Recent Progress in Super Hydrophobic/Hydrophilic ...

Polymer-Plastics Technology and Engineering

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Recent Progress in Super Hydrophobic/Hydrophilic Self-Cleaning Surfaces for Various Industrial Applications: A Review Sushanta Kumar Sethi & Gaurav Manik To cite this article: Sushanta Kumar Sethi & Gaurav Manik (2018): Recent Progress in Super Hydrophobic/Hydrophilic Self-Cleaning Surfaces for Various Industrial Applications: A Review, Polymer-Plastics Technology and Engineering, DOI: 10.1080/03602559.2018.1447128 To link to this article: https://doi.org/10.1080/03602559.2018.1447128

Published online: 12 Mar 2018.

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POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING https://doi.org/10.1080/03602559.2018.1447128

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Recent Progress in Super Hydrophobic/Hydrophilic Self-Cleaning Surfaces for Various Industrial Applications: A Review Sushanta Kumar Sethi and Gaurav Manik Department of Polymer and Process Engineering, Indian Institute of Technology, Roorkee, India ABSTRACT

ARTICLE HISTORY

The growing interest in self-clean coatings owes to their low maintenance cost, high durability, and immense potential applications. Such coatings, for instance, may offer useful resistance against fouling, icing, smear, corrosion and possesses oil-water separation capability. Hence, the design and development of self-cleaning materials is an area of significant interest to researchers worldwide. This exploration prospectus critically reviews and highlights various types of self-cleaning surfaces, their applications, working mechanisms, and fabrication techniques used globally. The comprehensive literature study suggests that an extensive study of surface behavior towards water and oil may jointly determine its self-cleaning nature.

Received 6 November 2017 Accepted 26 February 2018 KEYWORDS

Biomimetic; contact angle; super hydrophilic; super hydrophobic

GRAPHICAL ABSTRACT

1. Introduction Coatings are progressively being used to protect textured surfaces, clothing articles and for different open-air applications such as automotive applications, window glasses etc. This improvement has its underlying foundations in the innovation of new water repelling polymers. From the late 1950s to mid-1960s, when polyacrylates and polyvinyl acetate were supplemented by polyurethanes, coatings have turned out to be progressively prevalent for chic pieces of clothing. The growing interest in research involving self-clean coatings is evident from the data presented in Figure 1. The number of publications in different areas of coatings from ISI Web of Science. An extensive variety of procedures are accessible for applying the coatings.[1–4] The most feasible coatings procedure needs to primarily consider three important points:

(i) coating substrate type, (ii) coating formulation and (iii) end utilization of the article. This is, since, the coating-substrate interaction decides the coating adhesion to the substrate, and hence, its durability. At the same time, the other face of coating open to environment ensures self-cleaning through either its super hydrophobic or super hydrophilic behavior. The prediction of surface properties of novel waterrepelling polymers in the field of coatings, adhesives and biomaterials applications[5,6] is vital for future advances in product development. The origin of super hydrophobicity, which is very common to many natural surfaces, is based on their chemical composition and specific surface texture.[7] While surface hydrophilicity is attributed to the presence of hydroxyl, carboxyl or sulphonic groups, super hydrophobicity is dependent on the presence of non-polar groups such as methyl group.

CONTACT Gaurav Manik [email protected]; [email protected] Department of Polymer and Process Engineering, Indian Institute of Technology, Roorkee, India. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lpte. © 2018 Taylor & Francis

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S. K. SETHI AND G. MANIK

Figure 1. The statistics of the paper indexed in the “ISI web of science” based on studies related to (a) different coating types, and (b) those focused on self clean coatings from 1965 to 2017.

2. Advantages of using self-cleaning materials Self-cleaning is a standout amongst the several applications and functions of a super hydrophobic/ hydrophilic surface. The development of a super hydrophobic/hydrophilic material as the coating is critical to provide useful self-cleaning property to a surface. A surface, showing water contact angle (WCA) > 90° is termed as hydrophobic, whereas, the one with WCA > 150° is termed as super hydrophobic surface. Similarly, surfaces showing WCA < 10° are termed as super hydrophilic. Super hydrophobic surfaces can be obtained by combining small-scale or microscale and additionally nanoscale surface roughness with surface hydrophobicity. Major advantages of using such coatings are to reduce the maintenance cost, increase the durability, prevention of snow or ice adhesion and protection from environmental pollution. Such selfcleaning coatings possess enormous potential applications in areas of textiles, garments, automotive parts (like car bodies and mirrors), building (glass windows or doors and plastic roofs), agriculture (greenhouse covers), household (bathrooms, kitchen fittings and non-stick pans, fluid/ nourishment packings), optical applications (cameras, cellphones, lenses, optical sensors, marine applications (anti-corrosion protection) and aerospace (non-stick and ice-phobic surfaces).

