Fish-scale inspired superoleophobic membrane from cellulose: A Review

Nature-inspired technology has been investigated widely. Anti biofouling properties of fish scale has inpired to improve membrane performance. Biofouling affects an oil-water separation membrane. The bacterial, coagulant


Introduction
Nature has inspired researchers, engineers, chemists, architects, etc. to create valuable technology and products. For instance, the airplane was created by mimicking how birds fly, and the Shinkansen's front-end design mimics a kingfisher to reduce noise. This technology is called "bioinspired technology". The bioinspired technology is promising in terms of efficiency because it mimics natural structures that have been involved in natural selection for thousands of years (Abaie et al., 2021;Halim et al., 2022;Jin et al., 2022).
Recently, bioinspired technology has been used not only on the macroscale, but also on the micro-and mesoscale. For example, the nanofur of gecko feet inspired a dry adhesive pad (J. Fan et al., 2022;L. Wang et al., 2020; W. Wang et al., 2021), the nano bumb of wax layer on the lotus leaf inspired the superhydrophobic surface for oil-water membrane (Halim et al., 2023;Sam et al., 2021), the nanostructure of namib beetle inspired development of water harvesting from air(Gou & Guo, 2020; F. Zhang & Guo, 2020;Y. Zhang et al., 2022), and the fish scale principle inspired development of selfcleaning membrane (Halim et al., 2019(Halim et al., , 2022Halim, Lin, et al., 2020;Yang et al., 2021). The fish scale has oleophobic properties that repel oil and penetrate only water keeping the fish skin always clean even though lives in a muddy environment (Ángeles Esteban, 2012). The secreted protein of fish scale forms a hydrogel layer absorbing water (Waghmare et al., 2014) and as a result, creating superoleophobic properties.
By mimicking the fish scale structure, a functional membrane is developed to separate the oil/water mixture (Ao et al., 2017(Ao et al., , 2018(Ao et al., , 2021. This property is useful in the membrane to prevent membrane blocking by repelling oil and keeping the membrane surface clean (self-cleaning ability). Even if oil and water are immiscible liquids, the current separation process needs a long residence time, a large space, and high energy consumption.
The increase in industrial activities brings huge environmental consequences, such as increased wastewater effluent. Water is essential for life and strongly affects energy and food production (Kibler et al., 2018;Scanlon et al., 2017). Therefore, purification is the most important process. Water is purified by chemical, physical, or biological processes. Chemical processes for purification include electrolysis, coagulation, and flocculation. The physical processes such as sedimentation, distillation, or filtration contrast with the biological processes such as enzymatic or bacterial oxidation. The filtration membrane is one of the most promising processes due to its effectiveness in separating a wide range of pollutants from microscale to nanoscale. The principle of filtration is divided into three categories: sieving by size, electrical, and absorption difference (Padaki et al., 2015). However, the membrane surface faces a fouling problem that deteriorates the performance (Fathanah et al., 2020;Martini, 2022). Superoleophobic based membranes have been introduced to overcome this problem. The superoleophobic property is inspired by a fish scale that keeps the fish clean even when living in a harsh environment. This principle is adapted to the surface of the membrane so that membrane performances (i.e., separation efficiency, flow rate, and resistance to fouling) can be increased. Numerical simulations show that the hydroxyl group has a higher oleophobicity underwater than other groups such as methyl, amide, oligo (ethylene glycol), and ethanolamine (G. Cheng et al., 2017).
Cellulose is the most abundant biopolymer which widely distributed in plants, marine animals, bacteria, and even single-celled organisms like protozoa (Lavoine et al., 2012). Cellulose is comprised of β-d-glucopyranose units linked by β-1-4-linkages. Every cellulose monomer contains three hydroxyl groups that construct strong hydrogen bonds, thus offering graded structural properties. There are four different kinds of cellulose polymorphs, including cellulose I, II, III, and IV. Cellulose I is abundantly found in nature and consists of two different forms that mix, Iα and Iβ (Atalla & VanderHart, 1984). Triclinic Iα and monoclinic Iβ form is a variant present respectively in algal-bacterial cellulose and annual plants (Dinand et al., 2002). Cellulose II represent the two main polycrystalline forms of cellulose. It is prepared by precipitation in an alkaline solution. Those two kinds of cellulose II distinguished by antiparallel accumulation and parallel chain run to cellulose I (Dinand et al., 2002;Isobe et al., 2012Isobe et al., , 2013Sugiyama et al., 1991). Cellulose II is the most stable form of thermodynamics. The cellulose I and II which treated with ammonia would generate cellulose IIII and IIIII. Furthermore, modification of cellulose III decisively produces cellulose IV (Lavoine et al., 2012). Based on its high crystallinity and modulus, cellulose I most influences the mechanical properties that function as cellulose nanofillers, through a post-extraction lignocellulose bleaching process.
Due to the abundance of the hydroxyl group on its polymer chain, cellulose also poses underwater superoleophobicity (Halim et al., 2019). Compared to other polymers such as polyvinyl alcohol, cellulose offers advantages such as biodegradability, environmental friendliness, a carbon zero pathway, and being the most abundant polymer.
This review discusses the principle of the superoleophobic membrane. First, the wettability definition will be discussed to give some background on the superoleophobic surface. The second part will discuss the separation mechanism based on the superoleophobic surface. The third part will discuss about the polymer mainly used for superoleophobic surfaces and will be focused on cellulose. Four parts will discuss the minimum parameters for the evaluation of membrane performance. The final section will go over the challenges that must be addressed as well as the future development.

