Simulation for Contact Angle of Droplet on Riblet Surface

Tae Wan Kim 한국트라이볼로지학회 2017 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.33 No.5

In this study, the hydrophobicity properties for riblet surfaces that replicate shark skin are simulated. Riblet surfaces with surface roughness on riblets are generated numerically based on the measured data of real shark skin. We assumed that a rib on a scale is hemi-elliptical surface. The surface used in the simulation for the calculation of contact angle is composed of 9 scales like checkerboard type with a roughness. The contact angle of a water droplet can be calculated using the Wenzel equation and Cassie-Baxter equation for the generated riblet surfaces. The variation of contact angles with a fractional depth of penetration for the generated shark skin surfaces without and with coatings is demonstrated in the condition of solid-air-water. The results show that the contact angle for the surface without coating decreases with an increase of the fractional depth of penetration more drastically than that for the surface with coating. We compared the experimental and simulated results. It is shown that the measured contact angles of the shark skin template and the shark skin replica are within the simulated results. Therefore the contact angle of water droplet for rough surfaces can be estimated by the developed numerical method in this study.

Study on the lubrication state and pitting damage of spur gear using a 3D mixed EHL model with fractal surface roughness

Youhua Li,Lubing Shi,Zhongming Liu,Xiaopeng Wang,Xuetao Qiao,Zhihong Zhang,Shidang Yan 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.12

For high-speed, heavy-load gear units, the meshing tooth surfaces are generally under the mixed lubrication state, which is crucial for gear pitting or micro pitting damage. To clarify the effects of lubrication state on tooth pitting damage, carrying out both numerical and experimental studies on the contact severity of gears under different lubrication states is necessary. In this paper, a 3D line-contact elasto hydrodynamic lubrication model considering fractal surface roughness was developed and used to investigate the lubrication characteristics of involute gears. Both the distributions of pressure and film thickness fluctuated under the rolling contact of rough surfaces. The fluctuations became more substantial with the increase of load and roughness. Furthermore, from the calculated film thickness ratio λ, the contact of gear tooth surfaces with a fractal roughness of Ra = 0.8 μm was always under full lubrication state (λ > 1). When the roughness was increased to Ra = 3.2 μm, the contact was first transmitted into the mixed lubrication state (0.2 < λ < 1) under the load of 1000 N*m and finally deteriorated to boundary lubrication state (λ < 0.2) under the load of 2000 N*m. The boundary lubrication state that occurred under the contact of highly rough surfaces could induce the formation of gear pitting damage. The contact fatigue test showed the gear tooth surface roughness increased from 0.7 μm to around 2.7 μm after 8 million running cycles and then suffered pitting failure after another 2 million cycles, which was consistent with the simulation analysis prediction.

Static and dynamic contact angle measurements using a custom-made contact angle goniometer

Hanbyeol Pak,김정현 대한기계학회 2023 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.37 No.8

Contact angle is a physical quantity used to evaluate the interaction between a solid surface and a liquid. However, many research laboratories or educational institutions with budget constraints have limited access to a commercial contact-angle goniometer with a highresolution imaging system. In this study, we fabricated a custom-made contact angle goniometer with a smartphone and quantified the contact angles of water on various surfaces. We found that the receding contact angles on the surfaces were sensitive to the change in flow rates. The receding angle sharply decreases when the flow rate exceeds 50 μl/min, indicating that accurate flow control is required in contact angle measurements. The dynamic contact angles could also be quantified by the developed goniometer in an extremely low-capillary number regime. The dynamic advancing and receding contact angles on tested surfaces followed the molecular-kinetic theory.

Estimation of Surface Forces in Micro Rough Surface Contacts

Doo-In Kim,Hyo-Sok Ahn,Dong-Hoon Choi 한국트라이볼로지학회 2002 한국트라이볼로지학회 학술대회 Vol.2002 No.10

In a micro-scale contact, surface forces such as capillary force and van der Waals interaction significantly influence the contact between asperities of rough surfaces. Little is, however, known about the variation of these surface forces as a function of chemical property of the surface (hydrophilicity), relative humidity and deformation. of asperities in the real area of contact. A better understanding of these surface forces is of great necessity in order to find an optimal solution for reducing friction and adhesion of micro surfaces. We proposed an effective method to analyze capillary and van der Waals forces in nano-scale contact. In this method, Winklerian foundation model was employed to analyze the contact of rough surfaces that were obtained from atomic force microscopy (AFM) height images. Self-mated contact of diamond-like-carbon (DLC) coatings was analyzed, as an example, by the proposed model. It was shown that the capillary-force was significantly influenced by relative humidity and wet angle of the DLC surface. The deformation of asperities to a critical magnitude by external loading led to a considerable increase of both capillary and van der Waals forces.

