Jumping droplets take a lot of heat, as long as it comes in a cool way

Microscopic water droplets jumping between surfaces that repel and attract moisture could hold the key to a wide array of more energy efficient products, ranging from large solar panels to compact laptop computers.

Duke University engineers have developed a new way of producing thermal diodes, devices which regulate heat to preferentially flow in a certain direction, effectively creating a thermal conductor in the forward direction and an insulator in the reverse direction. While thermal diodes can be made from solid materials, these solid-state diodes are not nearly as effective as "phase-change" thermal diodes that rely on vaporization and condensation to transport heat.

These phase-change diodes can transfer over a hundred times more heat in the forward direction compared to the reverse, but with major limitations -- they are dependent on gravity or restricted by a tubular configuration. These limitations severely constrain the application of phase-change thermal diodes, for example, in mobile electronics which require orientation independence or solar panels which require a large surface area.

The Duke engineers believe they have figured out a way to solve these limitations to existing thermal diodes by exploiting self-propelled water droplets, which can jump from a superhydrophobic (highly water-repellent) surface to a superhydrophilic (highly absorbent) surface, but not the other way around. The results of the Duke experiments were published online in the journal Applied Physical Letters.

Chuan-Hua Chen, assistant professor of mechanical engineering and materials science at Duke's Pratt School of Engineering, and his research group was the first to actually videotape the self-propelled jumping motion of water droplets on a superhydrophobic surface. They found that the droplets literally jumped straight up and off the surface. In their current experiments, a superhydrophilic plate was placed opposite to the superhydrophobic one, creating an asymmetry crucial for the directional transport of heat in their thermal diode.

"When the superhydrophobic surface is colder than the superhydrophilic surface, the heat transport is very effective with phase-change processes, much like sweat taking away body heat; when the superhydrophobic surface is hotter, the heat flow is blocked and the diode behaves like a double-paned window," said Chen. "Because the jumping droplets in our system are very small, gravity has negligible effect on them. Therefore, devices based on this approach can be oriented in any direction without the need to worry about gravity."

Furthermore, Chen said, this approach can be easily scalable, which means technology based on this design can be used for thermal management of devices as small as computer chips and as large as building roofs. The jumping-drop approach uniquely combines large-area scalability, orientation independence, and effective thermal rectification into one device. This combination of properties is extremely useful for thermal diodes but has remained elusive until Chen's invention.

Typical phase-change thermal diodes rely on evaporating water to transfer heat from one surface to another, with gravity pulling the subsequent condensate down to restart the cycle again. For example, these so-called thermosyphons are in use in the Alaskan oil pipeline, in order to keep the heat from the pipes from melting the permafrost.

Chen believes that this new approach could make thermal diodes more practical and effective for a variety of applications. These applications range from energy-efficient solar panels to smart "skins" of thermally adaptive buildings. For example, in the summertime a diode panel on a building could let heat escape out but prevent heat from creeping in. In space vehicles, thermal diodes can be used to regulate diurnal thermal fluctuations in the outer space, or even to harvest solar energy for powering satellites.

Chen's research is supported by the Defense Advanced Research Projects Agency. Other Duke members of the team are and Yuejun Zhao.

Story Source:

The above story is reprinted from materials provided by Duke University. The original article was written by Richard Merritt.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Jonathan B. Boreyko, Yuejun Zhao, Chuan-Hua Chen. Planar jumping-drop thermal diodes. Applied Physics Letters, 2011; 99 (23): 234105 DOI: 10.1063/1.3666818

Note: If no author is given, the source is cited instead.

When it comes to churning out electrons, metal glass beats plastics

 Field emission devices, which produce a steady stream of electrons, have a host of consumer, industrial, and research applications. Recent designs based on nanotubes and other nanomaterials embedded in plastics show initial promise, but have a number of drawbacks that hinder their wide-scale application.

The embedded nanotubes, which serve as the source for the electrons, also enable the normally inert plastic to conduct electricity. This has the desired effect of producing a versatile and easily manufactured field emission device. But since plastics are, by nature, poor conductors of electricity, they require a high concentration of nanomaterials to function. Plastics also have low thermal stability and do not hold up well under the excess heat produced by prolonged operation.

