Scientists from the Institute for Manufacturing (part of the Department of Engineering) have developed a method for predicting whether a filament of fluid will condense along its length into a single droplet, or collapse into multiple droplets. The research, published today in the journal Physical Review Letters, could aid in designing processes for paint-spraying, ink-jet printing and the dispersion of drugs for inhalation.

Dr Alfonso Arturo Castrejon-Pita, first author of the paper, said: "For the first time in an experimental and quantitative way, the ultimate behaviour of a filament under nothing but the action of viscous and surface tension forces has been explored."

One of the greatest challenges facing the ink-jet printing industry is to develop print-heads that are capable of generating “clean” single, uniform size droplets within a large range of fluid properties (viscosity, surface tension).

The University of Cambridge team, led by Professor Ian Hutchings, developed a large-scale, fully controlled model of a print-head to recreate the process of droplet generation. Using a very simple fluid solution, made up of water and glycero, the team gradually increased the viscosity of the working fluid (by increasing the amount of glycero in the mixture). The entire process of droplet generation was then recorded using ultrafast imaging techniques to observe how long threads, or filaments behaved. For sufficiently viscous mixtures, these would be several centimetres long and one tenth of a rmillimetre thick. Under some conditions the thread condenses along its length to form a single drop, while for others, it breaks up into several drops.

Professor Ian Hutchings, said: “We now have a regime diagram that can predict how a thread of any liquid will behave. In simple words, it provides a rule of thumb to determine whether it will form multiple drops or only one.”

Dr Rafa Castrejon Pita, co-author of the paper, said: “Almost every situation in which droplets are involved, i.e. rain, splashes in the street, printers at home, water dripping from a leaky tap, pouring honey onto toast will, to some degree, follow the findings detailed in this work.

“What is also interesting is that many living organisms have evolved certain ways to generate droplets and jets rather similar to those formed from our experimental systems and to commercial printheads.”