when the surface is smooth, homogenous, planar or non-deformable. The final equilibrium drop configuration attained relies upon the properties of the surface, and in addition, the outside conditions such as temperature. This field is extensively categorized as the study of wetting and spreading phenomena and intends to decide how a fluid behaves on the surface of a substrate. The occurrence of wetting is boundless in nature and happens whenever a surface is open to atmosphere. The understanding of reasons why and when a liquid wet a surface can enormously upgrade our knowledge. In addition, this fundamental investigation can also help in the development of new materials and innovation of new products by both scientists and engineers. 3.1. Young’s equation Wettability of a solid surface is quantified by Young’s equation by considering the interface where a liquidvapor meets a solid surface. Practically, the contact angle hysteresis is observed by extending the advanced contact edge to receding contact edge. The equilibrium contact angle reflects wettability of the surface. While considering the presence of a fluid on a surface as shown in Figure 2, there are three frameworks that become an integral factor: surface, fluid and encompassing vapor. Young established[8] a relation among different surface tensions to that of contact angles which are

3. Theoretical basis The self-cleaning property is mainly related to contact angle that a liquid droplet forms when placed on a substrate and achieves an equilibrium configuration. The drop may either spread and cover the entire surface or stay as a drop or sometimes attempt to leave the surface. A stable equilibrated contact angle is achieved only

Figure 2. Contact angle platform.

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING

given in Eq. (1). cSV ¼ cSL þ cLV cosh

ð1Þ

Where, S, V, and L refer to solid, vapor and liquid surfaces. γ refers to the interaction between two surfaces and θ refers to the contact angle. To anticipate contact angle of a liquid droplet on a roughened surface several models have been developed, but not many proposed models incorporate depending on nano-scale surface roughness. At sub-atomic scale, the parameter of surface roughness can be rectified by regarding the solid-fluid interface as a hybrid Cassie-Wenzel state in which the part in the Wenzel state relies on the liquid contained inside the cavities. This would give a more accurate estimation of wettability when surface dimensions at nanoscale could significantly impact the surface phenomena.

to that needed to form a unit area of the liquid-air interface. The solid-liquid interfacial tension depends especially on surface condition. If the surface is roughened, the gain in energy in shaping solid-liquid interface will be σ (SV-SL), and the contact angle θ0 is given in Eq. (4). cos h0 ¼

cSL > cLV

ð2Þ

Where γSV, γSL and γLV are interfacial tension between solid-vapor, solid-liquid and liquid-vapor, respectively. The contact angle exists at the point when the above imbalance remains unsatisfied, thereby, the liquid droplet stays finite in size. The equilibrated contact angle θ, between the fluid and solid surface, without gravity impact is given by Eq. (3). cos h ¼

cSV cSL cLV

ð3Þ

The above condition demonstrates that the cosine of the contact angle gives the proportion of energy gained in forming a unit area of the solid-liquid interface Table 1.

ð4Þ

3.3. Wenzel model Wenzel[10] explored the impact of rough grooves on a solid surface where fluid droplet penetrates them and derived the contact angle equation given in Eq. (5). cos hw ¼

cSV

rðcSV cSL Þ cLV

The angle θ0 is termed as “apparent” contact angle whereas, θ is the “real” angle, expressed earlier through equation (3).

3.2. Cassie model When a liquid droplet is put on a substrate, the droplet achieves an equilibrium configuration either by remaining as a drop in a limited zone or by spreading uncertainly over that solid surface.[9] The state of spreading is that: the energy required to form a droplet should surpass the interfacial energy difference between solid-vapor and solid-liquid, i.e., shown in Eq. (2).

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rðcSV cSL Þ ¼ rcosh cLV

ð5Þ

In this condition, hw is the contact angle of a liquid droplet on a roughened surface, θ is the contact angle on a similar smooth surface. Here, hw is expressed to be dependent on cosine of Young’s contact angle and surface roughness factor, r, which is defined as the ratio of actual surface area to projected surface area. Hence, Wenzel equation mentions, that surface roughness reasonably affects the surface wettability and predicts that when θ < 90° it enhances and while θ > 90° lessens it. However, when θ > 90°, air bubbles occupy positions in such rough grooves and prevent water droplets to enter into hydrophobic pillars, and hence makes the surface super hydrophobic. This situation is well depicted by Cassie and Baxter.[11]

3.4. Transition between Cassie and Wenzel states Solid/fluid contact mode will transform from Cassie state to Wenzel state under the states of droplet pressure, impact or vibration.[12] Koishi et al.[13] investigation shows that the two states (Wenzel and Cassie) can co-exist together on a hydrophobic nano pillared surface, i.e., it can be in the bistable Wenzel/Cassie state.

A survey of different types of self-clean surfaces and their prime features as reported in literature.

