Research

Current Research

Our current research is focused on solving the critical energy, water and health issues in our society. We believe nature has already given us some amazing solutions. We will keep learning from nature and conducting interdisciplinary research to break the conventional academic barriers and provide new solutions to the scientific and engineering communities.

Our research is related to both science and engineering. Particularly, we have strong interests in bioinspired materials, superhydrophobic surfaces, polymer, energy transport, fluid dynamics and emerging technologies. 

We acknowledge our sponsors for their funding supports on our current research projects. 

Past research

Research achievements (summarized in 2016)

1. Created a bioinspired slippery rough surface with a broad impact

Water and energy issues are very important in our daily life. Surface design is critical for water harvesting and energy efficiency. Enhancing the mobility of water droplets (or even vapor) on rough surfaces has the potential to improve condensation heat transfer for power-plant heat exchangers, create a more efficient water harvesting system for arid regions, prevent over-icing and frosting on aircraft wings, and protect medical implants from gunk that can build up and ruin the material.

The mobility of a liquid droplet on a rough solid surface has been associated with how droplets wet the surfaces. When liquid drops are sitting on the top of the solid textures and air is trapped underneath, they are in the Cassie state. When the drops are impregnated within the solid textures, they are in the Wenzel state.

While the Cassie state has been associated with high droplet mobility and the Wenzel state with droplet pinning, our work challenges this overwhelming consensus by showing that both Cassie and Wenzel state droplets can be highly mobile on the slippery rough surfaces. The idea is inspired by pitcher plant with liquid as the lubricant to capture insects. Our surfaces were developed by engineering hierarchical nano/microscale textures and infusing liquid lubricant into the nanotextures alone. These slippery rough surfaces combine the advantages of large surface areas and slippery interfaces. We have demonstrated that our slippery rough surfaces outperform the state-of-the-art liquid-repellent surfaces in water harvesting and dropwise condensation applications.

Xianming Dai, Nan Sun, Steven O. Nielsen, Birgitt Boschitsch Stogin, Jing Wang, Shikuan Yang, and Tak-Sing Wong, “Hydrophilic Directional Slippery Rough Surfaces for Water Harvesting“, Science Advances 4(3), eaaq0919 (2018)​. [Video]
Highlighted by National Science Foundation Science360 as top story of the day.
Reported by New ScientistACS C&ENScience MagazineEurekAlert‎ (AAAS)ZME ScienceScience DailyXINHUA NET.

Birgitt Boschitsch Stogin, Shikuan Yang, and Tak-Sing Wong, “Slippery Wenzel State“, ACS Nano 9(9), 9260-9267 (2015). [Video]
Selected as one of the JALA Ten 2016, due to its deep impact on how technology is used across a wide range of disciplines.
Rank NO. 1 among the ACS Nano Most Downloaded Articles in the past 12 months.
Featured in NSF Science360 as the top story of the day.

2. Created a bioinspired hybrid material for advanced cooling

Advanced cooling technologies are highly desirable in our daily life, such as the thermal management for electronic devices, batteries and fuel cells, as well as the cooling for LED lights and photovoltaic systems. All these applications demand effective cooling to reduce the operation temperature, and in turn ensure the safety and reliability. For example, our computers require effective heat dissipations to maintain the proper functioning. One of the most important cooling technology is evaporation heat transfer.

Evaporation is a phase change heat transfer process that the liquid absorbs heat and becomes vapor. During the evaporation process, the heat can be transferred effectively by the latent heat. When the surface temperature is too high, fast evaporation will effectively take the heat away but may result in surface dryout. This needs capillary force to replenish liquid to the dry area and maintain continuous evaporation.

In this work, we developed a surface which can significantly enhance continuous evaporation and dissipate heat even when the temperature is extremely high. An ideal surface to enhance capillary evaporation must provide a large capillary force and a simultaneous small flow resistance. However, there is a trade-off between these two. Inspired by the transpiration of trees, we developed a hybrid surface with microchannels underneath and micro copper woven mesh screen on top of the channels. Such a combination can generate large capillary force and simultaneously reduce the flow resistance.

Xianming Dai, Fanghao Yang, Ronggui Yang, Yung-Cheng Lee, and Chen Li, “Micromembrane-enhanced capillary evaporation”, International Journal of Heat and Mass Transfer, 64 (2013) 1101-1108.

Xianming Dai, Levey Tran, Fanghao Yang, Bo Shi, Ronggui Yang, YC Lee, and Chen Li, “Characterization of hybrid-wicked copper acetone heat pipes”, Proceedings of the ASME/JSME 2011 8th Thermal Engineering Joint Conference (AJTEC2011), March 13-17, 2011, Honolulu, Hawaii, USA. (ISI index).

3. Carbon nanotube for enhanced boiling, evaporation and reduced friction

Thermal energy efficiency is critical in our daily life, such as the efficiency of power plants, photovoltaic systems, and renewable energy. All these applications demand high energy efficiency, particularly high heat transfer rate, which can result in considerable economic benefits and pollution prevention. To make the industrial heat exchanger small with high high efficiency, nanomaterial coatings are essential because they do not increase the volume or weight a lot.

During phase change heat transfer, the surface structure, interfacial wettability and the presence of micro/nano bubbles govern liquid replenishment and heat transfer rate. When the liquid can be continuously supplied to the heating area, efficient heat transfer can be maintained. Hydrophobic surfaces can reduce flow resistance for liquid supply, while Hydrophilic surfaces are excellent at providing driving-force for liquid flow.

