• 한국가시화정보학회지
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• 제19권3호
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• pp.15-21
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• 2021

In this study, we investigated the dynamics of a droplet impacting rough hydrophobic surfaces through high-speed imaging. Micrometer-sized structures with grooves and pillars were fabricated on smooth Polydimethylsiloxane (PDMS) surfaces by laser ablation. We used Newtonian and non-Newtonian liquid droplets to study the drop impact dynamics. De-ionized water and aqueous glycerin solutions were used for the Newtonian liquid droplet. The solutions of xanthan gum in water were prepared to provide elastic property to the Newtonian droplet. We found that the orientation of the surface structures affected the maximal spreading diameter of the droplet due to the degree of slippage. During the droplet retraction, the dynamic receding contact angles were measured to be around 90° or less. It resulted in the formation of the micro-capillary bridges between the receding droplet and the surface structures. Then, the rupture of the capillary bridge led to the formation of micrometer-sized droplets on top of the surface structures. The size of the microdroplets was found to increase with increasing the impacting velocity and viscosity of the Newtonian liquid droplets. However, the size of the isolated microdroplets decreased with enhancing the elasticity of the droplets, and the size of the non-Newtonian microdroplets was not affected by the impacting velocity.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 춘계학술대회논문집
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• pp.159-164
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• 2009

The water droplet motion and the interaction between the droplets in a PEMFC air flow channel with multiple pores, through which water emerges, is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid-gas interface is tracked by a level set method which is based on a sharp-interface representation for accurately imposing the matching conditions at the interface. The method is modified to implement the contact angle conditions on the walls and pores. The dynamic interaction between the droplets growing on multiple pores while keeping the total water flow rate through pores constant is investigated by conducting the computations until the droplet motion exhibits a periodic pattern. The numerical results show that the droplet merging caused by increasing the number of pores is not effective for water removal and that the contact angle of channel wall strongly affects water management in the PEMFC air flow channel.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.1891-1895
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• 2008

Water covers 70% of the earth's surface and the human body consist of 75% of it. It is clear that water is one of the prime elements responsible for life on earth. Over the last 30 years or so, numerous studies have attempted to find out more about the water microscopically. In this paper, we investigated how the receding and advancing contact angle of the moving water droplet changes on a solid surface having various LJ epsilon parameters. To observe the dynamic contact angle history, a body force applied to all water molecules after obtained the water droplet in equilibrium with the solid surface. We obtained the density profile and receding and advancing contact angle of the moving water droplet

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2390-2395
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• 2008

The water droplet motion in an air flow microchannel with pores through which water emerges is studied numerically by solving the equations governing the conservation of mass and momentum. The gas-liquid interface is tracked by a level set method which is based on a sharp-interface representation for accurately imposing the matching conditions at the interface and is modified to implement the contact angle conditions on the wall and pores. The numerical results show that the droplet growth and detachment pattern depend significantly on the contact angle and inlet air velocity. Also, the dynamic interaction between the droplets growing on multiple pores is investigated. The pore arrangement subject to droplet merging is found to be not effective for water removal.

• 한국분무공학회지
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• 제4권2호
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• pp.10-18
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• 1999

The droplet/wall impingement processes in the diesel-like environment are numerically modeled. In order to evaluate the predictive capability of the droplet/wall impingement model developed in this study, computations are carried out for two ambient temperature conditions. Numerical results indicate that the present droplet/wall impingement model reasonably well predicts the basic features of the impinging spray dynamics.

• 한국환경과학회지
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• 제22권4호
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• pp.407-413
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• 2013

The flow of sewage has been studied for hundreds of years. Reducing drag in pipes can allow sewer to be removed easily and quickly. Drag reduction is not only a macroscale issue. Physical and chemical properties of the nano-scale can affect flow at the macroscopic scale. In this paper the predictability of hydrophobicity at the nano-scale is studied. Molecular dynamics simulations were used to calculate the range of contact angles of water droplets in equilibrium on a pillared graphite surface. It was found that at a pillar height of two graphite layers there was the largest range of contact angles. It is observed that at this height the droplet begins to transition from the Wenzel state to the Cassie-Baxter state. Surfaces with larger pillar heights have much larger contact angles corresponding to a more hydrophobic surface. Silicon dioxide was also simulated in the water droplet. The contaminant slight decreased the contact angle of the water droplet.

• 설비공학논문집
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• 제22권2호
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• pp.64-69
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• 2010

We investigated the variation in contact angle of a nano-sized water droplet on a nano stripe-patterned surface using molecular dynamics simulation. By changing the height and width of the stripe pillar, and the gap width of the stripes, we observed the contact angle of water droplet in equilibrium. When the surface energies were 0.1 and 0.3 kcal/mol, the calculated contact angles were in good agreement with the Cassie and Baxter equation. However, when the surface energy is 0.5 kcal/mol, the contact angles are observed to be perturbed along the Cassie and Baxter equation.

• 설비공학논문집
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• 제30권1호
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• pp.33-43
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• 2018

This study numerically investigated statistic and dynamic behaviors of the water droplet on plate with or without various structured-pillars at nano-scale by molecular dynamics simulation. This study considered smooth plate, plate with the rectangular-structured pillar, and the plate with dual-structured pillar under various characteristic energy conditions. The static behavior of water droplet depending on the plate shape, plate surface energy, and the pillar characteristics were examined. After the water droplet reaches its steady state, this study investigated the dynamic behavior of the water droplet by applying a constant force. Finally, this study investigated the static and dynamic behaviors of the water droplet by measuring its contact angle and contact angle hysteresis. As a result, we found that the structure was more hydrophobic.

• 한국분무공학회지
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• 제17권1호
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• pp.27-34
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• 2012

In this paper an experimental study is presented to investigate the effect of a step edge and a stationary droplet on the dynamic behavior of impacting droplet on a wall. The main parameters are the distance from the edge and the center-to-center distance between two droplets. Photographic images are presented to show coalescence dynamics, shape evolution and contact line movement. The emphasis is on presenting the spreading length of droplet for the step edge and two coalescing droplets along their original centers. It is clarified that the droplet exhibits much different dynamic behavior depending on the location of the step edge. The momentum of impacting droplet was better transferred to the stationary droplet as the center- to-center distance between two droplets was reduced, which results in more spreading of coalescing droplet.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2008년도 International Meeting on Information Display
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• pp.622-626
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• 2008

We have investigated the inter-relationship between the ink-jet printability and the physical fluid properties by monitoring the droplet formation dynamics. Printability of the fluids was judged based on the inverse of Ohnesorge number ($Z^{-1}$) that relates to the viscosity, surface tension, and density of the fluid.