Omniphobic surfaces: state-of-the-art and future perspectives

Abstract

Limited oleophobicity of the Lotus-inspired superhydrophobic surfaces accompanied by low stability and self-healing inability, have led to the evolution of synthetic omniphobic surfaces that repels a variety of liquids. This review summarized recently developed concepts for preparation of such surfaces, based on engineering the surface properties. Future challenges and perspectives for this growing field of research have also been addresses.

Keywords:

• Omniphobic
• surface
• lotus- inspired
• preparation

1. Introduction

Designing a liquid-repellent surface, regardless of its complexity, would be a promising approach in future applications ranging from architecture and industry to medicine [1]. A deposited droplet on a surface forms a contact angle, terms as material contact angle that reflects the wettability of the surface. However, in reality, there are two contact angles, receding (θ_rec) and advancing (θ_adv), for a droplet in contact with a surface. The pinning strength of a droplet to the surface asperities is represented by the difference of these two angles (Δθ = θ_adv−θ_rec), is known as contact angle hysteresis [2]. Removing a droplet from the surface needs a distortion in drop shape. The required force for this distortion is usually expressed as the sliding angle: (1) $$\text{sin}\alpha =\frac{{w\gamma }_{\mathit{LV}}}{\mathit{mg}}(\text{cos}{\theta }_{\mathit{rec}}-\text{cos}{\theta }_{\mathit{adv}})$$(1)

W is the width of the droplet in contact with surface, m and g are the drop mass and gravitational acceleration, respectively, α is the sliding angle and γ_LV represents the surface tension of liquid. Reducing the liquid surface tension and contact angle hysteresis can lead to a low tilt (or rolling) angle, implying low gravitational force required to overcome the drop adhesion and therefore, easy removal of the drop [3].

Inspiring from Lotus leaves as a natural water-repellent structure [4,5], liquid-repellent synthetic micro structured substrate have been developed relying on the formation of a durable air-liquid interface, which presented water contact angle of higher than 150° and hysteresis of lower than 5°. In these artificial superhydrophobic surfaces, entrapped air among asperities of a rough surface is responsible for repellency and decreasing the droplets adhesion, via a non-wetting Cassie state [6].

Despite the promising potentials of superhydrophobic surfaces, trapped air within the texture is susceptible to pressure, temperature and low surface tension fluids. Inability of these surfaces for self-repairing along with high production cost restricted their applicability.

To overcome the aforementioned shortcomings, nature benefits from simple and effective alternatives, repelling not only water droplets, but also low surface tension liquids. Designing superhydrophobic and superoleophobic surfaces have been reported by different groups [7–11], but special surfaces capable of repelling a wide variety of liquids with different surface tensions require precise considerations. Mimicking nature strategies, researchers tried to generate omniphobic surfaces.

The present paper overviews current approaches in designing liquid-repellent surfaces for the first time. Each section describes fabrication techniques that have been used to obtain an omniphobic surface, based on a unique idea. Finally, remained challenges and future perspectives have been discussed to pave the way for the far-reaching approaches.

2. Slips

Inspiring from Nepenthes pitcher plant, Aizenberg [12] research group designed liquid-repellent surface for the first time and named it “slippery liquid-infused porous surface (SLIPS). To prepare this omniphobic surface, they coated a porous substrate with lubricating agent such as perfluorinated liquid (Figure 1(a,b)). Liquid could spread over the surface, wet the substrate via capillary wicking and form a smooth and stable interface, where inhibit pinning of the low or high surface tension liquids contact line to the surface.

Figure 1. (a) Schematic procedure of preparation of SLIPS by infiltration of a low-surface-energy liquid into the porous substrate. (b) SEM images of epoxy-based nano post arrays (left) and nano fiber network of Teflon (right) [12]. (A) Schematic concept of perfluorinated lubricant infiltration into the porous Teflon substrate. (B) Formation of bacteria layer on the prepared SLIPS after 7 days compared with PEGylated surface after 5 h, as presented in [14]. Reproduced by permission of ref. [11,13].

In order to design such SLIPS, first of all, lubricant should wet the surface and wick into the porous matrix. Secondly, substrate should be wetted by the lubricant not the test fluid. Moreover, lubricant and working fluid should be immiscible. To satisfy these requirements, a rough topography with high affinity to the lubricant would facilitate wettability and formation of a stable slippery layer throughout the substrate.