We created a surface coating that combines the advantages of both hydrophobic and hydrophilic surfaces. Specifically, we treated carbon nanotubes with nitric acid to grow some hydroxyl and carboxyl functional groups. The surface chemistry of each carbon nanotube was changed from water-repellent to be partially hydrophobic and partially hydrophilic. The treated carbon nanotubes combined the unique advantages of both hydrophilic and hydrophobic surfaces. Our experimental study shows the novel nanoporous coating with composite wettability can effectively improve boiling, evaporation and drag reduction.

Xianming Dai, Xinyu Huang, Fanghao Yang, Xiaodong Li, Joshua Sightler, Yingchao Yang, Chen Li, “Enhanced nucleate boiling on horizontal hydrophobic-hydrophilic carbon nanotube coatings”, Applied Physics Letters, 102(16) (2013) 161605.

Xianming Dai, Fanghao Yang, Ronggui Yang, Xinyu Huang, William A Rigdon, Xiaodong Li, Chen Li, “Biphilic nanoporous surfaces enabled exceptional drag reduction and capillary evaporation enhancement”, Applied Physics Letters, 105(19) (2014) 191611.

4. Microjet enabled flow separation technique for electronics cooling

The increased density of electronic components has pushed heat generation and power dissipation to unprecedented levels. Current thermal management solutions are unable to limit the temperature rise of these components. People can use larger cooling devices, but this results in complex military systems because the inefficiency of existing thermal management hardware will require the use of large volume and complex cooling systems.

One solution is to integrate the three-dimensional microscale channels within the electronic components. Then pump liquid through the microchannels to cool down the high-temperature devices. However, the outlet temperature of the microchannel is much higher than that of the inlet. Also, it requires a large amount of energy to pump the liquid through the channel because the size is very small.

To resolve these problems, we exploited microjets to separate the entrance fluid flow into a mainstream and multiple bypass flows. This can induce low-temperature fluids into the downstream and thus effectively reduce the outlet temperature. In addition, the flow resistance can be significantly reduced because the flow area is increased.Furthermore, the efficiency improvements for cooling could conceivably produce a significant saving of input power.

Xianming Dai, Fanghao Yang, Ruixian Fang, Tsegaye Yemame, Jamil A Khan, Chen Li, “Enhanced single-and two-phase transport phenomena using flow separation in a microgap with copper woven mesh coatings”, Applied Thermal Engineering, 54(1) (2013) 281-288.

Fanghao Yang, Xianming Dai, Chih-Jung Kuo, Yoav Peles, Jamil Khan, Chen Li, “Enhanced flow boiling in microchannels by self-sustained high frequency two-phase oscillations”, International Journal of Heat and Mass Transfer, 58(1) (2013) 402-412.

5. Scalable nanowires for enhanced microfluidic cooling

The cooling of numerous space systems (e.g. power systems on the aircraft) is critical. To ensure the safety and reliability, the cooling device needs to dissipate a large amount of heat in a small area and maintain continuous heat dissipation. Conventional cooling technologies are designed based on the regular gravity on the earth (1g).

However, under micro-gravity in space, these techniques are not effective. Specifically, owing to the weak buoyancy effects, microgravity is not favorable to enhance boiling heat transfer because the bubbles are difficult to depart from the heating surfaces. Once the bubbles merge and grow large, the whole surface will be covered by bubbles. This in turn retards the liquid replenishment and further heat transfer.

The objective of this work is to achieve the efficient cooling of high temperature space systems. We utilized nanowires to reduce multiple flow patterns into a single annular flow, which can minimize the existence of bubbles. Without bubbles, the cooling system becomes gravity-insensitive, which is good in micro-gravity. The utilization of nanostructures can effectively enhance the heat transfer owing to the increased surface areas, and delay the liquid dryout due to the improved liquid replenishment.

Fanghao Yang, Xianming Dai, Yoav Peles, Ping Cheng, Jamil Khan, Chen Li, “Flow boiling phenomena in a single annular flow regime in microchannels (II): Reduced pressure drop and enhanced critical heat flux”, International Journal of Heat and Mass Transfer, 68 (2014) 716-724.

Fanghao Yang, Xianming Dai, Yoav Peles, Ping Cheng, Jamil Khan, Chen Li, “Flow boiling phenomena in a single annular flow regime in microchannels (I): Characterization of flow boiling heat transfer”, International Journal of Heat and Mass Transfer, 68 (2014) 703-715.

6. Fundamental study of phase change heat transfer: surface structure and interfacial wettability

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Guided by the Wenzel’s law, the apparent contact angle can be dramatically reduced via increasing the roughness of a hydrophilic surface. As a result, engineered surfaces with microscale, nanoscale, and hierarchical structures were developed to enhance boiling and evaporation. However, it is challenging to distinguish the effect of augmented surface roughness and the effect of intrinsic wettability on heat transfer.

In our study, boiling and evaporation on atomic layer deposited (ALD) hydrophilic coating were studied. The intrinsic surface wettability is critical for to delay the boiling crisis and enhance the evaporation heat transfer owing to the improved wetting effect. Understanding the effect of intrinsic critical on boiling and evaporation will be very helpful for the surface design of heat exchangers, heat pipes and other energy conversion and transport systems.

Xianming Dai, Pengtao Wang, Fanghao Yang, Xiaochuan Li, Chen Li, “Decoupling the influence of surface structure and intrinsic wettability on boiling heat transfer“. Applied Physics Letters112(25), 253901 (2018).

Xianming Dai, Mehdi Famouri, Aziz I Abdulagatov, Ronggui Yang, Yung-Cheng Lee, Steven M George, Chen Li, “Capillary evaporation on micromembrane-enhanced microchannel wicks with atomic layer deposited silica”, Applied Physics Letters, 103(15) (2013) 151602.