Kenneth Librecht was a strange man who happily left the warm southern California of the United States in the middle of winter and headed for Alaska, the northernmost part of the United States. There, he put on a cotton coat, sat in the field with a camera and a foam board, waiting for the snow to fall.
Specifically, he was in search of the most well-defined and beautiful snowflakes that nature could produce. These snowflakes tend to form in the coldest places. Lybrecht was a physicist who studied celestial bodies, but he had a hobby - to study snowflakes, not only their appearance, but also their formation.
Snowflakes are much more complex and mysterious than we think. In order to solve this mystery, Librecht has been exploring for 20 years and finally solved the mystery of the formation of snowflakes.
Why are ice crystals hexagonal?
Snowflakes are a group of ice crystals formed by the crystallization of water vapor in the atmosphere, and ice crystals are the basic units that make up snowflakes.
Everyone knows that no two snowflakes are exactly the same. This fact stems from the way ice crystals form at high altitudes. While there are differences in temperature and humidity everywhere in the same cloud, these differences are negligible in the small area where a snowflake is formed. This is why the growth of ice crystals is usually symmetrical. On the other hand, each snowflake will take on a slightly different shape when it falls to the ground due to the effects of changing factors such as wind, sunlight, and collisions during its fall.
Although no two snowflakes are exactly the same, there are rules for the shape of ice crystals. Ice crystals, for example, are always hexagonal. Why is this so? We now know that the water molecule is made up of two hydrogen atoms and one oxygen atom. Since two hydrogen atoms in adjacent water molecules tend to be connected by hydrogen bonds, they tend to lock together to form a hexagonal array with oxygen atoms as the corners. The array has a structure that is open to all sides, so it can grow continuously, eventually forming snowflakes.
In a hexagonal structure, the average distance between water molecules is greater than the average distance in the liquid state, so this also makes the ice less dense than liquid water. This greatly affects the Earth's environment. Life on Earth would not have existed if the ice had not floated on water.
There are two main types of snowflakes
Snowflakes are grown from ice crystals. Ice crystals can grow both laterally and vertically. It's like a hexagonal floor tile that can be spread out or stacked. In general, the growth of ice crystals occurs simultaneously in both directions, but there are fast and slow movements. Depending on which direction grows faster, there are two main types of snowflakes: flaky and columnar. For other types, it's basically a combination of the two types (e.g. spool, columnar in the middle, flakes on both sides), or add some lace.
Interestingly, during the growth process, the ice crystals constantly adjust their growth strategy according to the surrounding temperature and humidity: flakes form around -2, columnars form around -5, then flakes again around -15, and columnar ...... again at -30At low humidity, lace is simple; At high humidity, the lace is complex.
So, why do ice crystals adjust their growth strategies as the temperature changes? What are the atomic processes that determine the growth of ice crystals into flakes or columns?
Liebrecht tried to solve these mysteries. For more than two decades, he has been trying to build a unified model for the growth of ice crystals. His dream is that by inputting a set of parameters such as temperature and humidity into the model, the computer will be able to display the various colorful snowflake patterns that we actually see. Now, his goal has largely been achieved.
A growth model of a snowflake
To observe the growth of ice crystals, Liebrecht creates sharply contoured and colorful snowflakes in closed containers by precisely controlling conditions such as temperature and pressure. In most cases, they are smaller than a human hair and can only be observed under a microscope. Because if they get too big, the shape will be too complex to be easy to study.
On the basis of extensive observations and experiments, Liebrecht finally developed a "surface energy-driven molecular diffusion model" to describe the growth of ice crystals.
This model is a bit complicated, and it is not convenient to go into detail here. But to give you a rough idea, let's imagine that when the water vapor first begins to condense, the condensed water molecules begin to form a rigid lattice, and each oxygen atom is surrounded by 4 hydrogen atoms. This lattice will grow by pulling water molecules from the surrounding air in to form more lattices. They can either expand in all directions or up and down. So, what strategy will it choose? It depends on the situation in which the surface of the crystal can be minimized.
For surface energy, we might as well understand it as the tension potential energy of a surface with tension. For example, the surface of a balloon is elastic, and when the balloon is inflated, its tension potential energy is lowest only when it is in the shape of a sphere. This is the reason why balloons are neither square nor diamond-shaped, but spherical.
Similarly, ice crystals also have surface energy. The surface energy of ice crystals is affected by temperature, pressure, and other factors, which determines what shape they will grow into under different temperature and pressure conditions. For example, under very high pressures, we will only get thin flake crystals, and columnar ones will not appear.
Librecht's model not only helps us solve the mystery of the shape of snowflakes, but also provides important insights for studying the growth of other crystals. Crystallization is a common means in industry. Pharmaceutical molecules, semiconductor chips, solar cells, and countless other applications rely on high-quality crystals resulting from crystallization.