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Home  > Raw materials & technologies  > Technologies  > Functional coatings  > Self-cleaning: repellence of hot water

Friday, 05 June 2020
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Raw materials & technologies, Technologies, Functional coatings

Self-cleaning: repellence of hot water

Monday, 27 July 2009

Superhydrophobic surfaces show reduced repellency to hot water.

Optical photos of cool water (25°C, a) and hot water (55°C, b) on lotus leaces; SEM images of hydrophobic lotus leaf surface (c, bar = 10µm) and 55°C hot water washed lotus leaf surface (d, bar =10µm), Source: Yuayg Liu
Optical photos of cool water (25°C, a) and hot water (55°C, b) on lotus leaces; SEM images of hydrophobic lotus leaf surface (c, bar = 10µm) and 55...

In a recent study researchers of the Hong Kong Polytechnic University, Hong Kong and the University of Minnesota, USA studied the repellence of hot water from superhydrophobic surfaces. Superhydrophobic surfaces that can repel hot liquids stand for a new generation of self-cleaning surfaces with broad applications. However, can current existing superhydrophobic surfaces repel hot water? In this paper, the repellent characteristics of current existing superhydrophobic surfaces to hot water (5080 °C) were investigated. Representative superhydrophobic surfaces from lotus leaves, Teflon, silicafluoropolymer composites and solgel processes were fabricated on silicon wafers and their repellency to hot water was evaluated based on water-contact angle (WCA) measurements and water uptake. These so-called superhydrophobic surfaces, which usually exhibit high repellency to cool water (around 25 °C), however, show remarkably decreased repellency to hot water. This may be attributed to a decrease of surface tension when the temperature of water increases and the lower surface tension of hot water makes it a better wetting agent to get into the pores and fissures of rough surfaces rather than bridging them with surface tension. The results in this paper also show that the wettability of a solid surface is governed by both the surface roughness and surface free energy, but the surface energy is more prevailing than the surface roughness in making the surface repel hot liquids. To fabricate hot water repellent fabrics, a nanocomposite of CNTs and Teflon was prepared and applied to commercial fabrics to produce scalding protection clothes. The repellency of the CNTsTeflon treated fabrics to hot beverages (water, milk, coffee and tea, 5080 °C) was studied. The spray test shows that although some superhydrophobic surfaces exhibit high repellency to static water they show reduced repellency to dynamic water. (Y. Liu, X. Chen, J.H. Xin, "Can superhydrophobic surfaces repel hot water?", J.Mater.Chem, 2009, DOI: 10.1038/b822168e)

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