Ploughing into the Science of Snowflakes this Christmas
Ever wondered if every snowflake truly is unique? Why snow is white? Or why ice is slippery?
A physicist at King’s College London is delving into the science of snow crystals – the tiny crystals that make up snowflakes – this festive period.
Professor Carla Molteni works at the interface of physics with chemistry, materials science and biology by investigating the properties of matter at the molecular level with the aid of powerful computers. One area she is researching is the growth of ice crystals – an area of science still not fully understood.
Professor Molteni has answered some fascinating questions about snowflakes – though she cannot guarantee whether we will have a white Christmas this year!
How do snowflakes form?
Professor Molteni said: “Snow crystals form in clouds at low temperatures. They begin to form when water molecules freeze, for example around dust particles. The shape of these snow crystals depends on the temperature and humidity conditions they encounter while growing. They first grow as small hexagonal prisms but may develop six intricate symmetric branches as they become larger.
“In the 17th century scientists like Johannes Kepler and Robert Hooke observed the hexagonal shape of snow crystals but could not explain it because the details of the structure of matter at the molecular level were still not understood. Today we know that this signature symmetry reflects the underlying hexagonal crystal structure – that is, the way water molecules interact and pack in a very organised way when freezing. The final complex shape is the result of different growth patterns, influenced by the environment.
“Fast forward to modern day and there are still many aspects related to how water molecules assemble to create snow crystals that are far from being fully understood. In my research group, we have been investigating the growth of ice crystals using state-of-the-art computer simulation methods to solve some of the puzzles behind their fascinating structures.”
Are all snow crystals unique?
“It’s true that, in atmospheric conditions, snow crystals really are uniquely different. This is because their growth is affected by the many slightly different conditions of temperature and humidity that they experience during their journey in clouds, resulting in a huge variety of shapes.
“However, in a lab, scientists can in principle make identical snow crystals by carefully controlling the environment where they grow.”
If ice crystals are transparent, why is snow white?
“Ice crystals are transparent because light can pass directly through them. However, with snow, light is scattered in different directions by many ice crystals assembled together, creating its signature white colour.”
Why is ice slippery?
“Ice is so slippery because a thin layer of liquid water forms on top of ice surfaces at very low temperatures. This is known as the ‘quasi-liquid layer’ and has very interesting properties. This layer acts as a sort of lubricant – causing the ice to be very slippery. We use salt on our roads and paths because it lowers the freezing point of water, which prevents ice from forming.”
Find out more about Professor Molteni’s research here – https://www.kcl.ac.uk/people/carla-molteni