Omniphobic re-entrant PVDF membrane with ZnO nanoparticles composite for desalination of low surface tension oily seawater Bhaskar Jyoti Deka, Jiaxin Guo, Noman Khalid Khanzada, Alicia Kyoungjin An * School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region article info Article history: Received 23 May 2019 Received in revised form 9 August 2019 Accepted 12 August 2019 Available online 13 August 2019 Keywords: Omniphobic membrane Zinc oxide nanoparticle Membrane distillation Re-entrant structure Low surface tension oily seawater desalination Anti-wetting abstract In this study, an omniphobic membrane was fabricated by electrospraying fluorinated zinc oxide (ZnO) nanoparticles (NPs) mixed with polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) on the surface of an organosilane functionalized polyvinylidene difluoride (PVDF) membrane. Our results revealed that the functionalized ZnO NPs membrane exhibited a rough hierarchical re-entrant morphology with low surface energy which allowed it to achieve high omniphobic characteristics. It was observed that the addition of 30% ZnO (w/w of PVDF-HFP) was found to be optimal and imparted a high repulsive characteristic. The optimized PVDF/ZnO(30)/FAS/PVDF-HFP referred as cPFP-30Z mem- brane exhibited a high contact angle values of 159.0 ± 3.1  , 129.6 ± 2.2  , 130.4 ± 4.1  and 126.1 ± 1.2  for water, sodium dodecyl sulfate (SDS) saline solution (0.3 mM SDS in 3.5% NaCl), ethanol, and vegetable oil, respectively. The low surface energy and high surface roughness (Ra) of optimised membrane was assessed as 0.78 ± 0.14 mN m 1 and 1.37 mm, respectively. Additionally, in contrast with the commercial PVDF membrane, the cPFP-30Z membrane exhibited superior anti-wetting/anti-fouling characteristics and high salt rejection performance (>99%) when operated with a saline oil solution (0.015 v/v) and SDS (0.4 mM) feed solutions. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction Membrane distillation (MD), an emerging thermal-based tech- nology, can be a viable option for the production of sustainable high-quality water from super-saline brine, shale gas or oily wastewater when provided with access to waste/low-grade heat or solar energy sources (Alkhudhiri et al., 2012; Lee et al., 2017). As MD is driven by the temperature and vapor pressure gradient in which water evaporates from the hot feed side and travels through a hy- drophobic membrane, the MD process is not affected by the feed concentration. Therefore, in recent years, MD has expanded its applications into different fields, including removal of volatile organic compounds from solutions, cleaning of environmental impurities, crystallization of valuable resources, biomedical, and food processing applications (Barbe et al., 1998; Kujawski et al., 2013; Motta et al., 2017; Villalba, 2013). However, while MD has shown significant potential in a wide range of applications, major hurdles such as wetting and fouling which result in flux decline and poor permeate quality still limits its largescale industrial applica- tion (Chen et al., 2017; Mishra et al., 2016; Rezaei et al., 2018; Wang et al., 2018). Whilst existing commercial MD membranes undergo severe wetting and fouling when dealing with organic and low surface tension contaminants (Wang et al., 2016), omniphobic membranes have gained attention due to their hydrophobic nature and ability to repel traditionally problematic low surface tension liquid substances, such as oil, surfactants, etc. (Lee et al., 2016b; Lin et al., 2014; Wei et al., 2005). The development of robust omniphobic membrane capable of repelling both water and oil while maintaining high water flux will therefore have remarkable scientific and commercial implications (Pan et al., 2013; Sheen et al., 2008). The very first report on the fabrication of an omniphobic membrane for MD application uti- lized silica nanoparticles (Si-NPs), which were coated over a hy- drophilic glass fibre membrane followed by surface fluorination. However, the membrane showed a contact angle (CA) below 90  with ethanol and a low water flux of 15.8 ± 2.7 L m 2 h 1 (LMH) (Lin et al., 2014). For a membrane to be omniphobic, the membrane must demonstrate a CA for water and oil of nearly 140  and 110  , respectively (Zheng et al., 2018). One method for achieving such high CAs is by grafting fluoroalkylsilanes (FAS) onto the membrane * Corresponding author. E-mail address: alicia.kjan@cityu.edu.hk (A.K. An). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres https://doi.org/10.1016/j.watres.2019.114982 0043-1354/© 2019 Elsevier Ltd. All rights reserved. Water Research 165 (2019) 114982