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Electric charge key to control the bubbles of boiling water

Posted: 03 Nov 2015     Print Version  Bookmark and Share

Keywords:MIT  electrical switch  power plant  boiling  bubble formation 

Normally regarded as a typical, end process of boiling water, the formation of bubbles that rise from to the surface is essential to most electric power plants, heating and cooling systems, and desalination plants. Now, a team of researchers at MIT has found a way to control this process through the use of an electrical switch.

The system, which could improve the efficiency of electric power generation and other processes, is described in a paper by MIT department of mechanical engineering professor Evelyn Wang, graduate student Jeremy Cho, and recent graduate Jordan Mizerak '14, published in the journal Nature Communications.

This degree of control over the boiling process, independent of temperature, Wang said, has not previously been demonstrated despite the ubiquity of boiling in industrial processes. Other systems have been developed to control boiling using electric fields, but these have required special fluids rather than water, and a thousand-fold higher voltages, making them economically impractical for most uses.

Bubble formation

Researchers found that sections of metal can be made to either promote bubbling (the two rectangles at the edges) or to inhibit bubbling (centre rectangles), simply by switching the polarity of voltages applied to the metal. (Courtesy of the researchers)

The feat was accomplished by adding surfactants to water, essentially creating a soapy liquid. The surfactant molecules, which carry an electrical charge, can be attracted to, or repelled by, a metal surface by changing the polarity of the voltage applied to the metal. This switches the metal surface between being hydrophilic and hydrophobic, Wang explained.

Adding the surfactant causes the surface to become more hydrophobic, which increases the rate of nucleation to form bubbles. But reversing the charge on the surface causes the surface to become hydrophilic, and inhibits the formation of bubbles. The researchers found that they could achieve a tenfold change in the rate of bubble formation simply by switching the charge.

Just as condensation, such as the formation of raindrops, requires a "seed," like a dust particle, to start the process of nucleation, the bubbles formed by boiling water also require nucleation. Tiny irregularities on a metal surface can provide those nucleation points, but if the surface is hydrophilic the formation of bubbles is inhibited.

"The whole concept relies on the fact that whether a surface is hydrophobic or hydrophilic will affect the rate of nucleation," Cho stated. "If it's hydrophilic, it's very difficult to nucleate bubbles." So by switching the polarity, the rate of bubbling can be precisely controlled.

Unlike other approaches to modifying the wettability of metal surfaces, which rely on the creation of precise kinds of nanoscale textures on the surface, this system makes use of the tiny irregularities that naturally exist on a metal surface and does not require special processing.

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