Journal of the Korean Society for Precision Engineering

Development of Droplet Sliding Control Surfaces on Multiscale Hierarchical Polymeric Structures: Min Ji Kim, Seong Min Kim, Ji Seong Choi, Seong Min Kang; J. Korean Soc. Precis. Eng. 2025;42(10):825-832.
Published online October 1, 2025; DOI: https://doi.org/10.7736/JKSPE.D.25.00005

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In this paper, we propose a novel method for controlling the anisotropic sliding behavior of droplets using multiscale hierarchical structures. First, we employed a silicon wafer mold containing micro-pillars and directional micro-line structures to induce the directional sliding of droplets. Additionally, we fabricated micro-cone patterns and integrated them into the structures to precisely control droplet movement. These two structures were replicated in polymer and subsequently fused into a single multiscale hierarchical mold through a partial curing process. The completed multiscale hierarchical surface was then replicated with PDMS to create anisotropy that governs the direction of droplet movement. We experimentally confirmed that the degree of sliding is influenced by the cone pattern. Our proposed structural design demonstrates that anisotropic wettability control is achievable even on surfaces made from a single material, indicating potential applications in various fields such as microfluidics, sensors, and functional surfaces.

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Dynamic Characteristics of Janus Drop Impact on the Inner Surfaces of Cylinders: Sungchan Yun; J. Korean Soc. Precis. Eng. 2025;42(8):665-672.
Published online August 1, 2025; DOI: https://doi.org/10.7736/JKSPE.025.067

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This study numerically investigates the spreading and retracting dynamics of Janus drops on the inner surfaces of cylinders using the Volume of Fluid method. The results indicate that increasing surface curvature enhances spreading in the axial direction and promotes the detachment of the low-viscosity water component, particularly under conditions of high viscosity ratio and Weber number. A regime map is constructed to identify the critical conditions for separation, revealing that surfaces with intermediate curvature exhibit higher separation efficiency compared to those with high curvature. The temporal evolution of axial momenta in the x and z directions highlights the role of viscosity contrast in inducing asymmetric deformation. A scaling law for residence time is proposed as a function of Weber number, which aligns well with simulation results in the high Weber number regime. These findings provide fundamental insights for optimizing surface curvature and fluid composition to enhance drop separation and may benefit applications such as selective liquid extraction, surface cleaning, and microfluidic manipulation.

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Periodicity of Droplet Impact Behavior by Liquid Viscosity on PDMS Surface: Dong Kwan Kang, Sangmin Lee; J. Korean Soc. Precis. Eng. 2022;39(11):857-862.
Published online November 1, 2022; DOI: https://doi.org/10.7736/JKSPE.022.070

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Periodicity behavior was observed after droplet collision in viscous solutions with controlled glycerol concentrations onto a PDMS surface. No periodic movement was observed in the droplets at glycerol concentrations of 50% or more. In contrast, the vertical diameter of the droplets increased and decreased periodically at glycerol solution concentrations of 40% or less. Moreover, there was little change in the periodicity of the impacting droplet movement, and the vibration frequency was measured at approximately 80 to 98 Hz in the entire range. The maximum droplet spreading factor after collision decreased significantly with increasing glycerol concentrations of 40% or more (Ohnesorge number 1.4 × 10^-2 or more). The results suggest that the effect of viscosity became greater than that of the solution surface tension with increasing glycerol concentrations.

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Study of Droplet Characteristics of Electrospray Coating Method as a Function of Ring Electrode Parameters
Ji Yeop Kim, Mun Hee Lee, Jun Yeop Kim, Jung Goo Hong
Journal of the Korean Society for Precision Engineering.2024; 41(2): 153. CrossRef

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Development of Multilayered Droplet Splitting Microfluidic System for Preparation of Microdroplet: Chul Min Kim, Gyu Man Kim; J. Korean Soc. Precis. Eng. 2022;39(6):425-431.
Published online June 1, 2022; DOI: https://doi.org/10.7736/JKSPE.022.015

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In this study, we present the multilayered symmetrical droplet splitting microfluidic system for preparation of microspheres. The microfluidic device was fabricated by conventional photolithography and PDMS casting. Multiple layers of microfluidic channels for symmetrical droplet splitting were stacked and integrated into a device. Each layer was designed to obtain 16 microdroplets from one droplet by droplet splitting. The droplet size was controlled with flow rate of dispersed phase (DI-water) and continuous phase (Mineral Oil with 3 wt.% SPAN80) by using a syringe pump. The droplet splitting behavior and production rate were analyzed by high-speed camera and inverted microscope in one layer of the microfluidic device. Additionally, the droplet size and size distribution were observed in each layer of the microfluidic device. The droplet size could be controlled by flow control of two phase flows with high uniformity of droplet size less than 5% coefficient of variation.

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Process for the Fabrication of Nickel Material High Aspect-ratio Digital PCR Partition
GeeHong Kim, HyungJun Lim, SoonGeun Kwon, Hak-Jong Choi
Journal of the Korean Society for Precision Engineering.2024; 41(8): 663. CrossRef

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Effects of Working Fluids on Spray Modes and Atomization Characteristics in Electrospray: Ji Yeop Kim, Sang Ji Lee, Gwang Yeol Baik, Jung Goo Hong; J. Korean Soc. Precis. Eng. 2021;38(1):61-68.
Published online January 1, 2021; DOI: https://doi.org/10.7736/JKSPE.020.100

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An electrospray is widely used in the industry due to uniform and continuous droplet generation. Most of the studies on the electrospray modes are conducted in the cone jet mode. The goal of this study is to confirm the spray patterns for each mode of the electrospray by setting various conditions, such as nozzle to substrate, nozzle diameters, fluid properties (Viscosity and Conductivity), and flow rate. As a result, 7 modes were observed by the applied voltage and flow rate. It was confirmed that the smallest droplet size was produced in the cone jet mode and working fluid V (High Conductivity).

