Enhanced Coalescence-Induced Droplet-Jumping on Nanostructured Superhydrophobic Surfaces in the Absence of Microstructures

Peng Zhang; Yota Maeda; Fengyong Lv; Yasuyuki Takata; Daniel Orejon

doi:10.1021/acsami.7b09681

Enhanced Coalescence-Induced Droplet-Jumping on Nanostructured Superhydrophobic Surfaces in the Absence of Microstructures

ACS Appl Mater Interfaces. 2017 Oct 11;9(40):35391-35403. doi: 10.1021/acsami.7b09681. Epub 2017 Sep 28.

Authors

Peng Zhang¹, Yota Maeda², Fengyong Lv¹, Yasuyuki Takata^{2

3}, Daniel Orejon^{2

3}

Affiliations

¹ Institute of Refrigeration and Cryogenics, MOE Key Laboratory for Power Machinery and Engineering, Shanghai Jiao Tong University , Shanghai 200240, China.
² Department of Mechanical Engineering, Thermofluid Physics Laboratory, Kyushu University , 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
³ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.

PMID: 28925681
DOI: 10.1021/acsami.7b09681

Abstract

Superhydrophobic surfaces are receiving increasing attention due to the enhanced condensation heat transfer, self-cleaning, and anti-icing properties by easing droplet self-removal. Despite the extensive research carried out on this topic, the presence or absence of microstructures on droplet adhesion during condensation has not been fully addressed yet. In this work we, therefore, study the condensation behavior on engineered superhydrophobic copper oxide surfaces with different structural finishes. More specifically, we investigate the coalescence-induced droplet-jumping performance on superhydrophobic surfaces with structures varying from the micro- to the nanoscale. The different structural roughness is possible due to the specific etching parameters adopted during the facile low-cost dual-scale fabrication process. A custom-built optical microscopy setup inside a temperature and relative humidity controlled environmental chamber was used for the experimental observations. By varying the structural roughness, from the micro- to the nanoscale, important differences on the number of droplets involved in the jumps, on the frequency of the jumps, and on the size distribution of the jumping droplets were found. In the absence of microstructures, we report an enhancement of the droplet-jumping performance of small droplets with sizes in the same order of magnitude as the microstructures. Microstructures induce further droplet adhesion, act as a structural barrier for the coalescence between droplets growing on the same microstructure, and cause the droplet angular deviation from the main surface normal. As a consequence, upon coalescence, there is a decrease in the net momentum in the out-of-plane direction, and the jump does not ensue. We demonstrate that the absence of microstructures has therefore a positive impact on the coalescence-induced droplet-jumping of micrometer droplets for antifogging, anti-icing, and condensation heat transfer applications.

Keywords: angular deviation; coalescence-induced droplet-jumping; dropwise condensation; microstructure density; nanostructured superhydrophobic surfaces; surface engineering.