Exploding ice droplets
A water droplet that freezes from the outside in exhibits a fascinating series of rapidly occurring physical phenomena before it violently explodes. Researchers from the University of Twente have revealed this in an article published in Physical Review Letters on 24 February 2017. Their publication was highlighted as 'exceptional news'.
A perfectly spherical droplet of water that freezes from the outside in initially forms a shell of ice that has the diameter of the droplet. The ice moves further inwards and this puts the remaining water 'in a tight spot', as this water also wants expand but is confined within the rigid shell. Eventually, the droplet will forcibly explode into smaller ice particles, but what happens prior to that explosion?
Shell formation
The researchers from the University of Twente produced a spherical droplet by placing the water on a hydrophobic surface. This setup was placed in a vacuum chamber and, by means of evaporative cooling, the temperature was brought below the freezing point of water, causing the droplet to be 'supercooled' but not yet frozen. The formation of the first ice crystal in the droplet was forced by briefly 'tapping' it with a tip of silver iodide. A shell of ice rapidly formed. This shell became thicker from the outside in. Video images revealed that cracks arise in the shell, an ice spicule appears and vapour cavities form under the surface. After some ice slivers have been shot off, the entire droplet eventually explodes into several fragments with speeds of about 1.5 m/s (5.4 km/h).
Meteorology
The mathematical model that the researchers formulated to explain their experiment revealed, among other things, that only droplets above a certain minimum size could explode. Droplets smaller than 50 micrometres in diameter do not exhibit this explosive behaviour. Meteorologists already knew that freezing explosions could occur in the cold tops of clouds. This phenomenon plays a role in the formation of precipitation and sometimes in the rapid transition of a cloud containing liquid droplets into a cloud containing ice particles.
'Dutch tears'
The experiments are similar to the method for producing tempered glass, which was already used by 17th-century glassblowers in the Netherlands. Drops of molten glass that are immersed in cold water solidify from the outside in, as a result of which a shell of solid glass forms around the core of molten glass. However, solidifying glass shrinks instead of expanding. That makes the spherical form so strong that even striking the glass with a hammer does not shatter it.
The research was carried out in the Physics of Fluids group of Detlef Lohse. The group is part of the MESA+ Institute for Nanotechnology at the University of Twente and will also be part of the new Max Planck – University of Twente Center for Complex Fluid Dynamics from 3 March 2017 onwards.
The paper 'Fast Dynamics of Water Droplets Freezing from the Outside In' by Sander Wildeman, Sebastian Sterl, Chao Sun and Detlef Lohse, was published in Physical Review Letters.