Miljkovic, UAE collaborators uncover phenomenon to enhance heat transfer performance
What began as an idea explored at a National Academy of Sciences workshop in November 2017 evolved into an international collaboration between engineers from MechSE and Khalifa University in Abu Dhabi, UAE.
MechSE Associate Professor Nenad Miljkovic and Khalifa’s Tiejun Zhang, an Associate Professor in Mechanical Engineering, along with postdoctoral associate Soumyadip Sett from MechSE and graduate students Qiaoyu Ge, Afika Raza, and Hongxia Li from Khalifa, have published new research, “Condensation of Satellite Droplets on Lubricant-Cloaked Droplets,” in ACS Applied Materials and Interfaces, one of the top journals in interfacial science.
The researchers found that condensation on lubricant-infused micro- or nano-textured superhydrophobic surfaces exhibits remarkable heat transfer performance due to the larger surface area available for the vapor to condense. The infused lubricant within the surface microstructures creates a liquid-liquid interface for the condensed water droplets, making them slip and be easily removed. Once removed and the condenser surface area cleared, vapor can come directly in contact, leading to higher rates of condensation.
When a low surface tension lubricant is used—such as Krytox oil, ionic liquid, and dodecane—it can spread on the condensed droplet and “cloak” it. Their findings describe a previously unobserved condensation phenomenon of satellite droplet formation on lubricant-cloaked water droplets using environmental scanning electron microscopy (ESEM). Using a novel method based on Raman spectroscopy, they found that the presence of these satellite droplets confirms the cloaking behavior of common lubricants on water. The spreading dynamics of cloaking and non-cloaking lubricants on water droplets show that ionic liquid has the capability to mobilize water droplets spontaneously.
Additionally, they observed satellite droplets on BMIm ionic liquid-infused surfaces—an unexpected phenomenon because BMIm was used in previous reports as a lubricant to eliminate cloaking during water condensation. They believe this is due to the rapid spreading during initial condensation, compared to the amount of time required for the lubricant to dissolve in water.
“Usually, cloaking of the condensate droplet is not favored during condensation since the cloaked lubricant layer creates additional thermal resistance between the vapor and the droplet, which tends to decrease the rate of condensation and heat transfer. However, as we see in this work, the cloaked lubricant layer in fact acts as an additional surface for vapor to condense, leading to the formation of satellite droplet. This increases the overall surface for vapor to condense, and may contribute to heat transfer enhancement, something to explore in the future,” Sett said.
Miljkovic indicated there are several applications potentially impacted by their findings.
“Due to its strong liquid-repellent properties, lubricant-infused surfaces are finding its way in several applications including enhanced condensation, anti-icing, anti-fouling, self-healing and in biomedical devices. Recently, we utilized these novel surfaces to attain 100% enhancement in heat transfer performance by promoting dropwise condensation of low surface tension fluids ethanol and hexane. However, one of the main questions is how the infusing lubricant affects condensation performance. This work throws considerable light on cloaked condensate droplet dynamics, and how it can affect heat transfer performance, suggesting promising guidelines for enhancing the efficiency of such operations,” he said.
This is Miljkovic’s and Zhang’s first publication sponsored by the Arab-American Frontiers Fellowship, with at least two more papers in development.