Wettability
As the superoleophobic membrane is based on the wettability of the surface, defining the term wettability is important. The wettability of the surface is generally assessed by the contact angle measurement of water or oil on a surface. A surface with a high water contact angle (>90°) is classified as hydrophobic, while a surface with a low water contact angle (<90°) is hydrophilic (Tuteja et al., 2007). The contact angle is influenced by three interfacial interactions, namely solid-liquid, solid-fluid,  and liquid-fluid interfaces. The total force acting on the interfaces determines the contact angle, as described in Figure 1. Due to the static state, the force acting on an interface is described by Equation 1.

cos = − (1)
Where and are apparent contact angle and surface tension, respectively. The subscribe of , , and refer to liquid, solid, and air, respectively. This equation is called Young's equation (Young, 1805). The equation provides an approximation of the contact angle of some surfaces. The contact area between solids and liquids affects the force, as shown in Figure 1b. This condition is known as the Wenzel state. Therefore, for rough surfaces, Equation 1 is modified to Equation 2.

cos = cos (2)
where is the Cassie-Baxter state contact angle, is the roughness of the solid-liquid interface, and the is the solid-oil fraction of the projected area. If water substitutes for the air environment, and the liquid is oil, the equation is modified to Equations 4-7 (Z. Wang et al., 2016) with subscribe , , and refer to water, oil, and air respectively. Therefore, is the contact angle of oil in the water environment. Figure 3 shows the schematic diagram of equations to predict the contact angle. The contact angle alone does not perfectly describe the wettability of the surface. For instance, the water drop on the surface of the rose petal shows a high contact angle, but the drop is highly attached to the surface (Bhushan & Nosonovsky, 2010;Feng et al., 2008). For a hydrophobic application, this property is not desirable. Therefore, one parameter that is the tilting angle is introduced. The title angle ( ) is the minimum angle of surface to roll the droplet. The value of tilting angle is similar to that of contact angle hysteresis, which is the difference between an advancing contact angle and a receding contact angle. Advancing contact angle ( ) is equivalent to the maximum contact angle if a droplet volume is increased, while receding contact angle ( ) is equivalent to the minimum contact angle if a droplet volume is decreased, as shown in Figure 2 (Z. Wang et al., 2016). The prediction of underwater contact angle is a complicated work. The modified Young's, Cassie, and Wenzel equation underwater is not robust enough to validate the contact angle underwater for cellulose because of surface dissociation. The absorption of water changes the roughness, and the dissociation of surface ions changes the surface energy (Halim, Lin, et al., 2020).

Separation Mechanism of Superoleophobic Membrane
Superoleophobic membrane shows promising separation efficiency and flowrate compared to conventional membrane. The improvement is cause by additional separation mechanisms instead of size sieving alone. Figure 4 depicts the separation mechanism of oily wastewater by the superoleophobic membrane. During the process, small oil droplets will collide with each other (1) or with a larger droplet (2) (Halim et al., 2022). These diverse sizes of droplets contact the oleophobic surface and are repelled. The cellulose hydroxyl groups absorb a lot of water and prevent the surface moieties from forming a superoleophobic layer (blue line). This layer of water will repel the oil droplets (3) and pass-through water (4). The repelled oil droplets collide with other oil droplets over and over. Pressure from gravity or a pump is the driving force behind the flow of an oil-water mixture. High pressure will force an oil droplet to impregnate membrane pores. The maximum pressure before the oil droplet impregnates the membrane pore is called breakthrough pressure. Breakthrough pressure is influenced by surface wettability, surface tension of the oil droplet, and membrane pore diameter. The maximum pressure to effectively filter oil droplets is predicted by Equation 8: where is the breakthrough pressure, is interfacial surface tension between oil and water, is the diameter of a pore, and is and OCA against water.

