Drop theory could aid nanowire production

US researchers have made a discovery about the formation of drops that could lead to new methods for making threads, wires and particles only a few nanometres wide.

A research team led by engineers at Purdue University and physicists at the University of Chicago have made a discovery about the formation of drops that could lead to new methods for making threads, wires and particles only a few nanometres wide.

Such nano-threads, wires and particles could, in turn, have numerous applications, including new kinds of composite materials, electronic circuits and pharmaceutical products, said Osman Basaran, a professor in Purdue’s School of Chemical Engineering.

The researchers made the discovery while studying how liquid drops and gas bubbles are formed by nozzles, such as those in inkjet printers. A widely accepted universal rule holds that, no matter what the liquid or gas is made of, drops and bubbles always break away from a nozzle the same way: As the drop is forming, it is attached to the nozzle by a thin segment of liquid or gas. This connecting segment grows progressively thinner, and as its width gets closer and closer to zero it breaks at a single point and the drop falls away from the nozzle.

‘This breaking region, which I and others have been studying, has some really amazing properties,’ Basaran said. ‘It always breaks the same way, no matter how big a nozzle is or how fast you are flowing the liquid.’

The researchers, however, have discovered an exception to this no-longer universal rule, Basaran said. Drops usually form in air, which has much lower viscosity than liquid. For example, water dripping from a faucet is more viscous than the surrounding air. If, however, a nozzle is immersed into a sticky liquid like honey or silicone oil, which is thousands of times thicker than water, the water drops form differently than they would in air.

‘First of all, the drops take much longer to form,’ Basaran said.

Moreover, instead of abruptly breaking off, the segment of liquid between the forming drop and the nozzle’s tip continues to grow into a narrow thread and eventually becomes much longer than it would if the drop were forming in air.

‘In this special case, this region doesn’t shrink to a point and break off like it ordinarily would,’ Basaran said. ‘Mathematically, we say that it ‘remembers’ its initial state, which is very unusual.’

Rather than separating from the nozzle at a single point, the liquid cuts away in two places: at the point where the drop has formed and at a point closer to the nozzle. The drop falls away, but an extremely thin thread of liquid or gas also separates from the nozzle.

If the liquid contains certain chemicals, the threadlike segment can be quickly solidified by exposing it to ‘photo-polymerising’ light, creating extremely thin filaments or fibres of uniform thickness.

Chemical engineers at Purdue have performed mathematical calculations and computer simulations to predict how long and thin the filaments will grow before they break away from the nozzle, and physicists at the University of Chicago have carried out experiments in which they have created fibres less than 100 nanometres wide.

The work is continuing at Purdue, with funding from the Department of Energy and private corporations.