Freezing droplets impact a surface by either normally sticking to it or bouncing away. Controlling this reaction is vital to many applications, including 3D printing, the spraying of certain surface coatings and the prevention of ice formation on structures such as wind turbines, airplane wings or power lines.
Presently, MIT Researchers have discovered an unexpected new twist to the mechanics involved when droplets make contact with surfaces. While most research has concentrated on the hydrophobic properties of such surfaces, it turns out that their thermal properties are also crucially important — and provide an unexpected opportunity to “tune” those surfaces to meet the exact needs of a given application.
The new results are presented today in the Nature Physics journal, in a report by MIT Associate Professor of Mechanical Engineering Kripa Varanasi, Former Postdoc Jolet de Ruiter, and Postdoc Dan Soto.
“We found something very interesting,” Varanasi explains. His team was examining a liquid’s properties of— in this case, drops of molten metal — freezing onto a surface.
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.
Kripa Varanasi, Associate Professor of Mechanical Engineering, MIT
According to conventional thinking, the way droplets acted on the two surfaces should have been similar, but rather it turned out to be considerably different.
This clip reveals the different behavior of droplets on materials that have different thermal properties. Identical droplets of molten tin impact a surface of fused silica (left) and one of zinc selenide (right). While the droplet on the left sticks to the surface, the one on the right displays fringes around the edge that show how the flattened droplet begins to curve upward and peel away. (Image: Varanasi Group/MIT)
On silicon, which conducts heat excellently, as a majority of metals do, “the molten metal just fell off,” Varanasi says. But on glass, which is a first-class thermal insulator, “the drops of metal stuck and were hard to remove.”
The finding revealed that, “we can control the adhesion of a droplet freezing on a surface by controlling the thermal properties,” of that surface, he says. “It’s a whole new approach,” to determining how liquids interact with surfaces, he adds. “It provides new tools for us to control the outcome of such liquid-solid interactions.”
To explain the variation in thermal conductivity of different materials, Varanasi offers the example of two flooring surfaces, one made of stone, and the other of wood. Even if both are at precisely the same temperature, wood will feel warmer than the stone when one walks bare feet on the wood. That is because compared to wood, stone has higher thermal effusivity (the rate at which a material can exchange heat), so it draws heat away from the feet more quickly, causing it to feel colder.
The experiments in the research were performed with molten metal, which is vital in certain industrial processes such as the thermal spray coatings that are applied to turbine blades and other machine parts. For these processes, the quality and consistency of the coatings can rely on how well each minute droplet sticks to the surface during deposition. The results probably apply to all kinds of liquids as well, including water, Varanasi states.
While coating surfaces, “the way droplets impact and form splats dictates the integrity of the coating itself. If it’s not perfect, it can have a tremendous impact on the performance of the part, such as a turbine blade,” Varanasi says. “Our findings will provide a whole new understanding of when things stick and when they don’t.”
The new insights could be practical both when it is advantageous to have droplets adhere to surfaces, such as in certain kinds of 3D printers, to help make sure each printed layer sticks very well to the previous layer, and when it is crucial to prevent droplets from sticking, such as on airplane wings in icy weather. The research could also be practical for cleaning and waste management of additive manufacturing and thermal spray procedures.
A droplet of molten tin is seen falling on a surface of silicon, left, which conducts heat well, and on glass, right, which is a thermal insulator. Under identical conditions, the solidified droplet on the silicon falls right off when the surface is tilted, whereas the droplet on glass adheres tightly to the surface. (Image: Varanasi Group/MIT)
Soto says the discovery happened when the team was analyzing the local freezing mechanism at the interface between the liquid and the substrate, using a thermal high-speed camera that exposed rapid effects during the cooling process that would not have been possible to see at longer timescales. The images revealed an advanced development of fringes around the droplets’ outer edges. “We then realized that the droplet was unexpectedly curling up and detaching from the surface as it froze,” he says. They described this occurrence as “self-peeling” of the droplets.
“The main ingredients for this phenomenon,” de Ruiter says, “are the interplay between short timescale fluid dynamics, which set the adhesion, and longer timescale thermal effects, which lead to global deformation.” The team created a design map that captures a variety of possible outcomes (self-peeling, sticking, or bouncing) in relation to key thermal properties: drop and substrate effusivities, and temperatures.
Since the degree to which droplets adhere or do not depends on the thermal properties of a material, it is possible to customize those properties based on the application, Soto says.
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.
Dan Soto, Postdoc, MIT
The sticking result can also be regulated just by altering the relative temperatures of the droplets and the surface, the team discovered. In a number of cases, these variations are counterintuitive: For instance, while one might imagine that the only way to prevent sticking of freezing droplets is by warming a substrate, the team found a new procedure, where cooling the surface can also result in the same outcome.
The study was supported by Alstom and a Rubicon fellowship from the Netherlands.