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Working in the Nano-World

One of the most common pieces of equipment in any biomedical laboratory is a Coulter counter, a device which counts and characterizes individual cells in a sample by drawing a liquid, such as blood, through a small pore and measuring their electric conductance.

Duke engineer Chuan-Hua Chen believes he can not only improve upon this device’s ability to count smaller objects, but also come up with a device that can create tiny droplets of water to carry payloads, such as proteins and drug capsules. The key to Chen’s approach is to combine Coulter technology with a nanoscale electrohydrodynamic jet that reliably detects the identity of the macromolecules and controls the number of molecules carried as payload.

The National Science Foundation also believes the potential of this approach, and awarded Chen a CAREER grant in April 2009 to support his research. Chen is Assistant Professor of Mechanical Engineering and Materials Science and director of Duke’s Microscale Physiochemical Hydrodynamics Laboratory.

Chuan-Hua Chen“It is quite challenging to come up with a way to accurately quantify and deploy these macromolecules because solid-state nanopores currently in use often get clogged up,” Chen explained. “We’re going around this issue by using electric fields to create a nanoscale jet from a millimetric nozzle, a phenomenon known as the electrohydrodynamic cone-jet transition. If successful, this single-molecule deployment technique could have a big impact, especially to biomedical research and clinical analysis.”

As an example, he mentioned flow cytometry, which has been used for years to identify and sort cells in a stream of liquid. Chen believes that once the electrohydrodynamic cone-jet technique is perfected, similar studies could be performed on much smaller objects down to the nanometer-scale.

Last year, he also received funding from the NSF for an exploratory project to develop a biomimetic electrospray vapor chamber. He hopes to fashion a revolutionary chamber that completely eliminates the wick structures required in conventional vapor chambers (two-phase heat spreaders).

The novel concept is a bioinspired condenser with hydrophilic patches on superhydrophobic surfaces that mimics the outer shell of desert beetles, which have developed a way to “harvest” water droplets for its own survival. The hybrid condenser promotes preferential condensation on the hydrophilic regions, and the water condensate is returned to the evaporator by electrostatic atomization, thereby enabling a wickless vapor chamber.

desert beetleAlong the same line, Chen recently received the Ralph E. Powe Junior Faculty Enhancement Award from Oak Ridge Associated Universities to develop a beetle-inspired water management system for polymer electrolyte membrane (PME) fuel cells. The goal is to mimic desert beetle's water-harvesting system to passively manage water to prevent flooding of the hydrophobic gas diffusion layers while facilitating ionic transport in the hydrophilic electrolyte membrane.

Taking advantage of the unique properties of the desert beetle is not the only way Chen has looked to nature for inspiration in controlling minute droplets of water. The leaves of the lotus plant are essentially self-cleaning because water is actually suspended on the rough surface of the leaf.

When examined at high magnification, the lotus leaf has a surface roughness with “two-tiered” structures, the top tier made up of row after row of tiny hairy nanoscale pillars and the bottom tier made up of irregular microscale textures. As a result of nature’s design, droplets of water condensate and are held aloft from the actual leaf surface and are consequently extremely mobile, a phenomenon known as superhydrophobicity.

“After closely studying these leaves, we developed a similar two-tier structure with carbon nanotubes on silicon posts, and the engineered structure turned out to be even better at keeping a surface dry,” Chen said.

To further his interests in nanostructured interfaces, Chen received funding from the Defense Advanced Research Projects Agency (DARPA) to develop, with colleague Stefan Zauscher, Associate Professor of Mechanical Engineering and Materials Science, a vapor chamber capable of adaptively supplying water to cool hotspots in microelectronics.

Chen came to Duke after two years as a research scientist at Teledyne Scientific (then Rockwell Scientific). Prior to that, he received his undergraduate training in applied mechanics from Peking University in China. He then went to Stanford University, where he received both an M.S. and Ph.D. in mechanical engineering.