Consequently, Aizenberg group utilized two different porous substrates: ordered nano-post arrays functionalized with polyfluoroalkyl silane and a network of Teflon nano fibers distributed randomly in the matrix. 3 M Fluorinated FC-70 was employed as a low surface tension perfluorinated liquid acting as lubricant, which is not miscible with a broad range of polar and non-polar fluids. The achieved SLIPS exhibited a smooth and homogenous surface in molecular level with roughness of around 1 nm, low contact angle hysteresis (Δθ ≤ 2.5°) and tilt angle of α ≤ 5°.

In onmiphobic surfaces inspired by lotus-leaf, reducing droplet surface tension increases contact angle hysteresis significantly, while smoothness and chemical integrity of liquid-liquid interface in SLIPS, hindered such dependence.

In another study [13], they infused perfluoropolyether into a PTFE membrane substrate to obtain a SLIPS material (Figure 1(A)). The prepared platform could inhibit deposition of around 99% of bacteria such as E. coli,S. aureus and P. aeruginosa, which was 35 times more than best-case-scenario, state-of-the-art PEGylated substrate [14] (Figure 1(B)). The prepared slippery surface remained stable under extreme conditions such as: concentrated brine, pH = 1–14 and UV exposure.

Glavan et al. [15] used fluoroalkyl trichlorosilanes to silanize three commercial cellulose papers with different topography in gas phase. The obtained omniphobic papers could repel low surface tension (28 mN m⁻¹) liquids, ascribed to the long fluorinated alkyl chains. After impregnation of the paper with a perfluoropolyether oil, a slippery surface (SLIPS), which was able to repel the very low surface tension (15 mN m⁻¹) liquids was formed (Figure 2).

Figure 2. (a) SEM picture of textured alumina and contact angle of water droplet on FDPA-modified surface (inset). (b) Representation of wetted Wenzel state. (c) Water contact angle on the textured FDPA-alumina infiltrated with PFPE. Reprinted with permission from ref. [15].

Ma et al. [16] treated the surface of alumina to form a nano-textured film and then immobilized perfluorodecylphosphoric acid (FDPA) on it. The obtained low energy film presented water contact angle of 160°, represented a superhydrophobic surface governed by Wenzel state. However, modifying the smooth alumina film could increase the contact angle only up to 113°. Despite the relatively suitable water repellency of the FDPA-coated rough alumina film, it suffered from low oleophobicity. Therefore, they infused perfluoropolyether (PFPE) lubricating liquid into the textured surface. Immiscibility of PFPE with ionic and non-ionic liquids and its remarkable chain mobility could inhibit deposition of liquid droplets in to the substrate and reduce the tilt angle to around 5 for non-polar liquids.

An interesting methodology for preparation of an antithrombogenic surface was proposed by Leslie et al. [17], which can be applied to smooth surfaces and resemble the SLIPS technology. They immobilized tethered perfluorocarbon (TP) on the substrate and coated it with perfluorodecalim (LP). Freely moving LP chains played as a liquid layer resembled a modified SLIPS method. They scrutinized the interaction of blood with the coated surface in a variety of static and dynamic situations, remarkably.

The prepared omniphobic surface not only could inhibit precipitation of microorganism and biofilm formation, but also presented outstanding antithrombogenicity prevented proteins and platelets adhesion in vitro as well as in vivo (Figure 3).

Figure 3. Blood repellency of TLP-coated substrate. Reprinted with permission from ref. [16].

Despite all efforts conducted in designing liquid-repellent surfaces, holding the lubricant in place during exposure of the non-sticking surface to foreign stresses remained a challenge. Therefore, Aizenberg group scrutinized the influence of the surface asperities size on retaining of lubricant under harsh situation [18]. They treated aluminum surface and created three roughness scales, 3.2 μm, 50–100 nm and a combination of nano and micro scale features (hierarchical), used as substrate. The surfaces were then functionalized with fluorine groups to facilitate further infiltration with fluorinated lubricant (K 100). Primary results showed that fluorinated hierarchically and nanometer textured surfaces presented higher superhydrophilicity compared to microscale roughened sample. Moreover, in comparison with flat surfaces, rough ones demonstrated better liquid-repellency after imbibing the fluorinated lubricant. To further investigate the effect of roughness scales on SLIPS performance under high shear stresses, they also subjected the samples to harsh conditions. When the length scales of the features can not surpass the lubricant capillary length, they can provide enough capillary force to retain lubricant in place and form a low-surface-energy liquid interface and prevent contact line pinning. Whereas, entrapped lubricant among the micro-scale asperities on the surface, can easily run away under high stress condition. Consequently, they proposed that uniform nanometer features on the surface can form a more stable SLIPS compared to micrometers and hierarchically textured surfaces.

