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Hydrodynamic analysis of the advancing dynamic contact angle in microtube
김설하,Tao Wang,Lei Zhang,Yuyan Jiang 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.11
We explored the hydrodynamic features of dynamic wetting both theoretically and experimentally. We studied the triple-line motions of glycerol-water solutions of various viscosities (85-456 mPa·s) in microglass tubes (300, 500 and 1000 μm in diameter). First, dynamic (advancing) contact angles were measured and compared with those of a well-known hydrodynamic model (O.V. Voinov, Hydrodynamics of Wetting, Fluid Dynamics (1976)). Second, the internal flow structures near moving menisci were visualized using micro-particle image velocimetry (μ-PIV). Several differences in flow shape (compared to those predicted by theory) were observed. Ultimately, we present a new method by which dynamic contact angles may be predicted, derived from analysis of wall shearing stress at the moving contact line to reflect on the liquid-solid interaction effect. Our analysis has the advantage of incorporating the effect of contact angle hysteresis on the dynamic contact angle. The modified approach yielded data in good agreement with our experimental results and other open-literature data. We thus fundamentally explored the hydrodynamic aspects of dynamic wetting.
STUDY OF LEIDENFROST MECHANISM IN DROPLET IMPACTING ON HYDROPHILIC AND HYDROPHOBIC SURFACES
김설하,강준용,안호선,조항진,김무환 대한설비공학회 2013 International Journal of Air-Conditioning and Refr Vol.21 No.4
Water droplets, 2mm in diameter, were allowed to fall freely onto hydrophobic and hydrophilic heated surfaces, and their impacts were imaged using high-speed cameras to investigate the droplet dynamics and heat transfer. As the heating power increased, the water droplets evaporated faster,eventually hovering over the surface due to the formation of a boiling ¯lm when the Leidenfrost point (LFP) was reached. The heat transfer from the surface into the droplet was evaluated, and LFP transition phenomena were investigated using time-resolved imaging of both side and bottom views. The hydrophilic surface showed a higher heat transfer rate and a higher LFP than the hydrophobic surface did. Furthermore, the droplet dynamics revealed very different shapes depending on the surface wettability; vigorous bubble nucleation and growth was observable for the hydrophilic surface, but not the hydrophobic surface. The rebound behavior of the droplets was analyzed based on the droplet free energy, including kinetic, potential, and surface energy terms.
Droplet impacting dynamics on wettable, rough and slippery oil-infuse surfaces
김설하,Tao Wang,Lei Zhang,Yuyan Jiang 대한기계학회 2020 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.34 No.1
In this study, we investigated droplet impact dynamics offalling water drops ( 0 D ~ 2.3 mm ) on slippery oil-infused surfaces and compared them to other features of the surfaces, to elucidate the wettability- and roughness-controlled characteristics. We prepared transparent substrates with the designed characteristics, so it would be feasible to visualize the droplet impact dynamics in detail. A wide range of impact kinetics ( ( ) 2 0 We ~ 800 / i w = rD V s ) was covered, which gave rise to several types of droplet-impact: gentle spreading, wavy (undulated fingers of spreading edges), droplet break-up, and splashing with small secondary droplets. The basic parameters of the droplet-solid interactions were measured, and events were mapped with respect to the sample surface and impact kinetic conditions. We found that, generally, surface wettability has a major influence on the triple line shape and instability during the impact and retraction process, and thus determines events in of the framework of the dynamic wetting-failure model. Furthermore, while rough conditions promote instability of the impacted droplet, slippery lubricant-infused features tend to dampen perturbations of the spreading/retracting edge.
시민 참여도 제고를 위한 정책 도구로서의 축제 평가 : 〈서울시 축제 평가제도〉를 중심으로
김설하 글로벌문화콘텐츠학회 2019 글로벌문화콘텐츠학회 학술대회 Vol.2019 No.1
서울시 축제 평가제도는 서울시가 개최하는 자체 축제, 서울시가 자치구를 지원하여 자치구에서 개최하는 축제 등 서울이라는 물리적 공간 안에서 서울시의 예산이 투입되는 행사를 대상으로 2012년 지표 설계 및 시범 평가를 거쳐 2013년부터 현재까지 매년 운영되고 있다. 이 글에서는 서울시 축제 평가제도의 체계와 지표를 중심으로 평가 제도와 서울시민의 축제 참여도의 상관관계를 분석하였다. 축제에 참여하는 시민의 역할을 크게 생산자와 향유자, 평가자로 나누어 해당 영역을 평가하는 방식을 검토하여, 그 중에서도 시민 참여도를 적극적으로 평가하는 수용자 주도성 지표가 실제로 주민 참여도를 높이는 효과를 도출하였음을 확인하였다. 서울시 축제 평가제도와 문화관광축제 평가제도의 내용 및 그 결과를 비교한 결과 시민 참여도를 평가하는 지표는 실제 축제의 성과 및 사회적 효과에 영향을 미치고 있음을 분석함으로써 서울시 축제 평가제도가 시민 참여를 견인하는 장치로서의 역할을 수행하고 있음을 고찰하였다. 이러한 변화를 주민참여 유형의 특징에 적용해 본다면, 단순히 정보를 제공하는 수동적 단계에서 주민 의견을 환류하고, 전반적인 과정과 내용을 정의하는데 적극적인 역할을 하는 능동적 단계로 변화하고 있다고 볼 수 있다. 이 과정에서 축제 평가제도는 축제에 대한 전문가의 모니터링과 자문을 넘어 주민들의 정책 과정 참여를 독려하는 도구로 활용될 수 있을 것이다.
김설하(Seolha Kim),이청화(Cheonghwa Lee),주백석(Baeksuk Chu) Korean Society for Precision Engineering 2020 한국정밀공학회지 Vol.37 No.1
In this study, slip phenomenon that occurs during trajectory tracking motion of an omni-directional mobile robot based on Mecanum wheels was analyzed. Mecanum wheels which generate the omni-directionality to the mobile robot comprise a centered rim wheel and passive sub-rollers. In forward and backward motion, they function like usual wheels to enable rolling along the ground. However, in sideways motion, they create lateral motion of the mobile robot from the rotational actuation using their peculiar structural configuration, during which slip of the sub-rollers occurs. Unnecessary over-slip of the sub-rollers causes tracking errors of the mobile robot motion. To analyze the properties and reasons for the slip phenomenon, squared and circular trajectory tacking experiments were performed. From the experiments, it was observed that sideways motion generated respectively larger tracking errors than forward and backward motion. The geometric analysis regarding the tracking error generation was discussed using the Mecanum wheel structure. Finally, it was confirmed that suspension mechanism to provide four Mecanum wheels of the mobile robot with even reaction forces on the ground is necessary.