Abstract
The complex droplet dynamics on hot surfaces play an important role in many industrial applications involving the phase change processes, such as spray cooling, fuel injection, microfluidic manipulation, and thermal management of electronic devices. In nature, droplet manipulation is a widespread phenomenon by regulating surface wettability or topological structure. For instance, droplet rapid detachment on the superhydrophobic surface is widely observed on insect wings, plant leaves, bird, and animal furs due to large contact angle and small contact angle hysteresis. Additionally, droplet directional transport under different environments has been extensively reported on many living organisms, such as desert beetle, spider silk, cactus, pitcher plant, shorebird, and desert lizard. Considering the natural inspiration, the similarity of fluid motion, and extensive advances in scientific technologies, this thesis is to design and fabricate some novel surfaces allowing for controlling droplet dynamics at high temperatures.First, we designed a uniform surface that can achieve directional guidance of the droplet to a preferential location at high temperatures and eliminate the meticulous control of the droplet released location. The novel surface consists of regularly patterned posts with Janus-mushroom structure (JMS) in which the sidewalls of the individual posts are decorated with straight and curved morphologies. It is revealed that such structural symmetry-breaking in the individual posts leads to directional liquid penetration and vapor flow toward the straight sidewall, and also reduces the work of adhesion, altogether triggering collective and preferential droplet transport at a high temperature. By surrounding a conventional cooling surface with JMS endowed with favorable directionality, it is possible to concentrate small impacting droplets preferentially onto a localized hotspot to achieve enhanced cooling efficiency in thermal management.
Second, we fabricated a copper surface with a hierarchical structure that can largely reduce the contact time of impacting droplets at high temperatures without the requirement of a low-pressure environment or high impacting velocity. Such a unique copper surface is composed of micropore arrays, in which the sidewalls of the individual micropore are decorated with copper nanoparticles and form a self-assembled dendritic structure. The impinging droplet on the heated surface is levitated by the intensive local vapor bubbles in the micropores and spreads to a pancake shape in the air. This novel bouncing regime that allows for a rapid droplet detachment can lead to an extremely reduced contact time at high temperatures in comparison with the traditional condition. The underlying physical mechanism for this approximate pancake bouncing will not only enrich our fundamental understanding of droplet dynamics but also can find promising applications in thermal-related fields.
Third, we demonstrated a new application for viscous Leidenfrost droplets as miniaturized chemical reactors. The Leidenfrost droplets exhibit the internal convective motion and thus, can mix rapidly within hundreds of milliseconds even with small volumes. By the introduction of the Leidenfrost effect, silver nanowires and silver nanoparticles can be fabricated quickly in microdroplets including the viscous solvent. This unique mixing mechanism opens up a new approach for nanomaterial synthesis, which can be further scaled up by successive operations in microfluidic manipulation.
In summary, we systemically investigated the complex droplet dynamics and the multifunctional applications at high temperatures. First, we proposed a global uniform patterned surface with symmetry-breaking JMS to achieve directional droplet transport. The introduction of mushroom structure can further enhance the directional motion. Second, we explored a novel bouncing process on the copper surface with a hierarchical structure at high temperatures, and such a unique design can break through the limitation of droplet contact time. Finally, we indicated the successful application of Leidenfrost droplets for viscous micro-reaction. We believe that our research will yield important insights to design artificial surfaces for diverse broad applications at high temperatures.
| Date of Award | 30 Nov 2020 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Zuankai WANG (Supervisor) |