Current Status and Future Prospects of Applying Bioinspired Superhydrophobic Materials for Stone Artworks Conservation

: The development of innovative materials is one of the most important focuses of research in heritage conservation. Eligible materials can not only protect the physical and chemical integrity of artworks, but also preserve their artistic and aesthetic features. Recently, as one of the hot research topics in materials science, biomimetic superhydrophobic materials have gradually attracted the attention of conservation scientists due to their unique properties. In fact, ultra-repellent materials are particularly suitable for hydrophobization treatments on outdoor artworks. Owing to their excellent hydrophobicity, superhydrophobic materials can effectively prevent the absorption, penetration of liquid water as well as the condensation of water vapor, thus greatly relieving water-induced decay phenomena. Moreover, in presence of liquid water, the superhydrophobic surfaces equipped with self-cleaning property can clean the dirt, dust deposited spontaneously, thereby restoring the artistic features simultaneously. In the present paper, besides the basic principles of wetting on solid surfaces, materials and methods reported for preparing bioinspired ultra-repellent materials, the recently proposed materials for art conservation are also introduced and critically reviewed. Lastly, the current status and the problems encountered in practical application are also pointed out, and the focus of future research is prospected as well.


Introduction
Surface water inhibition treatment is an indispensable action that has been widely taken for the conservation and restoration of various types of artworks which include stones, ceramics, mortars, woods, bronzes etc., since liquid water has been regarded as one of the most fundamental causes accounting for their degradation. Taking stone objects for example, liquid water not only causes the dissolution of water-soluble components inside stones, but also induces or promotes other waterrelated decay phenomena, such as freeze/thaw cycles [1], salt crystallization [2], deposition of atmospheric contaminants [3], leading to increased bioreceptivity and susceptibility towards biological colonization [4,5] and so on. In outdoor conditions, with the impact of other environmental factors e.g. atmospheric pollutants and spores, the infamous black crusts and microorganism colonization, as well as corrosion and decohesion phenomena, are commonly observed on stone heritage ( Figure 1). In order to halt or slow down stone degradation, hydrophobic surfaces are universally required.
By definition, protection is an active treatment that is operated directly on objects, providing hydrophobic properties as well as resistance to deterioration agents (e.g. pollution, micro-organisms etc.), without reducing the original vapor permeability [6]. As nowadays there are extensively large quantities of natural and synthetic substances available on the market, a series of criteria shall be considered when selecting appropriate product/s. According to standard Recommendation [7], the essential requirements for a substance to be considered as a protective agent for stone artifacts include: 1) hydrophobicity; 2) good adhesion to stone substrates; 3) good permeability to water vapor; 4) good chemical, physical compatibility with stone substrates; 5) good chemical, physical, thermal and photo-oxidative stability; 6) good solubility in environmentally benign and healthy solvents; 7) no perceivable color change of substrates; 8) reversibility etc. As a matter of fact, the history of surface treatment conducted on stone objects dates back to ancient Greek and Roman periods, when natural materials such as oils and waxes were used for protection and polishing purposes [8,9]. With the advance of science and technology, since the second half of the 20th century, synthetic polymeric materials have completely replaced natural resins in stone protection [9]. The main reasons for their popularity are their diversity in type and range of applications, and the possibility of individualised or performance-oriented adjustment during production [9]. Acrylic resins, silicon based resins, microcrystalline waxes, perfluoropolyethers (PFPEs), polyfluorourethanes, fluorinated polymers and fluoroelastomers are the representative compounds [8][9][10][11][12][13]. Recently, starting from the 21st century, some innovative materials such as structurally modified acrylic polymers and PFPEs, organicinorganic hybrids, nano-scaled and micro-emulsions of some polymers, nanoparticle composites etc. are being proposed and tested [14][15][16][17][18][19][20]. These products usually show good chemical, physical compatibility, good adhesion and good solubility, and in some cases also good consolidating effect [9,10,16]. Yet, they also have drawbacks in long-term use (e.g. low photo-oxidative stability, biodegradation, irreversibility) [10,16,[21][22][23][24]. In particular, their hydrophobicity and performance durability are not sufficient for protecting stones in outdoor environment, as black crusts, dust, dirt depositions and biological growth are frequently seen on some coated stone statues, buildings, mortars etc [25].
Nature is an unexhaustive source in inspiring humans to accomplish new scientific and/or technological achievements. Observing the unique wetting and antiwetting phenomena in nature, e.g. water rolling off on lotus leaves [26], dew, rain nonwettable and dust-free cicada wings [27], dirtfree gecko feet [28], water pinning on rose petals [29], etc. ultra repellent surfaces have attracted great interests in academic and industrial fields. Over the last two decades, artificial surfaces with "lotus effect" or "rose petal effect" properties have been studied and many successful biomimic surfaces have been prepared [30][31][32][33][34]. Among the two broad types of super-antiwetting materials, surfaces with "lotus effect" [static water contact angle (CA) 150° and low contact angle hysteresis (CAH< 10°)] have gained more attention, owing to their great value in both theoretical research and functional application in fields such as self-cleaning, anti-biofouling, anti-icing, oil/water separation etc. [35][36][37][38][39][40][41]. In fact, ultra water/oil repellent surfaces equipped with self-cleaning property are also the most ideal materials for protecting building stones which are exposed to open air, since they can well preserve the chemical, physical integrity of substrates while conserving the artistic features at the same time.
In this review, the basic principles of different wetting phenomena on solid surfaces and the definition of superhydrophobicity are introduced briefly. The most widely used fabrication materials and methods, along with the pros and cons of these proposed materials/methods for stone conservation, are described and critically reviewed. Last but not least, the obstacles need to be tackled in achieving superhydrophobic surfaces practically and the perspectives on future research (in both materials synthesis and application) are also presented.

