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John O | September 2017

Study reveals new way to enhance or reduce adhesion of freezing droplets


researchers at the massachusetts institute of technology (mit) in cambridge, mass. have discovered a new aspect of the mechanical process in which freezing droplets react when impacting a surface that focuses on the thermal property of the surface and could lead to engineered surfaces that are tuned to react to droplets in a particular way.

 


mit researchers have found a surprising new twist to the mechanics involved when
droplets come in contact with surfaces. pictured here is a microscopic top view of a droplet.
(varanasi group/mit)

 

according to a report from mit, previous studies of the interaction between droplets and a surface focused on the hydrophilic properties of the material, but mit scientists used two substrates with similar wetting properties, but different thermal properties, and discovered a “dramatic” difference in how drops of molten metal reacted on the surface.

 

on silicon, which has high thermal conductivity, the metal fell off, but on glass, which is a thermal insulator, the metal drop stuck and was hard to remove.

 

“the experiments in the study were carried out with molten metal, which is important in some industrial processes such as the thermal spray coatings that are applied to turbine blades and other machine parts,” the article explained. “for these processes, the quality and uniformity of the coatings can depend on how well each tiny droplet adheres to the surface during deposition. the results likely apply to all kinds of liquids as well, including water.”

 

another industry that will benefit from this discovery is 3-d printing where it is important to make sure that each printed layer adheres to another layer.

 

“the discovery came about when the team was studying the local freezing mechanism at the interface between the liquid and the substrate, using a thermal high-speed camera that revealed rapid effects during the cooling process that would have been impossible to see at longer timescales,” the article continued. “the images showed a progressive development of fringes around the droplets’ outer edges.”

 

the article added, “the team developed a design map that captures different possible outcomes (sticking, self-peeling, or bouncing) in terms of key thermal properties: drop and substrate effusivities, and temperatures. since the degree to which droplets stick or don’t depends on a material’s thermal properties, it’s possible to tailor those properties based on the application.”

 

in addition, the researchers found that changing the relative temperature of the droplets and the surface can also control whether droplets stick. they found that cooling the substrate prevented sticking of droplets.

 

the research was recently published in nature physics. the abstract read:

 

“whether an impacting droplet sticks or not to a solid surface has been conventionally controlled by functionalizing the target surface or by using additives in the drop.

 

“here we report on an unexpected self-peeling phenomenon that can happen even on smooth untreated surfaces by taking advantage of the solidification of the impacting drop and the thermal properties of the substrate.

 

“we control this phenomenon by tuning the coupling of the short-timescale fluid dynamics—leading to interfacial defects upon local freezing—and the longer-timescale thermo-mechanical stresses—leading to global deformation. we establish a regime map that predicts whether a molten metal drop impacting onto a colder substrate will bounce, stick or self-peel.

 

“in many applications, avoiding adhesion of impacting droplets around designated target surfaces can be as crucial as bonding onto them to minimize waste or cleaning.

 

“these insights have broad applicability in processes ranging from thermal spraying and additive manufacturing to extreme ultraviolet lithography.”

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