McCarthy and his team design, build and test surfaces that are becoming increasingly better at controlling the formation and removal of vapor bubbles during the boiling process, while also delaying the onset of and undesirable condition that engineers call critical heat flux
When a liquid reaches critical heat flux a thin layer of vapor blankets the heat-transfer surface. This vapor insulates the liquid from the heat source and it drastically reduces the surface’s ability to dissipate heat to the liquid. This results in “burnout,” which means dangerous increases in surface temperature occur very rapidly. After critical heat flux and burnout occur, it is extremely difficult to re-wet the surface and reduce its temperature.
McCarthy’s goal is to create nanostructured coatings for the heat-transfer surfaces that can delay or prevent the vapor barrier from forming in the first place. The ideal structure for higher heat transfer during boiling, according to McCarthy, is one that draws in the liquid and quickly rewets when the water does transform into a vapor.
“The only way to delay CHF is to keep the surface wet at higher and higher heat fluxes.” McCarthy said. To keep the boiling surface wet, McCarthy’s team is employing a technique that’s more frequently used to keep athletes dry. Wicking, or capillary effect, is the secret behind the high performance and thermal apparel that draws moisture away from the body. This material, which keeps people cool during a workout or warm in the winter, can also keep a boiling surface wet—thus staving off critical heat flux.
The trick to making a wicking, or hydrophilic material, is strategically increasing its surface area to draw the liquid down a path toward region of lower density. Sponges do this with their pores and air pockets. McCarthy’s team is creating its own super-hydrophilic surfaces by coating them with thousands of nanostructure tendrils. This is where the viruses come in.
Viral Building Blocks
The diagram shows virus-templated nanostructures (right), including a schematic of the core shell structure (left) where the Tobacco mosaic virus is shown in green, palladium in purple, and nickel shell in orange.
Once coated, the viruses are rendered inert. What they’ve left behind is a coating of evenly spaced tendrils—“metallic grass”. This “grass” creates a capillary effect, which allows the coating to wick liquids across it and keep them in contact with the heat-transfer surface.
It requires no electricity, power, heat, or special equipment—just a series of solutions at room temperature. After the coating process is complete, the inert viruses are fully encased, resulting in a conformal coating of high surface area metallic nanostructures.
The nanostructures we build using the TMV act to stabilize the boiling process at large heat transfer rates. “These coating essentially act like a sponge, when a vapor bubble forms on the surface they wick liquid underneath it using capillary forces to delay the dry-out phenomena associated with critical heat flux. The result is a greater than three-times increase in the critical heat flux, which allows safe operation at higher and higher heat transfer rates.”
The “metallic grass” coating also results in a tripling of the efficiency of the boiling process. So if two pots of water—one with a nanostructure coating and one without—were heated to the same temperature, the pot coated with nanostructures would produce twice as much water vapor as the uncoated pot.