Wetting properties and basic models
As a common interface phenomenon, the wetting of a solid surface by a liquid is mainly determined by the microstructure of the solid surface and the surface chemical composition (surface free energy) [27,30]. CA quantifies the wettability of a solid surface by a liquid via the Young's equation (Equation 1, Figure 2) where  is the interfacial tension, subscripts SV, SL and LV denote the solid-vapor (gas), solid-liquid and liquid-vapor (gas) interface respectively and θY (Young CA) is the equilibrium CA at three-phase point [42]. Surfaces are defined as hydrophilic when the water CA is less than 90°. If the CA ranges from 90° to 150°, the surface is hydrophobic, while surfaces are superhydrophobic when CA150° [43].

Smooth surface
Young's equation is an ideal model for perfectly smooth surfaces, since it neglects the influence of surface roughness, chemical heterogeneity, surface reconstructions swelling and dissolution etc. which do affect the wetting phenomenon. Among these factors, surface roughness is regarded as one of the most influential parameters on CA [44,45]. Two models i.e. Wenzel model and Cassie-Baxter model were developed later and generally employed to correlate surface roughness with apparent CA.

Rough surface
In the Wenzel cases (Equation 2), surface roughness factor r is considered (Equation 3), since it assumes the droplet follows the rough surface and fills in the asperities (Figure 2b) [44]. The roughness factor amplifies the effect of surface chemistry determined by cos. When Y 90°, an increase of r reduces apparent CA (w, Wenzel CA), while if Y 90°, an increase in r results in an increase of w. These means, a hydrophilic surface will become more hydrophilic when its surface is rougher, whereas a hydrophobic surface will be more hydrophobic when its surface is roughened. In general, droplets in the Wenzel state show strong adhesion to surface. Yet, for some surfaces, such as highly rough or porous structures, the absolute value of the right side in Equation 2 might be larger than 1. In such cases, the Wenzel model is not valid, and the Cassie model will be used.
cos θ w = r cos  Y (2) r = actual surface area planar surface area In the Cassie model, the droplet suspends on the surface and it does not penetrate into cavities (Figure 2c) [45]. Air pockets are assumed to be trapped in asperities. In this state, the apparent CA (c, Cassie-Baxter CA) is the results of contributions of varied phases (Figure 2c): Where fSL and fLV are the surface fractions of solid-liquid and liquid-vapor interface respectively, being s=w. If only one type of protrusion (which means homogeneous roughness) is present, fSL + fLV=1 and v = 180° , the equation becomes: In the Cassie-Baxter model, c of an either inherently hydrophilic or hydrophobic material increases with roughness and c can therefore become larger than 150° for any material.
In reality, the starting solid-liquid contact model (i.e. Cassie-Baxter model) does not persist forever. A transition from Cassie to Wenzel state is observed under external stimulus, such as droplet press, impact or vibration [46][47][48][49]. Droplets pinned on rose petals are in the Cassie impregnating wetting state in which water penetrates into cavities [50]. The transition or coexistence of these two models was visualized by large-scale molecular dynamic simulation method, as reported by Zeng et al [51].

