Nanotechnologists get a squirt gun, almost

It’s anything but a Super Soaker.

Propane molecules (blue) squeeze through a gold nozzle some 20 molecular diameters wide. Some molecules stick to the nozzle or boil away but most shoot off as a liquid jet. Moseler and Landman /Science

Snapshots of hypothetical jets show thinning before breakup. Contours from molecular-dynamics simulation (top) and modified hydrodynamic equation (bottom) closely match. Unmodified equation (middle) yields a much different profile. Moseler and Landman /Science

According to computer simulations by physicists at the Georgia Institute of Technology in Atlanta, a fantastically tiny squirt gun that can spit liquids a few hundred nanometers ought to work—if it can be built.

Michael F. Moseler and Uzi Landman developed the model of the miniature device to investigate a possible new technology and to confront a challenge facing scientists at the forefront of the much-anticipated nanotechnology revolution (SN: 3/1/97, p. S14: In the nanoworld, different forces prevail, compared with our everyday, macroscopic domain. Not only do intuitions of scientists and engineers fail at the nanometer scale, so do their equations.

Researchers welcome the simulations, but they also need equations, which are simpler and apply more generally, to describe submicroscopic happenings.

Moseler and Landman have done more than just simulate the movements of molecules in nanojets—liquids forced to spurt through nozzles with orifices smaller than viruses. They also created a so-called continuum description of the jets, which ignores the molecular details. They derived this description by adding the effects of random, thermal fluctuations to existing hydrodynamic equations. This modification of equations that were developed to describe macroscopic flows yielded results in line with the molecular simulations. The theorists report their findings in the Aug. 18 Science.

The newfound agreement of the two approaches means “that the powerful mathematical tools developed to solve the hydrodynamic equations can be put to use in the nanoworld,” comments Jens Eggers of the University of Essen, Germany.

On the practical side, Moseler and Landman predict that nanojets may make possible tiny fuel injectors whose smaller, more uniform droplets would burn cleaner in engines. Other minuscule machines might use nanojets to insert genes into cells or to lay down wires only a few nanometers wide in electronic circuits. “The major technological challenge will be to actually build nozzles of the required size,” Eggers remarks.

The new study shows that nanojets ought to behave differently from macroscopic ones. For instance, because thermal fluctuations add instability to the nanojets, the tiny spurts break into droplets after going only about half as far as large-scale jets would go under corresponding conditions, Moseler says. That’s why water nanojets are unlikely to become the nanoworld’s answer to Super Soaker squirt guns.

In another example of peculiar nanoscale effects, the spurting liquid becomes a plug. When the researchers simulated liquid propane gushing from a gold nanonozzle, they found that the fuel tended to stick to the outside of the nozzle, shutting off further flow.

By simulating heating of the nozzle or the presence of a slick exterior coating, the researchers fixed that problem. Still, the unexpected effect highlights the unfamiliar territory of the nanoworld, Landman says. There, surface forces, such as the attractive tug between the gold atoms and the propane molecules, become much more important than they are in the macroscopic world.

Next to come in the nanojet investigations are water, molten metals, silicon, and polymers, and liquid biomolecules. No one yet knows how those materials might behave, Landman notes.

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