Oct 5, 2017 | By Benedict

Researchers at MIT have found a way to control the activity of freezing droplets hitting surfaces, allowing them to determine whether a droplet will stick, bounce, or “self-peel.” The discovery could have an impact on future 3D printers and other technologies.

Being able to tune the properties of a surface can be beneficial for many reasons. You might, for example, want to create an extremely smooth surface to reduce the friction of a mechanical component, or adjust a surface’s thermal conductivity for a kitchen utensil.

For a group of MIT researchers, however, the ability to precisely control the properties of surfaces has allowed them to do something very unusual: control the behavior of impacting droplets.

Droplets in the process of freezing tend to do one of two things when they hit a surface: stick, or bounce off. And in certain engineering applications—metallurgy, some 3D printing processes, and even de-icing—the outcome of this binary can be hugely significant. The problem is that it’s hard to predict or control which way the droplets will go.

Until now, studies into the behavior of such droplets has tended to focus on the hydrophobic properties of the surfaces on which they fall: in other words, whether these surfaces tend to attract or repel water.

But the MIT researchers have another angle: they are looking at the thermal properties of those surfaces, believing that the “tuning” of the thermal properties of those surfaces could allow the control of droplets for various applications.

“We found something very interesting,” said MIT associate professor of mechanical engineering Kripa Varanasi. “We had two substrates that had similar wetting properties [the tendency to either spread out or bead up on a surface] but different thermal properties.”

To the researchers’ surprise, and contrary to common assumptions, the two substrates behaved in very different ways.

The molten metal droplets “fell off” the silicone substrate, but stuck fast to the glass surface. Since glass is a good thermal insulator, the experiment led the researchers to believe that the adhesion of a droplet can be controlled by adjusting the thermal properties of its impacting surface.

“It provides new tools for us to control the outcome of such liquid-solid interactions,” Varanasi said.

By thermal properties, the researchers aren’t just talking about temperature. Rather, factors like thermal effusivity—the rate at which a material can exchange heat—play a big part.

Thermal effusivity can be explained with an example of floor surfaces: it feels colder to step on a stone floor than a wooden one, even if the temperatures of each are identical. This is because the stone surface, with a higher thermal effusivity, draws heat away from your foot faster.

And the behavior of the droplets is similar. When they were studying the local freezing mechanism at the interface between the liquid and the substrate, the researchers noticed a progressive development of fringes around the droplets’ outer edges.

This was all picked up on a thermal high-speed camera, which showed “that the droplet was unexpectedly curling up and detaching from the surface as it froze.”

The researchers have coined the term “self-peeling” to describe this curling up and detaching phenomenon.

“The main ingredients for this phenomenon are the interplay between short timescale fluid dynamics, which set the adhesion, and longer timescale thermal effects, which lead to global deformation,” explained former postdoc Jolet de Ruiter.

Knowing this, the researchers were able to put together a “design map” that outlines how key thermal properties—temperature, drop and substrate effusivities, etc.—affect whether a droplet will stick, bounce, or “self-peel.”

The MIT researchers’ main experiments involved studying the behavior of molten metal droplets as they hit various surfaces. This was a particular area of interest because industrial processes involving the deposition of metal spray coatings depend on understanding and controlling the adhesion of metal droplets.

“These insights have broad applicability in processes ranging from thermal spraying and additive manufacturing to extreme ultraviolet lithography,” the researchers explain in their research paper.

“The way droplets impact and form splats dictates the integrity of the coating itself,” Varanasi said. “If it’s not perfect, it can have a tremendous impact on the performance of the part, such as a turbine blade. Our findings will provide a whole new understanding of when things stick and when they don’t.”

To make practical use of their findings, the researchers envision a system that would enable the instant adjustment of surface properties to make droplets either stick or bounce. Sometimes, this would be as simple as adjusting the temperature, though more complex adjustments could also be made.

“We can imagine scenarios where thermal properties can be adjusted in real time through electric or magnetic fields, allowing the stickiness of the surface to impacting droplets to be adjustable,” said postdoc Dan Soto.

The research paper, “Self-peeling of impacting droplets,” has been published in the journal Nature Physics.



Posted in 3D Printing Technology



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