Dynamic wetting behaviours
In real cases, to characterize the wetting properties of solid-liquid interfaces only by CA is not enough. CAH (contact angle hysteresis) and SA (sliding angle) are therefore introduced. When a droplet is inflated, the CA will increase but the contact area will not change until adv (advancing contact angle) reaches a critical value and then it begins to advance. Similarly, if a droplet is deflated, there is also a critical value called receding angle (rec) to be reached before the contact line begins to recede. By definition, the difference between adv and rec is termed as CAH. Surfaces with low CAH allow drops to slide over them easily (Cassie state), regardless of the equilibrium CA. On the contrary, surfaces wetted in the Wenzel state show high CAH and drops are adhered onto them. The quantitative relationship between the CAH and SA is governed by Equation 7 [52]: mg sin α = γ LV w(cosθ rec − cosθ adv ) where g is the gravity force, α is the tilt angle (or SA) while m and w represent the mass and width of the drop, respectively. For a water drop with fixed mass, the amount LV •w/mg in Equation 7 is constant, and therefore the SA for the Cassie model surfaces will result smaller than the Wenzel one. For slippery superhydrophobic surfaces (droplet in Cassie model), their CAH are usually smaller than 10. As the CAH is a measure of energy dissipation during droplet movement, the solid surface energy (s) can also be estimated exploiting CAH [53]: 2+cos θ adv +cos θ rec (8) where σ LV is the water surface tension (72.8 mN/m). For superhydrophobic surfaces, the surface energy is significantly smaller than 72.8 mN/m.

Fabrication of superhydrophobic surfaces
Based on the principle of surface hydrophobicity, the wettability of a material is determined by its surface free energy and surface roughness. At present, there are two main strategies for preparing superhydrophobic materials: a) to construct a microscopic roughness on the surface of a hydrophobic material; b) to add a layer of substance with low surface energy to the surface of a microscopically rough material [30,31,33]. In summary, surface roughness at microscopic scale (nano-, submicrometre) and low surface energy compounds are two essential conditions for obtaining superhydrophobic surfaces.

Chemicals
There are many types of raw compounds used for the synthesis of ultra-repellent materials, including organic, inorganic and organic-inorganic composite materials. Here we mainly focus on the superhydrophobic materials synthesized with the purpose to protect stone heritage, for which raw compounds such as organosilanes, siloxanes, silane coupling agents, SiO2 and TiO2 nanoparticles, fluorine-containing materials etc. are often employed.
In spite of the numerous chemicals available, silanes and silicones play very important roles in the preparation of ultra-repellent surfaces. Approximately 25% of the literature about superhydrophobic and superoleophobic materials is based on silanes and silicones [54]. They are frequently used for constructing appropriate surface topography, decreasing the surface energy, and as binders to link building blocks together or to bind materials of low surface energy to substrates [54]. In fact, the alkyl groups of silanes are mainly used to decrease the surface tension while the hydrolyzable groups, e.g. Si-Cl, Si-NH-Si, Si-OR, could react with water to form silanols. The silanols firstly could condense with other hydroxyl groups at the surface of substrates, while the remaining silanols may self-condense to form a unique micro/nanostructured silicone layer. Thus, it is justified the increased attention reserved to the possibility of tailoring the properties of the coatings by changing the hydrolyzable and alkyl groups of silanes, and the reaction conditions [55].