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Experimental Investigation of Electrospraying Properties Based on Ring Electrode Modification
Ji Yeop Kim, Sang Ji Lee, Mun Hee Lee, Jung Goo Hong
ACS Omega.2024; 9(1): 1125. CrossRef
Study on Correlation between Manufacturing Method of Thermal Insulation Fabric and Thermal Insulation Performance for Reducing Rail Temperature in Summer Season
Juyeop Park, Donghoon Kang
Journal of the Korean Society for Precision Engineering.2024; 41(4): 313. CrossRef
Study of Droplet Characteristics of Electrospray Coating Method as a Function of Ring Electrode Parameters
Ji Yeop Kim, Mun Hee Lee, Jun Yeop Kim, Jung Goo Hong
Journal of the Korean Society for Precision Engineering.2024; 41(2): 153. CrossRef
Periodicity of Droplet Impact Behavior by Liquid Viscosity on PDMS Surface
Dong Kwan Kang, Sangmin Lee
Journal of the Korean Society for Precision Engineering.2022; 39(11): 857. CrossRef
Spray Mode and Monodisperse Droplet Properties of an Electrospray
Ji Yeop Kim, Sang Ji Lee, Jung Goo Hong
ACS Omega.2022; 7(32): 28667. CrossRef
Viscosity Effect on the Electrospray Characteristics of Droplet Size and Distribution
Ji Yeop Kim, Sang Ji Lee, Gwang Yeol Baik, Jung Goo Hong
ACS Omega.2021; 6(44): 29724. CrossRef

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Anti-Icing Technology based on Drop Bouncing Dynamics for the Prevention of Freezing of Electric Power Equipment: Sang-Hyeon Lee, Jaeil Kim, Minho Seong, Hyejin Jang, Geonjun Choi, Hoon Eui Jeong; J. Korean Soc. Precis. Eng. 2020;37(12):917-928.
Published online December 1, 2020; DOI: https://doi.org/10.7736/JKSPE.020.086

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In winter, electric power facilities such as solar panels, substations, power towers, and power lines suffer from freezing or ice accumulation problems due to exposure to harsh external environments. These problems result in unstable power supply, high maintenance costs, and severe economic and social losses. To address these problems, diverse anti-icing or deicing techniques including physical, thermal, and chemical approaches have been developed. However, these conventional approaches have limitations such as requirements for additional external energy, environmental toxicity, and low applicability. Recently, novel anti-icing surfaces based on unique drop bouncing dynamics have been developed by mimicking nano/micro-structures of natural systems. These anti-icing surfaces have attracted attention due to their high energy efficiency and environmental friendliness. It has been found that the superhydrophobic surfaces with specific nano/micro-structures can effectively remove the water droplets before the formation of ice nucleation by minimizing contact area and contact time between the droplets and the surface, thereby demonstrating excellent anti-icing properties. This review introduces recently developed anti-icing techniques based on the drop bouncing dynamics, and briefly describes the future direction of the anti-icing technology for stable power supply.

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Design and Performance Test of Solenoid Actuator Type Jetting Dispenser: Jong Hoon Lee, Jae Sung Seo, Ho Sung Kang, Young Woo Lee, Chang Soon Park, Eung Suk Lee; J. Korean Soc. Precis. Eng. 2019;36(10):969-973.
Published online October 1, 2019; DOI: https://doi.org/10.7736/KSPE.2019.36.10.969

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From the past, productivity improvement has been a problem to be solved, and various methods have been continuously developed. Recently, the demand for electric vehicles has increased along with the domestic and international environmental policies. Researches on the electrode production facilities have been actively conducted to increase the productivity of secondary batteries. Dispensers required for bonding the separator of the secondary battery, the cathode material and the anode material are also increasing in demand. The dispensers that spray a certain amount of adhesive quickly contribute to productivity improvement. However, Piezo type, which is currently being developed and used widely, has a higher performance than some industry, and its more expensive. In this study, we design a dispenser with lower performance than the conventional piezo type and lower the unit price and analyze the results.

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Effects of the Liquid Surface Tension on the Ejected Droplet Volume in a Pneumatic Printing System: Sangmin Lee, In Ho Choi, Joonwon Kim; J. Korean Soc. Precis. Eng. 2018;35(6):635-639.
Published online June 1, 2018; DOI: https://doi.org/10.7736/KSPE.2018.35.6.635

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This paper presents results for effects of the liquid surface tension on the ejected droplet volume using a pneumatic printing system. The low surface tension of the solution causes the liquid wetting around the nozzle, and then the wetted nozzle also inhibits stable formation of droplets. First, we confirmed the maximum inlet pressure (i.e., balanced with capillary force on the outlet channel) corresponding to varied surface tensions of the solutions, prepared by controlling the concentration of a surfactant. The ejected droplet volumes with the surfactant concentrations was varied within approximately 7% at each maximum inlet pressure, and the volume variation decreased to a fifth as compared with a high surface tension liquid.

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