A molecular dynamics study of Wenzel state water droplets on anisotropic surfaces

Ambrosia, Matthew Stanley,Ha, Man Yeong Elsevier 2018 Computers & fluids Vol.163 No.-

<P><B>Abstract</B></P> <P>Some phenomena at the nanoscale are different from those at the macroscale. One of these phenomena is represented by the Wenzel equation which predicts a droplet's contact angle on a textured surface under certain conditions at the macroscale. However, at the nanoscale a different trend is observed in some cases. The Wenzel equation predicts contact angles to decrease as the surface roughness factor increases for droplets with a Young contact angle under 90°. However, for these cases at the nanoscale contact angles become larger as the surface roughness factor increases. In this study molecular dynamic simulations were run to investigate this phenomenon. Five surface energies and five surface roughness factors were considered for a graphite-like surface. Contact angles of nanoscale water droplets on an anisotropic surface with nanoscale texture are plotted and an equation related to the surface roughness factor and Young contact angle that fits the trend is proposed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Water droplets in the Wenzel state have larger contact angles at the nanoscale than the Wenzel equation predicts. </LI> <LI> At the nanoscale water droplets have a larger contact angles when the graphite layers are oriented vertically than horizontally. </LI> <LI> Water droplets at the nanoscale generally increase as the surface roughness factor increases regardless of the Young contact angle. </LI> <LI> A logarithmic equation dependent on the surface roughness factor models contact angles on pillared graphite at the nanoscale. </LI> </UL> </P>

Surface-to-Surface 접촉 모델을 적용한 콘크리트 쪼갬인장강도 시험 모델링

이상민,석승욱,최창식 한국콘크리트학회 2023 콘크리트학회논문집 Vol.35 No.3

본 연구에서는 콘크리트에 surface-to-surface 접촉 모델(surface-to-surface contact model)의 적용을 검증하기 위한 해석적 연구를 수행하였다. 가력판과 콘크리트 실린더의 접촉 표면에 surface-to-surface 접촉 모델을 적용하여 상용 유한요소해석 프로그램 ABAQUS로 3개의 쪼갬인장강도 시험을 시뮬레이션하고 3차원 환경에서 평면-평면 접촉 테스트(flat-flat contact test)와 평면-곡면 접촉 테스트(flat-circle contact test)를 수행하였다. 이후 2 mm 크기의 요소로 쪼갬인장강도 시험을 3D 유한요소해석을 수행하였다. 요소의 크기를 줄일 경우 해석 시간이 비약적으로 상승하였다. 따라서 2D 유한요소해석을 이용하여 0.5 mm 크기의 요소로 후속 해석을 수행하였다. 이러한 접근법을 통해, 요소의 크기가 감소함에 따라 해석 결과가 실험 쪼갬인장강도에 더 근접하였으며, 이는 제안된 모델링 기법이 타당함을 의미한다.

Simulation for Contact Angle of Droplet on Riblet Surface

김태완 한국트라이볼로지학회 2017 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.33 No.5

In this study, the hydrophobicity properties for riblet surfaces that replicate shark skin are simulated. Riblet surfaces with surface roughness on riblets are generated numerically based on the measured data of real shark skin. We assumed that a rib on a scale is hemi-elliptical surface. The surface used in the simulation for the calculation of contact angle is composed of 9 scales like checkerboard type with a roughness. The contact angle of a water droplet can be calculated using the Wenzel equation and Cassie-Baxter equation for the generated riblet surfaces. The variation of contact angles with a fractional depth of penetration for the generated shark skin surfaces without and with coatings is demonstrated in the condition of solid-air-water. The results show that the contact angle for the surface without coating decreases with an increase of the fractional depth of penetration more drastically than that for the surface with coating. We compared the experimental and simulated results. It is shown that the measured contact angles of the shark skin template and the shark skin replica are within the simulated results. Therefore the contact angle of water droplet for rough surfaces can be estimated by the developed numerical method in this study.

