• Journal of the Korean Society of Visualization
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• v.20 no.3
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• pp.19-26
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• 2022

In droplet microfluidics, precise droplet manipulation is required in numerous applications. This study presents ultrasonic surface acoustic wave (USAW)-based microfluidic device for label-free droplet separation based on size. The proposed device is composed of a slanted-finger interdigital transducer on a piezoelectric substrate and a polydimethylsiloxane microchannel placed on the substrate. The microchannel is aligned in the cross-type configuration where the USAWs propagate in a perpendicular direction to the flow in the microchannel. When droplets are exposed to an acoustic field, they experience the USAW-induced acoustic radiation force (ARF), whose magnitude varies depending on the droplet size. We modeled the USAW-induced ARF based on ray acoustics and conducted a series of experiments to separate different-sized droplets. We found that the experimental results were in good agreement with the theoretical estimation. We believe that the proposed method will serve as a promising tool for size-based droplet separation in a label-free manner.

• Korean Chemical Engineering Research
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• v.60 no.2
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• pp.277-281
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• 2022

The wetting dynamics of water droplets dispensed on the surface of dry and wet hydrophilic membranes were investigated experimentally from a microscale point of view. By using a high-speed, white-beam x-ray microimaging (WXMI) synchrotron, consecutive images displaying the dynamic motions of the droplets were acquired. Through analyzing the characteristics observed, it was found that the dry hydrophilic membrane showed local hydrophobicity at a certain point during the absorption process with apparent contact angles greater than 90. While on the other hand, the apparent contact angles of a water droplet absorbing into the wet membrane remained less than 90 and showed total hydrophilicity. The observations and interpretation of characteristics that affect the contact, wetting, recoiling, and dynamic behaviors of droplets are significant for controlling liquid droplet impingement in a desired manner.

• Bulletin of the Korean Chemical Society
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• v.35 no.9
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• pp.2704-2710
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• 2014

Catalase (CAT) decomposes hydrogen peroxide that is toxic to the body. In this study, simple and sensitive detector has been developed for observing catalase activity using liquid crystal droplet system. Microscale LC droplet patterns are formed by spreading aldehyde-doped nematic liquid crystal on pre-treated glass slides. When hydrogen peroxide is added, aldehyde is oxidized and amphiphiles are formed. Dodecanoates cause the pattern to transit from bright to dark as they self-assemble to form a carboxyalte monolayer at the interface. When a drop of pre-incubated CAT and hydrogen peroxide mixture is placed onto the pattern, bright fan-shape is observed. This planar optical appearance indicates that catalase has decomposed hydrogen peroxide. Compared to the detectors that have been previously developed, this system is more sensitive with detection limit of 1fM. This research suggests further studies to be on LC droplet patterning to develop highly sensitive and methodologically simple sensors for various chemicals.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1491-1498
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• 2003

Droplet evaporation can be used to transfer large amounts of energy since heat is transferred across a thin liquid film. Spreading the drop over a larger area can enhance this heat transfer. One method of accomplishing this is to dissolve gas into the liquid. When the drop strikes the surface, a gas bubble nucleates and can grow and merge within the liquid, resulting in an increase in the droplet diameter. In this study, time and space resolved heat transfer characteristics for a single droplet striking a heated surface were experimentally investigated. The local wall heat flux and temperature measurements were provided by a novel experimental technique in which 96 individually controlled heaters were used to map the heat transfer coefficient contour on the surface. A high-speed digital video camera was used to simultaneously record images of the drop from below. The measurements to date indicate that significantly smaller droplet evaporation times can be achieved. The splat diameter was observed to increase with time just after the initial transient dies out due to the growth of the bubble, in contrast to a monotonically decreasing splat diameter for the case of no bubbles. Bursting of the bubble corresponded to a sudden decrease in droplet heat transfer.

• Journal of the Society of Cosmetic Scientists of Korea
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• v.35 no.3
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• pp.179-191
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• 2009

The objective of this study was to investigate the effect of ethanol on the emulsion prepared by poly(oxyethylene) hydrogenated castor oils (HCOs)/oil/ethanol/water system. Emulsions were prepared using homogenizer as high-energy method. To evaluate effect of ethanol on the emulsion, physical properties such as droplet size and size distribution were determined and other components were almost fixed to analyze the effect of ethanol on the surfactant. In case of HCO-20, the droplet diameter was in micrometers and the droplet size was gradually deceased as the ethanol concentration was increased. The droplet diameter of nano-emulsion containing 4.00 % of HCO-30 was shown in nanometers and its mean droplet size was $128.15{\pm}1.06nm$ and the most stable at the 4.25 % of ethanol contents by the Form. 1 and $136.10{\pm}0.99nm$ at the 3.50 % of ethanol contents by the Form. 2. Similarly, the droplet diameter of nano-emulsion containing 4.00 % of HCO-40 and 4.50 % ethanol by the Form. 1 was $115.85{\pm}0.78nm$ and $121.15{\pm}0.35nm$ at the 3.25 % of ethanol by the Form. 2 and both size distributions were also narrow. Finally, the droplet size of nano-emulsion containing 4.00 % of HCO-60 and 2.25 % ethanol was $262.35{\pm}0.64nm$ and the most stable. The higher ethanol concentrations became the smaller size of emulsion became in the microscale emulsion but we determined nano-emulsion had a minimum size at a certain ethanol concentration. The results showed that the breakdown process of this nano-emulsion could be attributed to Ostwald ripening. This study about effect of ethanol on the emulsion showed that ethanol contents to prepare a stable emulsion could be determined as studying the effect of ethanol on the emulsion with the type of surfactants.