Silica nanoparticles
A common strategy to construct super-antiwetting surfaces is to use silica nanoparticles (NPs) to create micro/nanostructures, and then modifying them with low surface tension materials. Ethylene polymerized over self-assembled silica NPs [56], a perfluoroalkyl methacrylic copolymer coated on a hierarchical nano/submicron silica spheres stacking layer [57], poly(4-vinylpyridine)-bpolystyrene diblock copolymer surrounded on a silica core- [58] or a hydrophobic ionic liquid and negatively charged silica NPs alternatively absorbed onto glass substrates via layer-by-layer self assembly [59] are only some examples that show how to fabricate superhydrophobic surfaces.

Polydimethylsiloxane (PDMS)
Super-repellent surfaces can also be prepared by constructing micro-/nano-structures via rough PDMS surfaces. Two methods can be exploited to fabricate superamphiphobic surfaces based on PDMS: one involves PDMS surface roughening followed by surface modification with low surface energy compounds, while in the second method the hydrophobic properties of PDMS are enhanced by a direct surface roughening. In the first method, rough PDMS surfaces can be formed by soft lithography, plasma etching, H2O2 etching or their combination [54]. Robust superhydrophobic PDMS surfaces with fully controlled wetting properties and geometrical characteristics, for example, have been obtained via roughening PDMS in SF6 plasma followed by plasma induced fluorocarbon film deposition [60,61]. PDMS can also be used to negatively replicate the surface texture of plant leaves (e.g. lotus leaves) or, more generally, manmade surfaces (e.g. patterned silicon wafer) with well defined hierarchical structures [62]. Then, PDMS replicas (stamps), exploiting the principle of ''soft lithography" technique, can be used to fabricate superhydrophobic and functional polymeric composite films; as in the case where waterborne polymer suspensions were directly casted onto the PDMS stamps negatively replicated from fresh plant leaves [63]. Taking advantage of the hydrophobic properties of PDMS, microstructured and superhydrophobic surfaces have been also prepared by direct roughening of PDMS surfaces, achieved by various methods including replication of textured surfaces (e.g. plant leaves, fish skins, artificial molds), lithography or laser etching [54].
Besides, superhydrophobic surfaces can be prepared by chemical vapor deposition (CVD) of PDMS. The CVD process provides a coating over the entire surface of the porous material caused by the thermal degradation of PDMS which leads to a mixture of volatile, low molecular weight products [64].

Silanes and silicones
It was found that not only PDMS but also other silicones could be used as the precursor for the superhydrophobic coating [65,66]. Alkylchlorosilanes, alkylalkoxysilanes, perfluoroalkychlorosilanes, and other silanes can be used to decrease the surface energy via CVD or solvent deposition in the preparation of superhydrophobic and superoleophobic surfaces. Aminopropylalkoxysilanes, vinylsilanes and other silanes with functional groups, assembly of NPs modified with different silanes, PDMS and silicones can be used as binders between the substrates and coatings [54]. Silanes and silicones play several roles in forming superhydrophobicity and superoelophobicity simultaneously. On one hand, the reactive groups of hydrolyzed silanes and silicones through the cross-linking with each other can form certain roughness at multiple scales. On the other hand, the alkyl or perfloroalkyl groups of silanes decrease the surface tension, while other active sites can be used for further modification. For the simultaneous construction of micro-/nanostructures and decrease of surface energy, a single silane or silicone via a one-step process or mixture of silanes and silicones via a one-step or multi-step process can be used [54]. Silanes and silicones may also be used simultaneously as binders and for low surface energy materials. Likewise, mixtures of silanes can be used as binders and simultaneous decreasing of surface energy for constructing nanometric roughness. The frequently used mixtures are composed of tetraethyl orthosilicate (TEOS) and, alternatively, different silanes such as perfluoroalkylsilanes, alkylalkoxysilanes.

Titania nanoparticles
Similar to SiO2 NPs, TiO2 NPs are also very popular in preparing surfaces with special wettability (i.e. superhydrophobic and superhydrophilic), and they have been widely used to generate superhydrophobic surfaces with both low and high adhesion [67,68]. Owing to their unique electronic and optical properties, many synthetic routes are suggested for synthesizing nano-TiO2, among which sol-gel, hydrothermal and solvothermal methods are more frequently exploited [69]. In the fabrication of TiO2-based nanocomposite, a hydrophobic component is usually necessary due to the hydrophilic nature of TiO2 [70]. TiO2-based super-antiwetting surfaces can be multifunctional in addition to superhydrophobic self-cleaning properties, since they possess photocatalytic property which renders surfaces anti-fouling, de-polluting characteristics [70,71].

