Discovery could lead to new ways to create nanofibres and wires

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

Such nanothreads, 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.

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 tap 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. 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.

Researchers were surprised by the potential for practical applications. "Initially we just thought it was a new scientific discovery, which it is because it violates everything that was known," Basaran said. "This thin thread forms so slowly - which was also unexpected - that you have enough time to solidify it into a filament or wire."

Chemical engineers at Purdue have performed mathematical calculations and computer simulations to explain the phenomenon, and physicists at the University of Chicago have carried out experiments in which they have created fibres less than 100 nanometers wide.

Researchers deduced a mathematical formula that can be used to predict how long and thin the filaments will grow before they break away from the nozzle. The formula is essentially the viscosity of the outside liquid divided by the viscosity of the liquid inside the drop. This ratio of viscosities has been used experimentally to make filaments of varying lengths and widths. The greater the difference in viscosity, the thinner and longer the filaments become.

Scientists hope to eventually produce wires so thin that their diameter is smaller than the width of an electron's wavelength, which could be used to dramatically alter the flow of electricity and heat. It is possible that other researchers might use such wires to develop a new class of electronics, solid-state refrigerators, air conditioners and power generators.