflection) locally.
2. There is contamination at the surface (floating specks?) that is introduced when a shock is applied to the container (dust dislodged and falling onto the supercooled liquid).
3. The bottom of the container is under higher pressure than the top, and pressure melts ice near the freezing point, in water. Nucleation under pressure is slower than nucleation near the surface.

An unobserved crystallization would make an ice crystal with lower density than the surrounding water, which would float to the top of the container; I'm assuming that isn't happening here.

Here's another possibility. When I've done this experiment myself, I've noticed that ice forms in the cap. I didn't need to apply a strong shock to get formation of ice—I merely needed to tip the bottle so that the supercooled water made contact with the ice crystal adhering to the top of the cap. We can't know without looking at the cap, but I think it's possible that the water in the bottle freezes from contact with ice crystals in cap, not due to the shock.

• That seems like decent hypothesis but if you watch the video in link 3, the bottle is held sideways and the crystals form from side opposite of the cap. Commented Jan 29, 2019 at 15:51

The shockwave goes from the bottom to the top and when it hits the surface it will bounce back to the bottom due to a change of medium. I would say that at the moment of the bounce the first and second shockwaves produce a region of very low density where the crystals can start forming. Once started it's just a matter of time for the crystals to grow.