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한정훈(Jeonghoon Han),이충엽(Choongyeop Lee),남영석(Youngsuk Nam) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12
Recently, the droplet impact on solid surface is investigated over past decade because of various interests: anti-icing, Cassie to Wenzel transition, Leidenfrost effect and micro/nano-structured surface by surface modification. To reduce the contact time which affect to ice formation, the pancake shape of droplet without retracting is also reported. Droplet rebound phenomena can explain in two different well-known mechanisms: Leidenfrost effect and superhydrophobic surface which has high contact angle. The Leidenfrost effect is that the droplet impacts on superheated surfaces which spreading on gas layer by evaporation of the droplet. Meanwhile, the droplet impact experiment on superhydrophobic surface can also observe rebounded phenomena which is caused by low surface tension. In many previous studies, the experiments have been investigated in wide range of factors, for examples, changing the pitch, height, shape of micro-post and using different type of superhydrophobic coating. However, the droplet is spreading symmetrically on regular array of micro-post surfaces, so that the effect of the surface structure is same in all directions. To investigate more information of droplet spreading mechanism, we consider the hydrophobic grating surfaces and multi-scale superhydrophobic grating surfaces which have different surface tension and spreading friction according to across or parallel direction.
초발수 표면 위 마이크로 구조물의 기하학적 패턴에 따른 물 수집 성능의 영향
서동현(Donghyun Seo),이충엽(Choongyeop Lee),남영석(Youngsuk Nam) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
On superhydrophobic (SHPo) surfaces, either of two wetting states ?the Cassie state (i.e., non-wetted state) and the Wenzel state (i.e., wetted state) ? can be observed depending on the thermodynamic energy of each state and experimental conditions. Each wetting state leads to quite a distinctive dynamic characteristic of the liquid drop on SHPo surfaces and it has been of primary interest to understand or induce the desirable wetting state for relevant thermofluid engineering applications. In this study, we investigate how the wetting state of microstructured SHPo surfaces influences water harvesting performance of the SHPo surface via dewing by testing two different surface patterns of posts and grates with varying structural parameters. On grates, the observed wetting state during condensation is well described by the thermodynamic energy criteria and it is seen that the Cassie state is associated with the faster removal of smaller droplets due to the higher drop mobility compared with the Wenzel state. Meanwhile, on posts, the observed wetting state is dominantly the Wenzel state regardless of the thermodynamic energy of each state, and due to much larger pinning force of the Wenzel state the condensates are shed slowly only after they grow to a sufficiently large size. Based on a simple modeling, we attribute this difference in the droplet shedding characteristics to the different dynamic pathway from the Wenzel state to the Cassie state between posts and grates. Overall, the faster drop removal on the SHPo surface helps enhance the water harvesting performance of the SHPo surfaces by facilitating the condensation on the droplet-free area, as manifested by the best water harvesting performance of gratings on the Cassie state amongst the tested surfaces.
서동현(Donghyeon Seo),이정헌(Junghun Lee),이충엽(Choongyeop Lee),남영석(Youngsuk Nam) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
With a water shortage emerging as a world-wide issue, moisture in air has been receiving great attention as a clean and cheap water resource. In this study, we investigate how the wetting state of Cu based nano-engineered tube surfaces influences the water-harvesting performance via steam or dewing by testing bare, hydrophobic, superhydrophilic, superhydrophobic, and oil-infused superhydrophobic tube type condensers. On the basis of the force balance between the gravitational force and the pinning force, the sliding and dripping mass of condensates are quantified and the predictions were consistent with the experimental observations. It was found that the low viscosity oil-infused surface having low contact angle hysteresis exhibits the highest droplet mobility and the best water-harvesting performance in the steam condition. This is in contrast to a dewing harvesting result where the most efficient surfaces consist of bare and superhydrophilic surfaces. Despite of the similar super-saturation level, in the dewing environment, the droplet growth only depends on thermal diffusion between humid air and the substrate. Therefore the nucleation rate becomes as important as the droplet mobility in dewing harvesting. However, the saturated tiny droplets can easily stick and grow on the surface regardless of the nucleation barrier and eventually high droplet mobility becomes the most dominant factor for steam harvesting.
서동현(Donghyun Seo),오승태(Seungtae Oh),이충엽(Choongyeop Lee),남영석(Youngsuk Nam) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12
Frost formation and accumulation have widely caused serious performance degradation and economic/safety problems in a variety of industrial and residential applications including aircraft, ships, wind turbines, dams, power transmission liens, heat pumps, refrigerators and air conditioners. Recent studies have reported that oil-infused superhydrophobic (Oil-SHPo) surfaces have excellent anti-icing ability due to its high droplet mobility, but few studies have clearly demonstrated detailed mechanisms of freezing delay on the surfaces. Here, we apply the Oil-SHPo wettability on commercially available Al surfaces and investigate how such wetting property effectively suppresses ice spreading under condensation frosting condition. Observed in a microscope test setup, microscale icing behaviors on the surfaces exhibit that its high droplet mobility makes droplet distribution with small mass diffusion rate between frozen drops and liquid drops, it leads to low ice propagation velocity. In particular, it is shown that infused oil viscosity plays a critical role in condensation growth rate and small viscosity is more desirable for icephobic surfaces. Heat exchangers with the Oil-SHPo wettability exhibit high droplet removal rate thanks to the influence of its extremely low contact angle hysteresis and it leads to the best anti-icing ability when compare to other test surfaces.