Simulation for Contact Angle of Droplet on Riblet Surface

Kim, Tae Wan Korean Tribology Society 2017 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.12 No.4

In this study, the hydrophobicity properties for riblet surfaces that replicate shark skin are simulated. Riblet surfaces with surface roughness on riblets are generated numerically based on the measured data of real shark skin. We assumed that a rib on a scale is hemi-elliptical surface. The surface used in the simulation for the calculation of contact angle is composed of 9 scales like checkerboard type with a roughness. The contact angle of a water droplet can be calculated using the Wenzel equation and Cassie-Baxter equation for the generated riblet surfaces. The variation of contact angles with a fractional depth of penetration for the generated shark skin surfaces without and with coatings is demonstrated in the condition of solid-air-water. The results show that the contact angle for the surface without coating decreases with an increase of the fractional depth of penetration more drastically than that for the surface with coating. We compared the experimental and simulated results. It is shown that the measured contact angles of the shark skin template and the shark skin replica are within the simulated results. Therefore the contact angle of water droplet for rough surfaces can be estimated by the developed numerical method in this study.

Measurement of Adhesive Force of Urea-Formaldehyde Resins at Softwood Surface

( Minjeong Kim ),( Byung-dae Park ) 한국목재공학회 2021 한국목재공학회 학술발표논문집 Vol.2021 No.1

This study aims to demonstrate a way of measuring the surface adhesion between softwood and urea-formaldehyde (UF) resin adhesives, using a tensiometer. It is necessary to understand the surface adhesion of UF resins on wood because the interfacial adhesion between UF resin adhesives and heterogeneous wood is very complex. The surface adhesion force was measured on the surface of a glossy paper at different moving speeds such as 0.5, 7.5, and 1 mm/s. The speed of 0.75 mm/s provided consistent measurements, which was used for the measurement of the surface adhesion of UF resin adhesives on the surface of early-wood and late-wood of the sapwood or heartwood of Radiata pine. The surface adhesion force of UF resin droplets were measured using the tensiometer on the surface of different woods such as early-wood and late-wood of the sapwood or heartwood of Radiata pine. For a measurement, a liquid resin droplet placed on an RG 2 Du Nouy ring was contacted onto the wood surface. The liquid droplet was pulled off to record a force-distance curve during this procedure. The surface of a wood sample was moved up toward the UF resin droplet at a speed of 0.75 mm/s. Then, the wood sample was retracted until the resin droplet was detached from the surface of wood sample. The maximum surface adhesive force was obtained by the peak of a force-distance plot.

Surface Modification of Rigid Gas Permeable Contact Lens Treated by Using a Low-Temperature Plasma in Air

Hyung Sup Shin,Jun Kyu Jang,권용승,Ki Choong Mah 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.6

Rigid gas permeable (RGP) contact lenses, based on fluorosilicone acrylate, were treated by using a plasma in air to improve surface wettability. As a function of the plasma exposure time, the wetting angles were estimated, the chemical compositions were analyzed by using X-ray photoelectron spectroscopy (XPS), and the surface morphology was observed by using atomic force microscopy (AFM). As the contact lenses were treated by the plasma, the wetting angle decreased significantly in the initial stage and decreased to 40 % of the wetting angle of the untreated lenses. The C, F, and N contents decreased significantly below 20 %, and the O and Si contents increased above 200 % on the surface. The rate of decrease of carbon was similar to the decrease in wetting angle. The number of oxygen-containing hydrophilic radicals (C-O and O-C=O) decreased because the carbons on the surface were oxidized and volatilized. The silica-like structure (SiOx, x = 1.5 ∼ 2.0) increased greatly (above 50 %), and the surface became hydrophilic. The surface roughness Rms decreased from 7 nm to about 4 nm, and the wetting angles decreased. The improved wettability of the contact lenses treated by using a plasma in air was affected more by the formation of a hydrophilic silica-like structure than by the change in surface morphology. Rigid gas permeable (RGP) contact lenses, based on fluorosilicone acrylate, were treated by using a plasma in air to improve surface wettability. As a function of the plasma exposure time, the wetting angles were estimated, the chemical compositions were analyzed by using X-ray photoelectron spectroscopy (XPS), and the surface morphology was observed by using atomic force microscopy (AFM). As the contact lenses were treated by the plasma, the wetting angle decreased significantly in the initial stage and decreased to 40 % of the wetting angle of the untreated lenses. The C, F, and N contents decreased significantly below 20 %, and the O and Si contents increased above 200 % on the surface. The rate of decrease of carbon was similar to the decrease in wetting angle. The number of oxygen-containing hydrophilic radicals (C-O and O-C=O) decreased because the carbons on the surface were oxidized and volatilized. The silica-like structure (SiOx, x = 1.5 ∼ 2.0) increased greatly (above 50 %), and the surface became hydrophilic. The surface roughness Rms decreased from 7 nm to about 4 nm, and the wetting angles decreased. The improved wettability of the contact lenses treated by using a plasma in air was affected more by the formation of a hydrophilic silica-like structure than by the change in surface morphology.