Fabrication methods
The main approaches for preparing superhydrophobic surfaces can be divided into top-down and bottom-up strategies [31,33]. The former uses lithography, etching and other methods, such as photolithography, template method and plasma treatment method, to prepare multi-layer, microrough structures on the surface of the substrate [72][73][74]. On the contrary, the latter mainly involves self-assembly and self-organization methods (e.g. chemical deposition, layer-by-layer assembly, colloidal self-assembly, sol-gel, etc.), which control the microstructure and surface free energy of the material at an atomic or molecular level [75,76]. Yet, some researchers tend to distinguish these approaches on the basis of the chemical or physical nature of the method [31]. These preparation methods have both advantages and disadvantages depending on the field of application. Here, only the methods that have been proved to be more appropriate and efficient for fabricating surfaces for stone conservation are considered.

Sol-gel
The sol-gel method is essentially based on chemical processes which exploits a reactive precursor (alkoxy derivatives of metals, metal alkoxide, etc.) to carry out hydrolysis and condensation reactions in a suitable solvent. The materials used in this method can be applied with different methods, as shown in the following sections. During the sol-gel process, a transparent, stable sol is formed firstly, and then the sol polymerizes on the surface of the substrate to form a gel network, which becomes nanoscale microstructure spontaneously after drying. Both surface roughness and surface chemistry can be controlled by changing preparation conditions and reactants. For instance, superhydrophobic surfaces can be made by changing the functional groups of the gel or by dispersing the low surface energy material into the gel network. Sol-gel methods are widely used in the preparation of ultraliquid repellence surfaces on a variety of substrates, such as metals, silicon wafers, textiles, woods, stones etc. Wang et al. reported an effective way of generating superhydropohobicity on various substrates e.g. textiles, papers, glass etc [77]. The coating material was the fluorine-containing silica sol, which was prepared by the hydrolysis and co-condensation of TEOS and fluorosilane (FAS) in ammonia-ethanol solution (Figure 3). On the coated substrate, the silica NPs successfully constructed a rough surface with nanometer scales, whilst the fluorine-containing branch of the surface acted as a low surface energy material, giving rise to superhydrophobic and self-cleaning properties.

Layer-by-layer assembly
This method uses thin films (or layers), especially oppositely charged films, to construct a superhydrophobic material. A typical preparation of the layer-by-layer (LbL) assembly consists of an alternate immersion of the substrate in a positively or negatively charged solution to adsorb the charged material sequentially [33]. The removal of the excess or remaining solution, after each step, is done by a solvent rinsing which leaves a thin layer of charged material on the surface for subsequent adsorption. The connection between the layers can be achieved by various forces, such as electrostatic attraction, hydrogen bonding, coordination bonds, charge transfer, and characteristic molecular recognition. This method can also be used for the assembly of nanoparticles, for example, using different functionalized SiO2 NPs to make a composite surface with layered structure, and then hydrophobilize the last layer to reduce the wettability of the surface [78,79]. In general, the hydrophobicity of the surface is determined by the number of layers of SiO2 NPs and the surface topology. As shown in Figure 4, Tsai et al. produced a superhydrophobic nanocoating, which has a raspberry-like structure, by self-assembly of SiO2 NPs [80]. Firstly, a one-dimensional micro-rough structure was prepared on the substrate by using silica particles with a diameter of 0.5 μm, and then nanoscale silica particles were assembled on the particle film to form a two-dimensional micro-rough structure. Finally, surface hydrophobilization was carried out by using dodecyltrichlorosilane.

Solution immersion
It is a very simple method in which the substrate is immersed in a solution containing a lowsurface-tension agent to generate superhydrophobicity via one single step. Although the method is facile, it is time-consuming, and the microscopic roughness is not controllable. Zhang et al. constructed a nanocomposite coating on Ramie fiber by using multiwall carbon nanotubes (MWNT) and polybenzoxazine (BOZ) by soaking, washing and drying processes [81], while other researchers have prepared superhydrophobic surfaces on metal substrates by a one-step immersion method [82].