In order to pin the impregnated fluorinated lubricating liquid on the polybutadiene (PB) surface, Kamei et al. [19] patterned the surface in a honeycomb-like form by using self-organization. PB was fluorinated via the reaction of its vinyl groups with thiol groups of oerfluorooctanethiol (PFOT) to lower the surface energy and can retain the infused perfluoroalkylether as lubricant. The prepared liquid-repellent surface exhibited sliding angle and contact angle hysteresis of 5.4° and 1.9° for water, 3.3° and 2° for tetradecane, and 3.9° and 4.6° for ethylene glycol, respectively (Figure 4). The very low obtained angles were quite in agreement with the reported sliding angles for lotus-inspired omniphobic surfaces, which are normally lower than 5°.

Figure 4. (a) Fluorination of the film. (b) SEM image of the treated film. (c) Sliding angle and hysteresis of various liquids on the lubricated substrate. Reproduced by permission of ref. [18].

To reduce the evaporation loss of the liquid lubricant, low vapor pressure lubricants have priority. To have a stable SLIPS, immiscibility of the working liquid and lubricant is required. To do so, molecular configuration, surface tension and polarity of working-lubricant fluid pair needs to be similar, which minimize the interfacial tension and help rolling the droplet off the surface. However, for immiscible fluids, working liquid droplets may be cloaked (encapsulated) by lubricating agent and lead to the removal of lubricant.

Sett et al. [20] evaluated miscibility of a variety of working liquids and lubricants with a broad range of surface tensions to find a suitable lubricant for all working fluids. Cloaking possibility of the working liquid droplet by lubricant was investigated by measuring the coefficient of oil spreading over the drop, as follows: (2) $${\text{S}}_{\text{ol}}={\gamma }_l–{\gamma }_{\text{o}}–{\gamma }_{\text{ol}}$$(2) Where γ_l and γ_o are the working liquid and lubricant –vapor surface tensions, and γ_ol stands for the working-lubricating fluids interfacial tension. To have an effective and desirable lubricant in SLIPS, S_ol < 0 is preferable [21].

However, in addition to the depletion of lubricant by cloaking mechanism, shearing stresses generated during shedding of the working liquid could also result in lubricant drainage even for immiscible liquids, as observed also by other groups [22–24]. According to the reported data, a unique lubricant presenting immiscibility with all working liquids could not be proposed.

3. Socal

In order to overcome the deficiencies of the previously mentioned methods, Wang et al. [25] proposed a fast and simple method to obtain omniphobic surfaces. Slippery omniphobic covalently attached liquid (SOCAL) PDMS coating was created by polycondensation of dimethyldimethoxysilane (Me₂Si(OMe)₂) on a substrate in a solution of isopropanol and sulfuric acid (Figure 5). Freely motion of the homogenously grafted PDMS chains resulted in a very low contact angle hysteresis (Δθ ≤ 1°) upon contact with a variety of liquids. Reduction in liquid surface tension was accompanied by reduction in sliding angle, which was in accordance with aforementioned theories. However, the obtained results were in contrast with omniphobic SLIPS, where sliding angle is independent of liquid surface tension [12].

Figure 5. (a) Fabrication procedure of omniphobic PDMS coating. (b) Formation of liquid-like PDMS coating. Reprinted with permission from ref. [24].

4. Robust

Re-entrant structure has been observed on different plant leaves, stabilizing an interface with water and representing superhydrophobicity. But, they are susceptible to wetting by low surface tension oils. Tuteja et al. [26] optimized the Cassie scenario and made a metastable composite solid-liquid-air interface, which could repel various liquids with broad range of surface tension from 15.7 ≤ γ_lv ≤72.1mN/m. To do so, they created re-entrant texture on a silicon wafer by using photo-lithography. Engineering the re-entrant geometry and robustness parameters could control the wettability of the surface. To reduce the surface energy of the textured surface, it was silanized by using perfluorodecyltrichlorosilane or fluorodecyl POSS molecules.

However, re-entrant structure is supposed to repel low surface tension liquids, but it suffers from low mechanical stability. Unique geometry of the textured surface can enhance fragility and disturbance of the overhang parts due to high energy adsorption under external pressure or abrasion or even capillary force [27–30]. Mimicking the springtail cuticle and to obtain a liquid repellent as well as mechanically durable surface, Zhu et al. [31] textured the amphiphilic PVA membrane with interconnected cavities by using microfluidic emulsion templating (MET) method (Figure 6(a)). This procedure could induce omniphobicity on various substrates without any extra surface chemical modification. Water contact angle was around 120 in different pH, which states the chemical stability of the textured surface. Moreover, the re-entrant structure presented contact angle of θ > 90 in contact with a variety of polar and nonpolar liquids such as glycerol, olive oil, hexadecane and octanol, indicated the omniphobicity of the porous membrane (Figure 6(b)). The entrapped air pockets in micro-cavities could push the liquid droplets away and stabilize a reversible non-wetting Cassie state. In a similar study, Seo et al. [32] fabricated a repulsive air-spring structure by mimicking the springtail’s skin. The surface was consisted of a 500 nm hole arrayed flexible PDMS membrane and the micron-size pillar structures. The micropillar array was composed of cubes (length: 10 μm, width: 10 μm and height: 10 μm) or cylinders (diameter:10 μm and height: 10 μm). The thickness of the nanohole membrane was 0.5 μm. Which, was stably positioned on top of the micro-structures. The obtained surface showed water contact angle of 151° and mineral oil contact angle of 112°. Moreover, the contact angle of the blood was around 140°. When the surface tilted for 45°, blood droplets flowed rapidly without residual substance.

