We have all been taught that water freezes at the temperature of 0ºC. However, this is not strictly true. In fact, supercooled water can be liquid at below zero temperature. This is an unstable state which properties hadn´t been completely determined yet. A new study published by researchers at Universidad de Alcalá, Lulea University of Technology, Mag Soar SL and Saint George Tech LTD, confirms that supercooled water can remain liquid indefinitely if the proper humidity conditions are maintained.
The research team led by prof. Pérez-Diaz has demonstrated the correlation between the water surface freezing point and the relative humidity. In a first paper, Ice Surface Entropy Induction by Humidity or How Humidity Prompts Freezing, Prof. Perez- Diaz research team discovered this effects in water droplets. The main conclusion of this study was that the freezing process of supercooled water can be triggered or delayed by controlling the moisture levels of air. Additionally, it showed that the freezing of droplets begins on the surface. However, it remained as an unsolved question to determine if such behavior was just an exclusive property of the droplets or if it could be reproduced in bulk water as well.
In order to unravel this, the researchers introduced a container with deionized water in a climatic chamber and captured images of just the exact moment in which bulk water crossed the border between liquid and ice stages.
The new results demonstrate that humidity prompts freezing of bulk water in a similar way as it does in water droplets. Additionally, the cameras captured the formation of 2D single ice crystals on the top of the water surface, so that for a few milliseconds a solid ice layer is interposed between the liquid supercooled water of the container and the humid air above.
This discovery represents another step forward in the further understanding of the mysterious behavior of water. Freezing water phenomena affect a lot of key processes in food industry, aeronautics, energy, and transportation sectors. These finding could be also important for a better comprehension of the meteorological behavior of clouds.