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Thin and Flat

One often-overlooked quality of stellar snow crystals is that they are remarkably thin and flat. This photo shows one seen from the side. This is why we call them snow flakes.

Then again, snow crystals are not always thin, flat, plates. Some look like slender needle crystals. The different forms appear at different temperatures, as shown in the snow crystal morphology diagram.

This all begs the question: How is it that snow crystals can grow into such extreme shapes -- extremely thin, flat plates and extremely long, slender needles?

The Sharpening Instability

Part of the answer to this question lies in an edge-sharpening growth instability. It sounds complicated, and it is a bit. Turns out the growth of a faceted surface depends on the width of the facet. When last molecular terrace is less than 100 molecules wide (roughly), it becomes especially easy for molecules to attach to the top of the terrace. Strange, but that seems to be part of how ice works.

This quality tends to sharpen the crystal edges. As shown in this diagram, when a corner grows, it produces narrow faceted terraces. A narrow terrace grows faster than a wide terrace, and the growth then adds more terraces that are even narrower than before. The result is an edge-sharpening growth instability.

One big problem remains with this: we do not know the molecular cause of the terrace-width dependence in the growth, which underlies the edge-sharpening instability.. More than anything else, this is what currently limits our understanding of the snow crystal morphology diagram.

Plates on Needles

We study how all this works by growing ice crystals in the snowflake lab. The images on the left shows a thin plate growing on the tip of an electric ice needle. The composite image shows a side view at different times. The other photo shows the plate face on. The edges of the plate grow out rapidly once the edges become thin and sharp.

From the imaging data we measure the grow velocities, and then try to model everything on the computer. The sharpening instability is necessary to make sense of all these measurements, so we are fairly confident this is a real thing. Still, we are collecting more data, under all different conditions, just to be sure.

Cups on Needles

This shows a photo of a cup-shaped crystal growing on the end on an electric ice needle. This time it is the edges of the column that become thin and sharp from the edge sharpening instability.

It's unfortunate that ice crystal growth is so complicated, with faceting, branching, and edge sharpening all going on simultaneously. But, all this complicated physics is why snow crystals look so intriguing. Next time you see it snow, try to imagine all the things going on in the winter clouds.

Scientific references to the edge-sharpening instability:

Explaining the formation of thin ice crystal plates with structure-dependent attachment kinetics, by Libbrecht, KG, JOURNAL OF CRYSTAL GROWTH  Volume: 258   Issue: 1-2   Pages: 168-175   Published: OCT 2003.
[This describes the basic idea of the instability.]

Measurements of surface attachment kinetics for faceted ice crystal growth, by Libbrecht, Kenneth G.; Rickerby, Mark E., JOURNAL OF CRYSTAL GROWTH  Volume: 377   Pages: 1-8   Published: AUG 15 2013.
[A related paper, measuring the growth rates of faceted ice crystals.]

Observations of an Edge-enhancing Instability in Snow Crystal Growth near -15 C, by Kenneth G. Libbrecht, arXiv:1111.2786 (2011).
[Describes quantitative evidence for the edge-sharpening instability.]

An Edge-Enhancing Crystal Growth Instability Caused by Structure-Dependent Attachment Kinetics, by Kenneth G. Libbrecht, arXiv:1209.4932 (2012).
[Better evidence for the edge-sharpening instability. These data are difficult to explain any other way.]

Toward a Comprehensive Model of Snow Crystal Growth Dynamics: 2. Structure Dependent Attachment Kinetics near -5 C, by Kenneth G. Libbrecht, arXiv:1302.1231 (2013).
[More experimental evidence with quantitative modeling of the instability, this time at -5C.]

Toward a Comprehensive Model of Snow Crystal Growth Dynamics: 1. Overarching Features and Physical Origins, by Kenneth G. Libbrecht, arXiv:1211.5555 (2013).
[A related paper, describing my ideas about an overarching physical model for snow crystal growth.]