Forum Article www.acsami.org
Water-Repellent Properties of Superhydrophobic and LubricantInfused “Slippery” Surfaces: A Brief Study on the Functions and Applications Moyuan Cao,† Dawei Guo,† Cunming Yu,‡ Kan Li,§ Mingjie Liu,*,† and Lei Jiang*,†,‡,§ †
Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, P. R. China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, and §Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, P. R. China S Supporting Information *
ABSTRACT: Bioinspired water-repellent materials offer a wealth of opportunities to solve scientific and technological issues. Lotus-leaf and pitcher plants represent two types of antiwetting surfaces, i.e., superhydrophobic and lubricant-infused “slippery” surfaces. Here we investigate the functions and applications of those two types of interfacial materials. The superhydrophobic surface was fabricated on the basis of a hydrophobic fumed silica nanoparticle/poly(dimethylsiloxane) composite layer, and the lubricant-infused “slippery” surface was prepared on the basis of silicone oil infusion. The fabrication, characteristics, and functions of both substrates were studied, including the wettability, transparency, adhesive force, dynamic droplet impact, antifogging, self-cleaning ability, etc. The advantages and disadvantages of the surfaces were briefly discussed, indicating the most suitable applications of the antiwetting materials. This contribution is aimed at providing meaningful information on how to select water-repellent substrates to solve the scientific and practical issues, which can also stimulate new thinking for the development of antiwetting interfacial materials. KEYWORDS: superhydrophobic, superoleophilic, lubricant-infused, antiwetting, droplet impact
1. INTRODUCTION Interfacial materials with special properties offer great opportunities to develop advanced science and technologies.1,2 In the past decade, water-repellent substrates have emerged as potential materials in applications of antifogging, self-cleaning, and anti-icing.3−8 Learning from nature is an exciting method for designing smart and functional materials.9−12 In nature, a number of organisms have evolved superior antiwetting properties to survive in harsh environments.13,14 Notably, the lotus leaf (Nelumbo nucifera) has been well recognized as a classic antiwetting substrate, viz. the superhydrophobic substrate, which has a contact angle of larger than 150° and a roll-off angle of less than 10°.15,16 The superhydrophobic substrate in the Cassie−Baxter state generally possesses an “air cushion” when it interacts with a water droplet (Figure 1a). The trapping of air directly results in a short and discontinuous water/solid/air three-phase contact line (TCL) and realizes an ultralow water-adhesive force.17,18 The cooperative effect of the hydrophobic natural wax and the micro/nanoscale surface roughness contributes to the high water repellency and selfcleaning ability of the lotus leaf (i.e., the “lotus effect”). To date, artificial superhydrophobic materials were massively fabricated by convenient processes, including dip-coating, spray-coating, electrochemical processing, electrospinning, etc.19−23 Taking © XXXX American Chemical Society
advantage of their special properties, superhydrophobic materials have been proven as powerful tools for solving scientific and practical issues, such as oil/water separation and anticorrosion.24−31 In contrast, superhydrophilic substrates in an aqueous environment can also exhibit obvious oil repellency, revealing the secret of the oil-shielding ability of fish scales.32,33 On the basis of the superior liquid-absorbing ability of superwetting surfaces, the strategy that using liquid to repel another immiscible liquid was proven to be a different and effective method for designing novel antiwetting substrates (Figure 1b,c). In the air, the inner surface of the pitcher plant (Nepenthes sp.) was spontaneously lubricated by water. The other liquids, even the hydrophobic insect feet, can rapidly slide off this surface under a tiny incline angle, originating from the immiscible liquid film on the structured surface.34 This liquidwetted surface possesses a movable water/liquid/air TCL, which is quite distinct from the superhydrophobic surface. In Special Issue: Applied Materials and Interfaces in China Received: August 24, 2015 Accepted: September 29, 2015
A
DOI: 10.1021/acsami.5b07881 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Forum Article
ACS Applied Materials & Interfaces
Figure 1. Illustration of design principal and contact modes of (a) a superhydrophobic surface in air (lotus leaf), (b) a liquid-1-repellent surface of in the liquid-2 phase (fish scale), and (c) a liquid-1-repellent surface based on the liquid-2 layer in air (pitcher plant).
silicone-based superhydrophobic (SUB) surface can be conveniently converted to the lubricant-infused “slippery” (LIS) surface. According to the experimental phenomena, the advantages/disadvantages of the two surfaces were discussed, indicating rational applications in the field of interface science. This work can provide useful information on how to select suitable antiwetting substrates targeting different requirements and offer more options for their research and development.
2010, an amphiphobic and multifunctional pitcher-plantinspired surface was first fabricated via infusion of fluorinated oil into a porous substrate.5 Through the use of different infused liquids and porous structures, the rational designed “slippery” surfaces are able to be amphiphobic, self-healing, adaptive, and stimuli-responsive.35−37 This smart idea and design principle can facilitate versatile applications in fluid manipulation and vapor condensation.38−41 Taking inspiration from the antiwetting surfaces in nature, lotus-leaf-inspired superhydrophobic and pitcher-plant-inspired lubricant-infused “slippery” surfaces have been extensively fabricated and reported, and some of the products have already had their practical value. A deeper investigation and comparative study on the antiwetting performances of those surfaces can provide us important information on their advantages/disadvantages and instruct us to select appropriate materials for different requirements. In accordance with the previous reports, a brief analysis on both surfaces is listed in Table 1, indicating the main differences between those
2. EXPERIMENTAL SECTION 2.1. Preparation of Superhydrophobic and LubricantInfused Surfaces. The superhydrophobic mesh was produced with a typical dip-coating process. Coating solution: 1 g of PDMS (Dow Corning SYLGARD184, with 10% curing agent) and 1.25 g of HFS (Evonik R-974; average diameter of ∼16 nm) were well dispersed in 20 mL of hexane under sonication. The pristine substrate, such as the glass slide, was rinsed with ethanol and dipped into the coating solution for 30 s. Then, the substrate was slowly withdrawn, remaining a thin solution film on its surface. After the solvent was mostly evaporated, the coating layer was cured at 80 °C for 2 h to obtain the SUB substrate. Silicone oil was able to spontaneously wet the SUB surface in 1 min. Therefore, the LIS surface can simply be prepared through absorption of silicone oil (Sigma-Aldrich; viscosity of ∼20 mPa·s) with a planar density of about 10 mg/cm2. 2.2. Instruments and Characterization. The morphology of the substrates was observed by an environmental scanning electron microscope (ESEM; Phenom G2-Pro, Phenom-World, The Netherlands). The contact angles of the gauzes were recorded with a chargecoupled device of a contact-angle analyzer (OCA 20; Data-physics, Germany). The adhesion forces of the substrates were measured by a dynamic contact-angle analyzer with a microbalance (DCAT 21; Dataphysics, Germany). The detailed processes of droplet behaviors were recorded by a high-speed video camera (i-speed 3; Olympus, Japan). For the antifogging test, the fog flow with a velocity of ∼70 cm/s was generated by a humidifier (Angier, China).
Table 1. Comparison Analysis on Superhydrophobic and Lubricant-Infused Surfaces superhydrophobic substrates materials type design principle materials structure contact mode water contact angle (deg) sliding angle (deg)
lubricant-infused substrates
superantiwetting air cushion micro/nanoscale roughness water/air/solid >150
superwetting immiscible liquid phase porous surface or gel