A team of researchers from Monash University in Australia, in collaboration with colleagues from CSIRO Process Science and Engineering, has developed a promising and easily manufactured replacement for plastics: amorphous bulk metallic glass (ABM). These ABM alloys form amorphous materials as they cool, giving them more of a glass-like behavior. In a paper accepted for publication in the AIP's journal Applied Physics Letters, the researchers used an alloy made from magnesium, copper, and gadolinium.

This metallic glass has many of plastics' desirable features. It can conform to a variety of shapes, be produced in bulk, and serve as an effective matrix for the nanotubes. Besides its high conductivity, the metallic glass' highly robust thermal properties mean that it can withstand high temperatures and still retain its shape and durability. According to the researchers, these advantages, alongside excellent electron emission properties, make these composites one of the best reported options for electron emission applications to date.

Though other composites of bulk metallic glass and carbon nanotubes have been reported before, this is the first time that such a system is being used for a functional device, such as for field emission. Electron microscopes, microwave or X-ray generation, nano-electronics, and modern display devices are all examples of the potential applications of this technology, the researchers note.

Story Source:

The above story is reprinted from materials provided by American Institute of Physics.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Pejman Hojati-Talemi, Mark A. Gibson, Daniel East, George P. Simon. High performance bulk metallic glass/carbon nanotube composite cathodes for electron field emission. Applied Physics Letters, 2011; 99 (19): 194104 DOI: 10.1063/1.3659898

When the fat comes out of food, what goes in?

In the article Melody Bombgardner, C&EN Senior Business Editor, explains that food processors usually face the problem of reproducing the texture or "mouth feel" of products that have cut back on fat, sugar and gluten. More and more of these products are appearing on supermarket shelves in response to changing preferences of health-conscious consumers. Food companies are in a quandary in selecting replacements, because of a parallel consumer backlash against products with long complicated lists of ingredients with the names of tongue-twisting chemical compounds.

The article describes how a host of ingredients derived from Mother Nature, are assuming increasingly important roles in giving those processed foods a satisfying taste. It includes a "mouth map" used to help formulate "light" foods so that they taste like the full-fat versions. The article also features one sidebar on natural food ingredients used to give processed foods a satisfying texture and another on food ingredients that do double-duty as ingredients in toothpastes, shampoo, skin creams, and even oil and gas drilling.

More information: Call In The Food Fixers - http://cen.acs.org/articles/89/i44/Call-Food-Fixers.html

Provided by American Chemical Society (news : web)

Hollywood Comes To ACS At #ACSAnaheim

Hollywood writers brought a touch of glamour to a standing-room-only symposium at the ACS national meeting yesterday. Writers for “Breaking Bad,” “Eureka,” “House M.D.” and other TV series admitted they found their audience of chemists intimidating but with self-deprecating good humor shared their philosophy for trying to make their shows scientifically sound.

“Breaking Bad” follows a high school chemistry teacher dying of lung cancer who cooks and sells crystal meth to support his family after his pending death. Moira Walley-Becket, one of the show’s seven writers, said that “getting the science right is of the utmost importance to us.” After all, she noted, “we need to know how to dissolve a body in acid.”

She said the writers turn for help to “the brilliant and tolerant” Donna J. Nelson, a chemistry professor at the University of Oklahoma, in Norman. Nelson volunteered for the gig after reading in C&EN that the show had to do its research on the Internet because it couldn’t afford a paid science adviser. Here’s a typical knotty problem: “Using the P2P method, how much meth could you synthesize with 30 gallons of methylamine?” (Answer: 223 lbs.)

Other scientists have found their way to Hollywood through the Science & Entertainment Exchange, a National Academy of Sciences program that connects entertainment industry professionals with scientists and engineers to help bring cutting-edge science to their stories.

Kevin R. Grazier, a research scientist at the Jet Propulsion Lab who clearly relishes his role as science adviser to “The Zula Patrol,” “Battlestar Galactica,” and other TV series, conceded that many scientists hesitate to work in Hollywood because it’s perceived as shallow. But as ACS President Nancy B. Jackson noted in her introduction to the symposium, there are many innovative ways of communicating with the public about science, including storytelling.