Spray
This is the simplest and practical way to prepare nanocoatings, even on large surfaces such as those of monumental buildings. The coating is prepared by spraying the coating material dispersion, solution or emulsion directly onto the substrate by using a spray gun. Many literatures regarding the spray process have already been published [83,84]. Wu et al. prepared a [CH3(CH2)10COO]2Cu superhydrophobic coating on the surface of different substrates, such as glass and aluminium, by spraying the emulsion with nitrogen gas [84]. More recently, spray methods, including the spraying LbL asembly, have focused on studying the adhesion between coatings and substrates, controlling the surface morphology or nanoscale roughness of coatings and tailoring the deposition of layers with defined composition [83][84][85].

Other methods
Other methods, including chemical etching, laser etching, chemical vapor deposition, physical vapor deposition, lithography, hydrothermal method, composite techniques etc., have their own characteristics and corresponding fields of application. Among them, physical or chemical etching methods are more suitable for specific types of substrates, such as silicon wafers, metals, etc.; physical or chemical deposition methods require complex and expensive equipment, and the preparation process is complex and under harsh conditions (e.g. high or low pressure, high temperature, etc.). Hence, their applications are not universally exploited. Aiming to develop advanced materials for art conservation, some basic requirements must be accomplished; for example, the original appearance (artistic, aesthetic) of the cultural objects cannot be impaired after applying protective materials. Moreover, all physical and chemical methods that irreversibly modify the substrate during preparation cannot be adopted, such as chemical etching, laser etching, covalent or ionic LbL assembly. Additionally, the experimental procedure shall be simple and easy to perform, in any condition where the cultural surface is present (indoor, outdoor). As a consequence, preparation methods which require extreme experimental conditions (e.g. high temperature and high pressure) such as a chemical vapor deposition method or a hydrothermal method, are not suitable. It is noticed as well, methods that are only suitable for substrates with limited dimensions (e.g. solution immersion) are not practical for art conservation. At this stage, the sol-gel method is the most appropriate and convenient method for preparing materials for conservation, since it has several indispensable advantages: 1) availability of a wide variety of raw materials; 2) low cost of raw materials, 3) simple reaction conditions and equipment; 4) no physical or chemical modification of the substrate, 5) application process can be carried out in indoor and outdoor environment, 6) application can be done by simple methods such as spraying and brushing. All these characteristics meet the basic requirements for conservation and restoration practices.

Working mechanisms of superhydrophobic materials for stone conservation
Self-cleaning is one the most significant applications of superhydrophobic surfaces which demonstrate ultralow surface adhesion, typically known as the "lotus-effect". On a common hydrophobic surface, the droplets slowly fall across the dirt particles, and the attached particles are mainly re-distributed and redeposited behind the droplet, due to the non-slip boundary condition Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 March 2020 doi:10.20944/preprints202003.0331.v1 ( Figure 5) [26,70]. On the superhydrophobic surfaces, since there is very small interfacial area between particle and rough surface (where air is trapped inside), adhesion between solid particles and surface is minimized [26]. Water forms a spherical droplet that quickly runs over the surface, collects dust, dirt particles and rolls off easily. This specific property prevents surface from contamination, and is referred as "self-cleaning".

Figure 5.
Schematic illustration of the working mechanism of self-cleaning effect. Reprinted with permission from [26], Copyright (1997) Springer.
Bioinspired ultra water/oil repellent surfaces with self-cleaning property are the most ideal materials for protecting almost all kinds of stones artworks (e.g. stone objects, historical buildings, mortars, plasters etc.), and the reasons are threefold: 1) liquid water adhesion, penetration and vapor condensation are inhibited on such surfaces, and water-driven damaging effects are therefore prevented; 2) undesirable surface depositions e.g. atmospheric pollutants, dusts, bird poops etc., can be removed exploiting the self-cleaning properties in presence of liquid water; 3) risks of suffering from biological colonization are dramatically reduced, since spores, pollens are absent or deprived of water which is necessary for germination. Equipped with such surfaces, the physical and chemical integrity together with the aesthetic properties of stone heritage can be well maintained.

