– extracts from the article –
Spooning soft honey from a jar generates long, dangling threads of the liquid, but researchers have not understood why columns of a viscous fluid get so long without breaking up. In Physical Review Letters, a team explains the mystery using a combination of theory and experiments with dripping silicone oil. They found that while viscosity doesn’t affect the large-scale motion of the falling liquid very much—consistent with previous theories—it does affect the way that small, random waves in the fluid become amplified over time, which leads to breakup.
The results may be relevant for industrial processes that involve pulling long strands of viscous fluid, such as the fabrication of optical fibers. For perturbations that start at the nozzle, this influence of viscosity doesn’t count for much, because they get rapidly stretched out as the jet descends, before they can grow and create pinch-off. But irregularities appearing further down the jet can grow in amplitude before they get stretched too much, so viscosity matters for them.
Delayed breakup of liquid columns is important for the industrial process of fiber-spinning, where a viscous liquid such as a polymer or molten glass is drawn out into a long, thin strand to make textiles or optical fibers. The theory should help to predict the maximum length for such fibers, Eggers says.
Stretching of liquid jets also happens naturally in some volcanic environments, where molten rock is formed into glass fibers known as “Pele’s hair.” “This phenomenon shares with our falling viscous jets the elements of strong stretching and high viscosity,” says team member Neil Ribe of the University of Paris and the French National Center for Scientific Research (CNRS). “However, it also involves fast cooling and consequent strong increases in viscosity. This means that threads that have been stretched very thin will tend to solidify before they can break up.”