Wetting and adhesion energy of droplets on wettability gradient surfaces
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Wetting and adhesion energy of droplets on wettability gradient surfaces Cheng-xiao Xu1, Zhi-hai Jia1,*, and Xu-hui Lian1 1
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
Received: 24 January 2020
ABSTRACT
Accepted: 24 March 2020
Wettability gradient surfaces can be used for heat and mass transfer in many fields, which has aroused extensive interest of research. However, since numerous studies only focused on the surfaces with identical microstructures, the wetting and adhesion energy of droplets on wettability gradient surfaces are still unclear. In this work, three types of microstructured surfaces with gradient wettability were fabricated via photolithography and etching methods. We found that the contact angle, contact angle hysteresis and critical sliding angle will increase with area fraction. Meanwhile, the gradient microstructures will affect the triple line and wetted area of droplets. In addition, it is found that the critical sliding angle depends on not only the droplet volume but the size and types of microstructures, which is consistent with our force analysis. Based on the morphological characteristics and actual wetted area of droplets, an adhesion energy model for microstructured surfaces with gradient wettability is proposed. The model is also verified by other types of wettability gradient surfaces, which may be helpful to design wettability gradient surfaces with low adhesion energy for practical applications.
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Springer Science+Business
Media, LLC, part of Springer Nature 2020
Introduction Dropwise condensation can aggregate independent droplets efficiently and separate them from the surface fleetly, contributing to superior heat exchange efficiency compared to film condensation [1–3]. In addition to thermal applications [4–6], dropwise condensation can be extensively applied in various fields, such as anti-freezing [7–9], self-cleaning [10–12], flow control [13–15] and biomedicine [16, 17]. Multiple wettability gradient surfaces [16–30] have been rationally designed in order to meet the needs of
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https://doi.org/10.1007/s10853-020-04607-5
numerous engineering applications. Due to asymmetric microstructures and continuously varying surface free energy, wettability gradient surfaces can be regarded as a feasible solution to control flow. Chemical coating [30–32] and physical modification [24–27, 33, 34] are two prevalent techniques for the fabrication of wettability gradient surfaces. Depending on the surface material and droplets controlling methods [22–24, 35–39], wettability gradient surfaces can be theoretically applied in distinct fields. However, due to liquid imbibition and boundary pinning effect [40, 41], the surface
J Mater Sci
wettability is altered, especially in the cases of condensation [42, 43]. Given that, it is challenging to satisfy the demands for practical applications. The underlying physical mechanism with respect to wetting stability on wett
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