Surface properties

Comments

Hydrophobic/Oleophobic

Surfaces possess both water and oil resistant property.

Hydrophobic/Oleophilic Hydrophilic/Oleophobic

Surfaces have water-resistant property but not oil resistance. Surfaces having oil resistant property but not waterresistance. Surfaces which attract both water and oil.

Hydrophilic/Oleophilic

References Hayn et al.,[14] Ma et al.,[15] Fabbri et al.[16] and Pilotek et al.[17] Wang et al.,[18] Feng et al.,[19] and Korhonen et al.[20] Yang et al.,[21] Rohrbach et al.,[22] Howarter et al.[23,24] and Xue et al.[25] Cao et al.[26] and Wang et al.[27]

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The authors reported the existence of a critical pillar height beyond which the water droplets can exist in both states. On basis of statistical mechanics method and kinetic raining simulation, the authors computed the free energy barrier which separates the Wenzel and Cassie states. Types of self-cleaning surfaces have been shown in Table 1.

undeniably hot research subject. Several animals have been shown to exhibit proper harmonization and unification between structure and function.[33] This section condenses the current development in bio-inspired unique wettability like self-cleaning lotus effect, creepy crawlies, flower petals, mosquito eyes, gecko feet, etc.

4.1. Lotus leaf inspired

4. Current progress in biomimetic special wettability surfaces Nature is a school for scientists and researchers and inspires them for development of novel materials for society needs. Through billions of years of species development, many creatures in nature have developed unique wettability features. “Biomimetic”, derived from the Greek word “bios” (life, nature) and “mimesis” (impersonation, duplicate)[28] literally means copying nature for designing of new surfaces or improving existing ones. In general, biomimetic architecture is the extraction of useful physical structures and chemical details from nature to make several artificial yet useful advanced materials.[29–32] Overview of natural, artificial and natural to artificial self-cleaning surfaces has been depicted in Figure 3. These days, bio-inspired super hydrophobic surfaces have pulled large interest both in major fundamental studies and engineering applications and have turned into an

Many plants leaves display extraordinary waterrepellency.[7,29,34,35] Among them, the most exemplary case is lotus leaf which has motivated numerous scientists to create novel surface materials for industrial needs. Lotus leaves show WCA over 161° and little sliding angle around 2°.[36] Due to of this lower sliding angle, raindrops roll out effectively from leaf surface along with dust particles. The inherent hydrophobic epicuticular wax and isotropic dual (micro-nano) scale roughness papillae, shown in Figure 4, add remarkable super hydrophobic self-cleaning effect to its surface.[23,24,34,35] Such effect is now well documented as “lotus effect”. The huge applications of such super hydrophobic surfaces are in the field of anti-biofouling paints for watercrafts, biomedical applications, microfluidics, corrosion resistance, restraint on ice/snow adhesion, oxidation, etc.[38–45] This provides sufficient motivation to mankind to replicate nature to get amazing benefits.

Figure 3. Overview of natural, artificial and natural to artificial self-cleaning surfaces.


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Figure 4. SEM image of (a) Lotus leaf surface showing its papillae (b) Enlarged view of a papilla (c) Lower surface of leaf (d) Contact angle variation with mean diameter to length of papillae. Reproduced from[37] with permission from the Advanced Materials- John Wiley and Sons.

Nowadays, special anisotropic wettable surfaces have also pulled great interest in extending and employing their self-cleaning property to microfluidic applications.[30,32,46] The surface free energy and its texture are two most vital elements of a super hydrophobic self-cleaning surface.[1,47–50] Liu et al.[51] fabricated artificial lotus leaf on a cotton surface. The authors used pristine MWCNT and modified CNT’s (PBA-gMWCNT) as building blocks so as to mimic the surface texture of a lotus leaf. The obtained WCA was higher than 150°, and hence, showed super hydrophobicity. Likewise, Sun et al.[52] replicated the complex lotus leaf topography pattern on to a poly dimethyl siloxane (PDMS) surface using the leaf as a negative template and obtained a dual (nano and micro) scale roughness on the base polymer. Since super hydrophobicity is dependable both on chemical composition and surface texture, so the WCA on flat PDMS suddenly rose from 110° to 160°.

surface, it helps the water droplet to roll off easily in the leaf edge direction. Inspired by rice leaf, Zhao et al.[54] fabricated a super hydrophobic Au surface mimicking the rice leaf topography. The obtained biomimetic nano structured Au surface was then modified with low surface energy polymer to provide improved hydrophobicity with WCA ∼ 136°. Gao et al.[55] fabricated a super hydrophobic surface by a two-step replication method using rice leaf as a negative template. The authors used rice leaf as a negative template to mimic the surface topography on PDMS film, and then, mimic the PDMS topography on poly(N-isopropylacrylamide) (PNIPAAm) film. The authors reported the existence of good thermally responsive film with anisotropic wettability and exhibiting a WCA of 119 � 10° along the direction parallel to the grooves at 50°C and 77 � 9° at 20°C. In perpendicular direction, the WCA was measured to be 87 � 9° at 50°C and 49 � 6° at 20°C.