Materials proposed for stone conservation
In literature, some hydrophobic materials which can produce surfaces with "lotus effect" behavior, have been synthesized and proposed for building stones, including stone artefacts. Mosquera et al. reported the fabrication of a superhydrophobic coating on sandstone via a sol-gel process [86]. The starting sol was prepared by mixing ethoxysilanes (TES 40 WN from Wacker), colloidal silica particles and a hydroxyl-terminated polydimethylsiloxane (PDMS) in the presence of a surfactant (n-octylamine). The sol obtained was then applied on sandstone surfaces by brushing. The as-prepared surface had water CA of 1492 and CAH of 71 (Table 1), showing better performance than a commercial product (BS290) and the nanocomposite without colloidal silica particles (UCA-TP). The amount of water absorbed by stone after coating was dramatically reduced, and water did not penetrate into stone after immersion for 48 h. The coating also preserved the aesthetic features of stone substrates. However, the amount of coating material applied, which affected the performance of coating, was not reported, as well as the vapor diffusivity of stone after coating was not evaluated. Generally, these two parameters are related one to another: higher the amount of coating is, lower the vapor diffusivity. So, with a very high amount of coating material introduced, superhydrophobicity can be achieved, yet simultaneously, the open pore network could be damaged which induces severe reduction of vapor diffusivity. A drastic losing of vapor diffusivity facilitates water condensation below the coating layer, which consequently accelerates a series of physical decay effects such as free-thaw cycles, differential stress, salt recrystallization etc. In addition, the minimum amount of active product required for achieving superhydrophobicity also have great influence on the cost in practical application.  [87]. The coating was done by simply spraying the suspension on various substrates i.e. silicon, glass, silk, aluminium, wood, marble, and concrete. The effect of silica NPs was assessed by varying the concentration (i.e. 0.1, 0.3, 0.5, 1.0, 1.5 and 2.0%) ( Figure 6). The asprepare surfaces showed water CA ranging from 154-164, together with CAH ranging from 3-5. Similar to the example above mentioned, the exact amounts of coating materials applied were not reported, even if the authors stated that the spray time was relevant to produce films with appropriate performance. Likewise, the vapor diffusivity variation of porous substrates (e.g. marble and concrete) before and after coating was not evaluated. Later, Karapanagiotis et al. prepared an emulsion with a water-soluble siloxane and silica NPs (7 nm) to spry on marble and sandstone surfaces [88]. The as-prepared surface showed water and oil CA150 with tilting angle 7, exhibiting superamphiphobic and self-cleaning properties. It was also demonstrated that the structure of the coatings dramatically changes from a continuous rough structure, when the concentration of NP of silica is less than 2% (w / w), to a network of increasingly large grooves, as the concentration of NPs exceeds 2%. These grooves did not have influence on static CA, but they were responsible for the transition from Cassie-Baxter state to Wenzel state of liquids with low surface tension (e.g. oil, ethylene, glycol drops). Subsequent monitoring of water evaporation showed the drops followed the constant CA model (Figure 7), which was in agreement with superhydrophobic surface with low surface adhesion. This sol-gel process was very simple, and the application method (spray) was also easy to handle, showing good potential in practical application. As the previous publications, the authors did not evaluate the vapor diffusivity change after coating, but fixed the application conditions (i.e. amount of applied dispersion, diameter of nozzle, distance from the sample surface). Besides, to manifest the versatility of these coatings, application on stones with high porosity e.g. calcarenite, tuff is ought to be tested. Moreover, in order to be applied in outdoor environment, the resistance of as-prepared coating against mechanical abrasion, chemical corrosion, environmental weathering shall be assessed.  magnified to clearly demonstrate the drop of R and the temporary slight increase of θS. Reprinted with permission from [88], Copyright (2016) Elsevier.
