| LIAO Yang-bo,HUANG Xian-fu,LU Ying-fa,YU Ying-song.Experimental Investigation of Electrowetting of Aqueous Sodium Dodecyl Sulfate Droplets on Micro-grooved Non-wetting Surfaces[J],52(12):178-187 |
| Experimental Investigation of Electrowetting of Aqueous Sodium Dodecyl Sulfate Droplets on Micro-grooved Non-wetting Surfaces |
| Received:August 20, 2023 Revised:October 09, 2023 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.12.016 |
| KeyWord:sodium dodecyl sulfate droplet actuation voltage saturation voltage surface roughness contact angle hysteresis |
| Author | Institution |
| LIAO Yang-bo |
School of Civil Engineering, Architecture and Environment,Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakes, c.Innovation Demonstration Base of Ecological Environment, Geotechnical and Ecological Restoration of Rivers and Lakes, Hubei University of Technology, Wuhan , China |
| HUANG Xian-fu |
State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing , China;Guangdong Aerospace Research Academy Nansha, Guangzhou , China |
| LU Ying-fa |
School of Civil Engineering, Architecture and Environment,Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakes, c.Innovation Demonstration Base of Ecological Environment, Geotechnical and Ecological Restoration of Rivers and Lakes, Hubei University of Technology, Wuhan , China |
| YU Ying-song |
School of Civil Engineering, Architecture and Environment,Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakes, c.Innovation Demonstration Base of Ecological Environment, Geotechnical and Ecological Restoration of Rivers and Lakes, Hubei University of Technology, Wuhan , China |
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| Abstract: |
| Electrowetting on dielectric (EWOD) has found wide applications in micro-/nano-fluidics for its precise and accurate manipulation of minor droplets. Micro-grooved non-wetting surfaces exhibit great anisotropy in surface wettability and have been used as surfaces for directional transport of liquid. The work aims to study the electrowetting characteristics of aqueous sodium dodecyl sulfate (SDS) agueous droplets on micro-grooved polydimethylsiloxane (PDMS) surfaces, which were obtained by the peeling-off method. The solid fraction of micro-grooved PDMS surfaces was 0.50, 0.33 and 0.20, respectively. SDS concentration was fixed at 0, 0.41, 0.82, 1.23 and 1.62 mmol/L, respectively. Firstly, wettability of SDS aqueous droplets containing 1 mmol/L KCl on micro-grooved PDMS surfaces without the application of a direct current (DC) electric field was measured. It was found that all droplets were at the Cassie-Baxter wetting state, micro-grooved surfaces exhibited a strong anisotropy in surface wettability, and the apparent, advancing and receding contact angles were all larger in the transverse direction than the corresponding values in the longitudinal direction. Secondly, eletrowetting of aqueous SDS droplets containing 1 mmol/L KCl on micro-grooved PDMS surfaces was experimentally studied by varying SDS concentration and surface roughness of the substrate. Platinum wire was inserted into the mixture droplet as an electrode and micro-grooved PDMS surfaces were placed on the conducting layer of ITO glass. DC electric field was applied to the system at the increase speed of 20 V/s and droplet shape analyzer DSA30 was adjusted as soon as possible to record the profile of sessile droplets at the speed of 1 frame per second. The non-wetting surface of micro-grooved array showed strong wettability anisotropy. Compared with the apparent contact angle (110°≤θe≤141°), advancing angle (116°≤θa≤144°) and retreating angle (99°≤θr≤137°) in the longitudinal direction, the apparent contact angle (142°≤θe≤165°), advancing angle (159°≤θa≤177°) and retreating angle (118°≤θr≤140°) in the transverse direction were larger. It was found that there existed two characteristic values of applied voltage. The first one was the actuation voltage, which was widely accepted to be related to contact angle hysteresis. Another one was the saturation voltage which might originate from the trapping of charge. In this work, it was found that actuation voltage for SDS aqueous droplets was more sensitive along the longitudinal direction, indicating that there was less contact angle hysteresis along the longitudinal direction than along the transverse direction because more energy was needed to be overcome when the droplets spread along the transverse direction. Actuation voltage was found to decrease with increasing SDS concentration, Moreover, saturation voltage and the voltage for the transition from the Cassie-Baxter wetting state to the Wenzel one were also found to both decrease with the increase of SDS concentration. At the same time, actuation voltage of droplets along the longitudinal direction was found to decrease with the decrease of the solid fraction of micro-grooved PDMS surfaces, while actuation voltage of droplets along the transverse direction increased with the decrease of the solid fraction of micro-grooved PDMS surfaces. Moreover, saturation voltage decreased with the decrease of the solid fraction. Besides, for the case of micro-grooved surfaces with a solid fraction of 0.20 or 0.33, it was observed that a phenomenon of multistage stepped reduction in the instantaneous contact angle along the transverse direction was observed and it could be attributed to the fact that more energy barrier due to contact angle hysteresis must be overcome when the droplets spread in the direction perpendicular to the microgrooves. It can be concluded that addition of SDS molecules into liquid can effectively reduce the actuation voltage and the applied voltage necessary for the wetting transition, resulting in the variation of characteristics of electrowetting of aqueous SDS droplets on micro-grooved PDMS surfaces. |
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