It's a basic fact of the kitchen: you can drizzle long strands of honey or syrup from a bottle, but you can't do the same with water. Surprisingly, no one had a satisfying explanation for this – until now. The solution to the mystery lies in the behavior of tiny waves. You might think that the viscosity of a liquid would be a key factor in forming strands during pouring. However, previous research suggested that the way gravity pulls a liquid into a long string is largely independent of viscosity, making it unclear why honey and water behave so differently.

Now the international team of scientists, including Daniel Bonn from the UvA, have worked out another way in which viscosity affects fluid behavior – and this one seems to offer an explanation for the strands.

They used a mathematical model to predict how small waves of disturbance that occur naturally within fluids affect the drizzles. It is these waves that determine the length of the drizzles, because if the waves grow large enough they break the fluid stream and form droplets. The waves grow more slowly in viscous fluids, so in theory strands should stretch further the more viscous a fluid is. However, as more viscous fluids also fall more slowly under the influence of gravity, it wasn't clear that viscous strands would really be any longer.

But the team found that for extremely viscous fluids, like honey, the speed at which the fluid falls under gravity outstrips the speed with which the waves within the fluid grow. This means honey can form into long thin strings before the waves are large enough to break the strands. The team then confirmed their mathematical predictions through experiments using silicone oils with a range of viscosities.