Nanowires drop out of fluid research
Nov 17, 2003
Researchers in the US and the UK have made a breakthrough discovery about the behaviour of bubbles and droplets in fluids. The finding could have applications in areas such as microtechnology, nanostructures and medicine (P Doshi et al. 2003 Science 302 1188)
There is a widely accepted universal rule which states that drops and bubbles always break away from a nozzle in the same way, regardless of the liquid or gas. As the drop forms, it is attached to the nozzle by a thin segment of liquid or gas. This segment grows thinner before breaking at a single point, allowing the drop to fall away from the nozzle.
Now, Osman Basaran at the University of Purdue and colleagues have discovered an exception to this rule. While studying how liquid drops and gas bubbles are formed by nozzles - such as those in inkjet printers - the researchers found that for a nozzle immersed in a viscous liquid such as silicone oil, water drops formed differently to the way they would in air. The drops formed much more slowly and the segment of liquid between the growing drop and the tip of the nozzle grew longer than it would in air, before cutting away from the nozzle at two points rather than one.
“In this special case, this region doesn’t shrink to a point and break off like it ordinarily would,” said Basaran. “Mathematically we say that it ‘remembers’ its initial state, which is very unusual.”
The liquid separated from the nozzle at both the place where the drop formed and at a point nearer to the nozzle. This left a drop of liquid, along with an extremely thin liquid thread. “This thin thread forms so slowly that you have enough time to solidify it into a filament or wire,” said Basaran. By adding a prepolymer to the water and then exposing the liquid to light to bring about photopolymerization, the scientists were able to create solid fibres less than 100 nanometres wide.
The researchers calculated that the length and width of the fibre depends on the viscosity of the fluid outside relative to the viscosity of the fluid inside the drop - the greater the difference in viscosity, the thinner and longer the filaments.
“One can make every drop very tiny - as small as pico- and even femto-litres - and identical, even if you were to produce a trillion drops in a row from the same nozzle,” said Basaran. “The drops have great potential in advanced applications. These include microreactors to make ceramic particles, microcapsules for controlled-release applications in medicine, and to produce nanostructures.”
About the author
Liz Kalaugher is Editor of Nanotechweb.org