7/30/2019 Julia Stackler
Written by Julia Stackler
Yan, who came from China as a visiting scholar in December 2016 as part of the Chinese Scholarship Council program, then returned to Illinois to study as a postdoc under Miljkovic, has focused his research on functional surfaces, condensation heat transfer, and droplet jumping.
“I will never forget how exciting it was when I captured my first high-speed video of droplet coalescence, jumping, and oscillation, or how inspiring Professor Miljkovic and Dr. Sett (another postdoc in the lab) were when we discussed potential projects after I arrived. I felt truly lucky to work under the guidance of Professor Miljkovic, who has always been supportive of my research ideas. I also appreciate the great help from my collaborators in MechSE and the support from my PhD advisors Drs. Huang and Chen at Tsinghua University when I was working on these papers as a visiting PhD student,” said Yan.
Abbreviated abstracts of the three publications are noted below.
“Droplet Jumping: Effects of Droplet Size, Surface Structure, Pinning, and Liquid Properties,” January 2019
Coalescence-induced droplet jumping has the potential to enhance the efficiency of a plethora of applications. Although binary droplet jumping is quantitatively understood from energy and hydrodynamic perspectives, multiple aspects that affect jumping behavior remain poorly understood. In this study, Yan and colleagues developed a visualization technique utilizing microdroplet dispensing to study droplet jumping dynamics on nanostructured superhydrophobic, hierarchical superhydrophobic, and hierarchical biphilic surfaces. They developed a droplet mismatch phase diagram showing that jumping is possible for droplet size mismatch up to 70%. On the hierarchical superhydrophobic surface, jumping behavior was dependent on the ratio between the droplet radius and surface structure length scale. Surface structure length scale effects were shown to vanish for large droplets. On the hierarchical biphilic surface, similar but more significant scattering of the jumping velocity and direction was observed. Droplet-size-dependent surface adhesion and pinning-mediated droplet rotation were responsible for the reduced jumping velocity and scattered jumping direction. Furthermore, droplet jumping studies of liquids with surface tensions as low as 38 mN/m were performed, further confirming the validity of inertial-capillary scaling for varying condensate fluids.
“Atmosphere-Mediated Superhydrophobicity of Rationally Designed Micro/Nanostructured Surfaces,” April 2019
Superhydrophobicity – with its significant potential in self-cleaning, anti-icing and drag reduction surfaces, energy-harvesting devices, antibacterial coatings, and enhanced heat transfer applications – can be obtained via the roughening of an intrinsically hydrophobic surface, the creation of a re-entrant geometry, or by the roughening of a hydrophilic surface followed by a conformal coating of a hydrophobic material. Intrinsically hydrophobic surfaces have poor thermophysical properties, such as thermal conductivity, and thus are not suitable for heat transfer applications. Re-entrant geometries, although versatile in applications where droplets are deposited, break down during spatially random nucleation and flood the surface. Chemical functionalization of rough metallic substrates, although promising, is not utilized because of the poor durability of conformal hydrophobic coatings. Here Yan and colleagues develop a radically different approach to achieve stable superhydrophobicity.
By utilizing laser processing and thermal oxidation of copper (Cu) to create a high surface energy hierarchical copper oxide (CuO), followed by repeatable and passive atmospheric adsorption of hydrophobic volatile organic compounds (VOCs), Yan showed that stable superhydrophobicity with apparent advancing contact angles ≈160° and contact angle hysteresis as low as ≈20° can be achieved.
Yan and colleagues exploited the structure length scale and structure geometry-dependent VOC adsorption dynamics to rationally design CuO nanowires with enhanced superhydrophobicity. To gain an understanding of the VOC adsorption physics, they utilized X-ray photoelectron and ion mass spectroscopy to identify the chemical species deposited on surfaces in two distinct locations: Urbana, IL, United States and Beijing, China. To test the stability of the atmosphere-mediated superhydrophobic surfaces during heterogeneous nucleation, they used high-speed optical microscopy to demonstrate the occurrence of dropwise condensation and stable coalescence-induced droplet jumping.
“Hierarchical Condensation,” July 2019
With the recent advances in surface fabrication technologies, condensation heat transfer has seen a renaissance. A significant bottleneck to achieving higher condensation efficiencies is the difficulty of shedding sub-10 μm droplets due to the increased role played by surface adhesion and viscous limitations at nanometric length scales. To enable ultra-efficient droplet shedding, Yan and colleagues demonstrated hierarchical condensation on rationally designed copper oxide microhill structures covered with nanoscale features that enable large (~ 100 µm) condensate droplets on top of the microstructures to co-exist with smaller (< 1 µm) droplets beneath.
Using high-speed optical microscopy and focal plane shift imaging to show that hierarchical condensation is capable of efficiently removing sub-10-μm condensate droplets via both coalescence and divergent-track-assisted droplet self-transport towards the large suspended Cassie-Baxter (CB) state droplets, which eventually shed via classical gravitational shedding and thereby avoid vapor side limitations encountered with droplet jumping.
To elucidate the overall heat transfer performance, an analytical model was developed to show hierarchical condensation has the potential to break the limits of minimum droplet departure size governed by finite surface adhesion and viscous effects through the tailoring of structure length scale, coalescence, and self-transport.
Damena Agonafer, Assistant Professor of Mechanical Engineering and Materials Science at Washington University in St. Louis, highlights the significance of Yan’s research. “Dr. Yan’s work is at the intersection between material science, fluid dynamics, and thermal science. Studies on hydrophobic surfaces have been around for several decades, but the durability of these engineered surface features and its impact on the thermofluidic characteristic of complex phase change phenomenon remain important fields to be explored. Dr. Yan’s exceptional work, well addressing these research questions, can facilitate new concepts to the rational design of advanced surface engineering for enhancement in phase change heat transfer. His work will have a large impact on applications related to cooling micro- and power-electronic systems.”
Miljkovic agrees. “Xiao is a very talented researcher and a great leader. I think his case is a great example of how international student exchanges can really foster great collaborations. He came during his PhD studies to my lab and brought interesting laser-textured samples from China that we were not developing here in MechSE. Our group’s strength was characterization, while his adviser’s lab strength was manufacturing. Xiao was the glue between our two groups. Through his hard work and innovative mindset, he was able to conduct research over an 18-month timespan that culminated in these three great articles. After his 14-month visit ended in 2018, I had no doubt that he should return as a postdoctoral scholar after defending his PhD thesis and I jumped at the opportunity to hire him back. He will be a great researcher one day, hopefully in the U.S. where he can contribute to our economy and global lead in science,” he said.