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Narayan Pandurang Sapkal,박수청,Yeon-Won Lee,Dong In Yu 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.11
Droplet splashing phenomena are observed experimentally on the welldesigned hydrophobic micro and micro-/nano-textured surfaces. The critical Weber numbers (We cr ) for splashing are investigated by considering the geometrical surface conditions. The splashing was facilitated with large micropillar spacing and diameter and suppressed with small ones. Large pillar spacing and diameter enabled easy penetration of liquid by reduced capillary force and increased the outlet of airflow. This air-liquid velocity difference creates instability at the edge of the spreading droplet, thereby generating splashing based on the Kelvin-Helmholtz instability mechanism. Besides, earlier splashing was observed on micro/nano textures than on microtextured surfaces. Since the impacting droplet could not penetrate the nanopillars due to higher capillary pressure and slip boundary condition formation, it reduces airflow friction. Hence an increase in the air-liquid velocity ratio renders splashing.
Narayan Pandurang Sapkal,이연원,Su Cheong Park,유동인 대한기계학회 2023 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.37 No.7
Critical Weber numbers (We cr ) for droplet splashing are investigated by considering the geometrical morphologies of hydrophobic micro/nano-textured surfaces and the viscosity of lubricants. A decreasing trend of We cr with respect to pillar spacing is evident for all textured and lubricant-infused surfaces (LISs), but it is independent of the pillar diameter effect. Additionally, the overall We cr of the LIS exceeded that of the micro/nano-textured surfaces. The results show that the early droplet splash on micro/nano-textures may be attributed to the slip condition, decreasing the friction loss of airflow and increasing U air /U 0 and K-H instability. Moreover, the slip velocity on the LISs decreases as the lubricant viscosity increases in the order u slip (101) > u slip (105) > u slip (107). Therefore, an increase in slip velocity facilitates earlier droplet splashing on low-viscosity LIS101 than on high-viscosity LIS105 and LIS107. Furthermore, the droplet retraction behavior beyond the maximum spreading varied significantly based on the surface wetting characteristics.
소수성 미니채널 내 물-에탄올 혼합 액적 슬러그 거동에 따른 접촉선에서의 압력강하에 대한 실험적 연구
서경덕,Narayan Pandurang Sapkal,유동인 한국기계기술학회 2019 한국기계기술학회지 Vol.21 No.4
At a two phase flow, according to gas and liquid phase flow rates, various flow regimes are developed such as bubbly, slug/plug, churn, annular, droplet flow and so on. At a two phase flow in small scaled channels, among various flow regimes, the intermittent flows such as bubbly, slug/plug flow are developed in the broad regions of two phase flow pattern map. In particular, the flow regimes are influenced by surface wettability. In a case of slug flow in hydrophobic small scaled channels, gas and liquid phases are perfectively separated by interfaces and contact line. The pressure drop of the two-phase flow is largely generated at moving contact line. Therefore, to well design two-phase flow system with small scales, it is important to estimate the pressure drop at moving contact line. In this study, on the basis of previous research, the pressure drop at moving contact line is experimentally measured for a various fluids (0-40% water-ethanol mixtures). And, the previous correlation to estimate pressure drop at moving contact line is verified by experimental data. In an addition, we discuss interfacial broken phenomena of slug flow in a minichannel. (D=1.555mm).
친수성과 소수성이 복합된 마이크로/나노 혼합 구조 표면에서의 수조비등 연구
조학래(Hak Rae Cho),김영현(Young Hyun Kim),장진규(Jin Gyu Jang),Narayan Pandurang Sapkal,박수청(Su Cheong Park),주형민(Hyeong Min Joo),유동인(Dong In Yu) 대한기계학회 2020 대한기계학회 춘추학술대회 Vol.2020 No.12
Surface roughness and wettability is important to enhance not only critical heat flux (CHF) but also boiling heat transfer coefficient (HTC). Generally hydrophilic micro/nano structures contribute on enhancing liquid supply to dry spot and delay CHF and hydrophobic surface contribute on promoting nucleation and enhancing HTC. Therefore, excellent pool boiling performance can be achieved by combining hydrophilic structures and hydrophobic structures. In this study, we fabricated hydrophilic micro pillar (MP) surface, biphilic micro pillar (BMP) surface, hydrophilic micro/nano pillar (MNP) surface and biphilic micro/nano pillar (BMNP) surface. To investigate the effect of surface roughness and heterogeneous wettability on critical heat flux phenomena, pool boiling experiments were conducted using fabricated surfaces and bare surface.
Yu, Dong In,Kwak, Ho Jae,Park, Chulmin,CHOI, Chiwoong,Sapkal, Narayan Pandurang,Hong, Jiwoo,Kim, Moo Hwan American Chemical Society 2019 Langmuir Vol.35 No.10
<P>In this study, the existing knowledge on the wetting criterion, that is, the intrinsic contact angle, for distinguishing between hydrophilic and hydrophobic textured surfaces is verified experimentally. A precise apparent contact angle is measured on micro-, nano-, and micro-/nanotextured surfaces to quantitatively define the surface-wetting conditions. In particular, X-ray tomography is introduced to measure precise geometric morphologies of nano- and micro-/nanotextured surfaces, and the wetting state of the textured surfaces is clearly visualized using synchrotron X-ray imaging. By comparing previous theoretical models and experimental results, it is verified that the intrinsic contact angle for distinguishing between hydrophilic and hydrophobic textured surfaces should be corrected from 90° to 43°. In addition, nonwetting phenomena in the region of the intrinsic contact angle between 43° and 90° are discussed.</P> [FIG OMISSION]</BR>