Figure 6. (a) Schematic of the preparation procedure of porous membrane.(b) Contact angle of a variety of liquids on the porous substrate. Reprinted with permission from ref. [29].

5. Other methods

Based on the self-aggregating property of fluorinated polymers and also their ability to form a slippery surface with low surface tension [33], Junyang et al. [34] functionalized poly(methyl methacrylate) polymer with fluorine groups and prepared casting films with different surface topography, to evaluate the influencing factors on omniphobic properties of the obtained films (Figure 7).

Figure 7. SEM images of casted DFHM-ef-PMMA film surface; and enlarged view (inset). Reproduced with permission from ref. [32].

It is known that smooth surface repels liquid droplets by sliding, while rough topography governed by Cassie state induces liquid repellency by rolling mechanism [35,36]. Dodecafluoroheptyl methacrylate DFHM-ef-PMMA chains were able to form different morphologies of aggregations in different solutions. Fluorinated segments of these polymers could migrate to the surface due to phase inversion phenomenon occurs during casting the aggregate solution and film formation. However, utilizing various amount of functionalized PMMA with different fluorine content and also various aggregate morphologies in film formation influenced the surface topography. This report claimed that surface smoothness accompanied by high content of fluorinated groups (29.6 ∼ 34.8%wt) could induce high liquid repellency and also dewetting property on the film surface. They could reach a contact angle (CA) of about 108° ± 2° and 53° ± 2° and also sliding angle (SA) of around 33° ± 2° and 13° ± 2° upon contact of the film surface with water and hexadecane, respectively. Whereas the obtained samples with extreme smoothness and low fluorine content (13.2%w) or rough surface possessing even high content of fluorinated groups did not exhibit omniphobic property.

Neelakantan et al. [37] prepare a rough surface by coating the substrate with a mixture of ZnO/PDMS with different ratios and evaluated the influence of surface topography on contact angle. In best case, they could reach a water contact angle and sliding angle of around 150° and 5°, respectively, whereas the refrigeration oil RL-68H (γ = 27.7 mN/m) was pinned on the surface. In order to enhance the oleophobicity of the prepared superhydrophobic surface and render it omniphobic, immobilization of Teflon AF and perfluorodecyltrichlorosilane (FDTS) were examined, separately. Presence of a Teflon layer over coated on the textured substrate could result in contact angle of about 157° and 137° for water and RL-68H, respectively. But, oil repellency of this surface was not satisfactory. In comparison, FDTS coated surface presented contact angle and sliding angle of around 144° and 5° for water, and 148° and 17° for oil, respectively. This liquid repellency was ascribed to the lower surface energy of FDTS compared to Teflon, related to higher fluorine content.

6. Conclusions and outlook

Engineering the bio-inspired surfaces and adopting the clever solutions that nature has found for complex issues can be the most trustable and direct way for future developments. Mimicking different plants leaves, altering the surface topography and free energy as well as chemistry, resulted in the creation of prestigious strategies to make omniphobic surfaces, which have been briefly overviewed here. Working environment needs to be considered before selection of an appropriate strategy for surface modification. However, mechanical, chemical and long-term durability under harsh conditions, low-cost and easy procedure for large scale preparation remained as the main challenges.

Future research needs to go towards instant lubrication, where the external stresses can not deplete the surface of lubricating agent. In the author’s opinion, by mimicking the exocrine glands such as sweat or intestinal glands that release their secretion near the surface, some degradable reservoirs containing the lubricating liquid can be embedded in a porous substrate. By controlling the degradation rate, the amount of the released lubricant can be regulated for a long time. Theses reservoirs can be placed inside the matrix by using a multi-step preparation concept similar to hybrid structures fabrication, where the container can be a considered as an intermediate layer. However, alternative ways can also be applicable.

Disclosure statement

No potential conflict of interest was reported by the author.