A single drop falling onto a freezing surface has a much greater impact than a mere drop in the ocean: it freezes into a strange pointed shape. Together with co-workers from Paris, Brussels and Munich, researchers at the University of Twente (MESA+, Physics of Fluids) worked out how this shape is created, using video images and mathematical models. An understanding of the way that droplets solidify is vital to the effective control of 3D printing techniques, for example. The researchers are to publish their findings in a leading scientific journal, Physical Review Letters, at the beginning of August.
This experiment can be performed in your kitchen at home, using a metal plate cooled in the freezer - preferably to a temperature of less than minus 15 degrees Celsius. A drop of water falling onto the surface of this plate freezes from the bottom up, so it won’t stay round, instead it gradually assumes a conical shape. Existing theories were unable to account for that shape. However, with the aid of video recordings and mathematical models, the researchers from the Physics of Fluids group have now come up with an explanation. They have also been able to calculate the sharpness of the tip of the cone: the cone angle. This study has improved our understanding of how aircraft wings ice up, for example, and of 3D printing techniques involving the controlled solidification of molten metal droplets.
Perpendicular
Trying to see inside a drop, to get a good idea of what is going on, is no easy matter. This is because the drop itself acts like a lens, producing a highly distorted view of the three-dimensional image. For this reason, the University of Twente researchers shot video images of “flat drops” in 2D, by allowing drops flattened between two sheets of glass to fall onto a cold plate. Under these conditions, the freezing process can be clearly seen. So what did they find? The “solidification front” (the dividing line between liquid and solid) is always at an angle of about 90 degrees to the surface of the water. This has the effect that, relative to the plate, this front initially has a convex shape, which gradually becomes concave. Freezing also increases the volume by about 10%, so the expanding ice develops into a point. Heat transfer calculations have shown that the angle of the cone is always about 140 degrees, regardless of the drop’s temperature or size.
The study was conducted by PhD student Oscar Enríquez and Prof. Jacco Snoeijer, both of whom work in the Physics of Fluids group. This group, which is part of the MESA+ Institute for Nanotechnology at the University of Twente, has a range of research interests, including drops, bubbles in liquid, and complex phenomena such as turbulence. Mr Enríquez and Prof. Snoeijer collaborated with co-workers in Paris, Brussels and Munich.
Their paper, entitled “Universality of tip singularity formation in freezing water drops”, was published online on 1 August at the Physical Review Letters website.