4.2. Rice leaf inspired

4.3. Mosquito eyes inspired

Like lotus leaf, rice leaf also possesses super hydrophobic property. In particular, rice leaf has an interesting property known as anisotropic wettability. It consists of dual (micro and nano) scale surface roughness in which the papillae of 5–8 µm in diameter are arranged parallelly to the leaf edge,[53] shown in Figure 5, which provides super hydrophobicity with a value of 157 � 2°. Since the papillae are arranged parallel to the leaf edge

Fogging is a serious discomfort for drivers during driving. It happens when condensation of moisture forms droplets with a diameter bigger than the visible light wavelength. Such issues can be resolved by two possible methodologies; (i) making the surface super hydrophobic or by (ii) making the surface super hydrophilic. Youngblood et al.[24,56] first developed anti-fogging self-cleaning coatings by utilizing perfluorinated

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Figure 5. Snapshots of (a) few water droplets on a rice leaf (b) zoomed view of a water droplet floating on a rice leaf, and (c, d) SEM images of rice leaf at different magnifications (50 and 1 µm). Reproduced from[53] with permission from the Plant Science- Elsevier.

polyethylene glycol oligomers (f-PEG) and silica. On account of super hydrophilic approach, these coatings can essentially stifle the fogging effect by spreading water droplets instantly on their surfaces, thereby, eliminating light scattering which occurs because of water droplets. The mosquito eyes comprise of hexagonally nonclose-stuffed nipples at the nanoscale and hexagonally close-packed hemispheres at the microscale level as shown in Figure 6. Both the structures contribute

equally to offer a perfect super hydrophobic surface with a natural, antifogging property. This prevents the moisture to form droplets on its eye surface and hence, provides a clear vision to the mosquito. Inspired by this, several authors have developed artificial mosquito eyes by considering the impacts of hierarchical dual scale roughness in lithography. Moreover, it was accounted for that the counter intelligent nano nipples on insect’s ommatidia can bring

Figure 6. Illustration of (a) A SEM image (b) A hexagonally close-packed micro-hemisphere (ommatidia) (c) Structure of neighboring ommatidia, and (d) Hexagonally non-close-stuffed nano nipples covering an ommatidia surface of a mosquito eye. Reproduced from[42] with permission from the Advanced Materials- John Wiley and Sons.


about the dangerous interference of reflected light from the ommatidial surface to upgrade the productivity of photon catch,[57,58] which ensures mosquitoes a great vision in diminished lighting environment. Since the adhesion between tidy particles and micro-nanostructured surface is not as much as that with liquid droplets hence, condensed fog drops may remove the tidy particles, and thereby, exhibit self-cleaning property.[59] Gao et al.[42] developed a bio-inspired novel antifogging surface by using the soft-lithography technique. First, the authors used lithography technique to obtain a rough surface using PDMS followed by use of silica nano particles (100 nm diameter) arranged in hcp (hexagonally closed pack) structure. Subsequently, the prepared compound-eye like structures are chemically modified with FAS (Fluoro alkyl silane) molecules. The prepared surfaces exhibited superior static WCA of 155° and sliding angle of 15°. The authors first spin coated a positive photoresist on glass slides at 2000 rpm and kept them in the oven at 88°C for 18 min. this was again followed by spin coating with the photoresist to obtain a thicker coating on the glass slides. In order to generate circular photoresist posts, the authors kept the coated glass slides under a 1000 W UV light source through a transparent mask patterned with 20 µm circles spaced 5 µm apart. After the samples were heated at 160°C for 5–15 min the circular photoresist posts melted because of the

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minimization in interfacial energy and formed hemispheres. As shown in Figure 7 the compound eye like structure was imitated through a large number of synthetically prepared PDMS hemispheres of a diameter 22 µm and arranged in an hcp structure. 4.4. Insects and butterfly wings inspired Apart from isotropic natural leaves, different cases of anisotropic surfaces were also found in nature, for example, the legs of a few insects like arthropods, pigeon feathers, butterfly wings[46,60–62] etc., which remain dirt free naturally. This is accomplished due to a specific interaction between the dual scale topography of their wings with water droplets. Water droplets roll outward because of its two periodic structures: the epidermal scale (∼40–80 microns) and the micro-relief of raised ridges covering each wing scale (∼1200–1500 nm in width) which together create distinctive adhesive force.[63] Soz et al.[63] fabricated super hydrophobic surfaces of WCA 150° and fairly low hysteresis contact angle values (