Cellulose as Superoleophobic Membrane
The contribution of cellulose as a superoleophobic membrane is divided into two categories: supporting materials and active materials, as shown in Figure 5 (Halim et al., 2022). In the first category, cellulose acts as a supporting material. The most commonly used cellulose material is woven cotton fabric (Ao et al., 2017;Dai et al., 2019). Other materials such as paper-like (J. B. Fan et al., 2015), 3D structures such as aerogel (Fu et al., 2020;Z. He et al., 2016) and sponge-like (Halim et al., 2019;Halim, Xu, et al., 2020;G. Wang et al., 2015), and non-woven cotton. For 3D structure, raw cellulose is dissolved and regenerated to form a desired structure. The cellulose is usually coated with particles such as ZnO (T. Fan et al., 2018;Gao et al., 2018), CaCO3 (Yang et al., 2021) and Fe3O4 (Shen et al., 2018) to increase its roughness and give additional properties.
For instance, TiO2 gives photocatalytic properties (Y. Yin et al., 2016) and Fe3O4 imposes magnetic properties. The coating is also applied to increase the oleophobicity and decrease the pore size of the membrane. Precipitation of precursor particles, hydrothermal processing, and direct coating of commercially available particles are among the coating methods.
Second, cellulose acts as a coating agent that acts as a functional material to create the  Rohrbach et al., 2014;Xi et al., 2021). Supporting materials are generally synthetic polymers such as nylon (Lu et al., 2014), and metal mesh such as stainless steel (L. Zhang et al., 2013). The supporting material gives physical strength. In comparison to cellulose as a supporting material, cellulose as an active material provides more options and greater physical strength.
For biodegradability reasons, cellulose was also reported as an active and supporting material. (Almeida et al., 2020;Halim et al., 2019).

Performance Evaluation
The membrane's performance is evaluated by its filtration performance and material performance ( Figure 6). The filtration performance is evaluated by contact angle, flowrate, separation efficiency, and pressure. The flowrate is a trade-off for separation efficiency, where a higher flow rate will produce a lower separation efficiency, vice versa (Halim et al., 2019). The pressure includes breakthrough pressure where the maximum pressure before the oil penetrates the membrane and minimum pressure to perform the separation (Chen & Xu, 2013; C. F. Wang et al., 2017). The flow rate usually corresponds to the amount of energy needed to operate the membrane. A low flowrate needs vacuum or other pressure to increase the flow rate. The many superoleophobic membranes are reported to only use gravitational force alone (Rohrbach et al., 2014;Fan et al., 2015;Wang, Yang, and Kuo, 2017;Halim et al., 2019).
The separation efficiency is qualitatively observed by the Tyndall effect (Q. Cheng et al., 2017) or microscope observation (C. F. Wang et al., 2017)

Summary and Future Outlook
Several reports have validated the functionality and effectiveness of superoleophobic membranes. Cellulose, with its abundant hydroxyl group, has good performance as a superoleophobic material, either as a supporting material or as an active material.
Some additives such as nanoparticles or organic crosslinking; are added to increase roughness and physical strength, respectively. However, the utilization of cellulose as a membrane due to its sustainability must consider the by-product and life end of the membrane, especially when the membrane is composited with nonsustainable materials. Another future challenge is the validation of the technology in a real pilot plant. The utilization of membrane in the specific case needs to be investigated. High pressure resilient membranes are necessary for industrial systems; however, medium pressure might be enough for household applications.  Yang, J., Xie, A., Cui, J., Chen, Y., Lang, J., Li, C., Yan, Y., & Dai, J., 2020. An acidalkali-salt resistant cellulose membrane by rapidly depositing polydopamine and assembling BaSO4 nanosheets for oil/water separation. Cellulose 27 (9), 5169-5178. https://doi.org/10.1007/s10570-020-03114-9