Binary mixture droplet wetting on micro-structure decorated surfaces

Khaloud Moosa Al Balushi; Khellil Sefiane; Daniel Orejon

doi:10.1016/j.jcis.2021.12.171

Binary mixture droplet wetting on micro-structure decorated surfaces

J Colloid Interface Sci. 2022 Apr 15:612:792-805. doi: 10.1016/j.jcis.2021.12.171. Epub 2021 Dec 29.

Authors

Khaloud Moosa Al Balushi¹, Khellil Sefiane², Daniel Orejon³

Affiliations

¹ Institute for Multiscale Thermofluids, School of Engineering, The University of Edinburgh, Edinburgh EH9 3FD, Scotland, UK; Department of Engineering, The University of Technology and Applied Sciences, Suhar 311, Oman.
² Institute for Multiscale Thermofluids, School of Engineering, The University of Edinburgh, Edinburgh EH9 3FD, Scotland, UK.
³ Institute for Multiscale Thermofluids, School of Engineering, The University of Edinburgh, Edinburgh EH9 3FD, Scotland, UK; International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Electronic address: d.orejon@ed.ac.uk.

PMID: 35065463
DOI: 10.1016/j.jcis.2021.12.171

Abstract

Liquid surface tension as well as solid structure play a paramount role on the intimate wetting and non-wetting regimes and interactions between liquids droplets and solid substrates. We hypothesise that the coupling of these two variables, independently addressed in the past, eventually offer a wider range of understanding to the surface science and interfacial communities. In this work, intrinsically hydrophobic micro-pillared surfaces varying in the spacing between structures, and pure ethanol, pure water and their binary mixtures (as well as acetone-water and ethylene glycol-water mixtures) are utilised, accessing a wide range of substrate solid fractions and liquid surface tensions experimentally. Wettability measurements are carried out at different azimuthal directions to exemplify the wetting/non-wetting behaviour as well as the droplet asymmetry function of both liquid composition and structure spacing. Our findings reveal that high water concentration droplets, i.e., high surface tension fluids, sit in the Cassie-Baxter regime while partial non-wetting Wenzel or mixed-mode regimes with enhanced droplet asymmetry ensuing for medium and high ethanol concentrations, i.e., low surface tension fluids, below certain micropillar spacing. Beyond micropillar spacing s ≥ 40 µm, the impact of the surface structure on the droplet shape is negligible, and droplets adopt a similar contact angle and circular shape as on a flat smooth hydrophobic surface. Wetting and non-wetting regimes are then supported by classical wetting theories and equations. A wetting regime map for a wide range of surface tension fluids and/or their mixtures on a wide domain of solid fractions is then proposed.

Keywords: Binary mixture; Cassie-Baxter; Geometrical footprint; Interfacial phenomena; Microstructured surfaces; Non-wetting Wenzel; Wetting; Wetting regime map.