Cappelletti et al. also reported a sol-gel method to produce superhydrophobic surfaces on various stones [89]. The sol was prepared by mixing a sol of TiO2 NPs containing a non-ionic surfactant with a silane polysiloxane (Alpha SI30) in varied weight percentage polysilixane/TiO2 sol. The coatings were applied by spraying the mixture on the stone surfaces under controlled conditions (fixed spray pressure and spray time). Superhydrophobicity was only observed on Carrara marble surfaces with water CA150, while the CAH was not measured omitting to indicate the adhesive or slippery property of this coating. Again, the residual vapor diffusivity of the coated rocks was not considered. Instead, the resistance of the coatings to environmental weathering and artificial UV exposure was proved. However, further in-depth testing of as-prepared coatings, in terms of performance on stones with higher porosity, inhibition efficiency against long-time capillary absorption, strength against chemical and mechanical damage, is expected.
Recently, we developed several innovative coatings based on different partially fluorinated oligoamides (e.g. oligo ethylene-succinamide, -adipamide or -suberamide with two pendant perfluoropolyether segments), which have good solubility in alcoholic and hydro-alcoholic solvents and can be easily applied by common methods (i.e. brush or spray) [18][19][20]. The as-prepared coatings manifested near superamphiphobicity (water CA 150° and oil CA 140°) on highly porous Lecce stone, yet the surfaces also possessed high adhesion property (CAH 90°). Self-cleaning function was not achieved, since water droplet could pin tightly on surfaces even when the substrates were turned upside down. These sticky surfaces showed good application prospects in other fields such as dew collection, non-loss liquid transport, sticky tape etc. However, as explained previously, to better preserve outdoor stone materials, self-cleaning property is considered essential in order to eliminate dust, dirt, pollutants and pollen accumulation and to eradicate black crusts and biological growth. Another desired property for cultural heritage surfaces conservation is the superoleophobicity. Though super oil-repellent surfaces are not common in nature, superoleophobicity can be achieved artificially by preferentially adopting fluorinated or perfluorinated compounds [90][91][92][93]. Unlike some acrylic coatings, which tend to absorb particulate matters, oily dirt from atmosphere spontaneously and gradually become stained, superoleophobic surfaces are very efficient in reducing the deposition and adsorption of oily and greasy contaminants. When the superhydrophobic and self-cleaning coating is also superoleophobic, the cleanness of substrate is well kept. More recently, we further developed a superamphiphobic coating agent synthesized via one single step [94]. The coating agent was obtained by reacting an aminosilane with a perfluoroether derivative at room temperature and room pressure. By simply deposing the coating dispersion (in 2-propanol), superamphiphobic surfaces were obtained on natural stones (e.g. carbonate, silicate stones) with low and high porosity, and on artificial stones like bricks, tiles, ceramics. On as-prepared surfaces, the CA of water, oil and complex water dispersion systems (e.g. tea, milk, coffee, coke) were all above 150° whereas CAH<10° (Figure 8), thus demonstrating superamphiphobic and self-cleaning properties. The surface morphology of as-prepared coating was characterized by SEM analysis (Figure 9). It is clear that the coating material which has a slender, needle-like structure (diameter 100 nm) had created a nanoscale roughness structure on the porous substrate, satisfying the physical condition for achieving superhydrophobicity. Meanwhile, exploiting the fluorine segregation phenomenon of the coating material at the surface/air interface, the surface energy of the substrate was significantly reduced. These two fundamental conditions work together to generate the superamphiphobicity and self-cleaning properties. In practical test, the coating was effective in preventing capillary water absorption for a long time, with water inhibition efficacy >90% after 24 h (Table 2). It should be mentioned as well, the coating also preserved the physical properties of stone substrates, by well maintaining the original water vapor diffusivity, surface color and pattern. Moreover, the novel coating also demonstrated strong resistance against chemical corrosion, water immersion, mechanical abrasion and environmental weathering, exhibiting great potential as protective coating for objects in outdoor environment (including rainy areas and regions where acid rain occurs).   There are some other publications presenting the preparation of superhydrophobic coatings for stone maintenance [95][96][97][98]. However, similar to the above mentioned ones, they have both advantages and drawbacks. Their shortcomings in common are: unclear coating amount; unknown vapor diffusivity change after coating; not versatile on stones with both low and high porosity; no indepth characterization of structure or performance etc.

Conclusion and future prospects
The biomimetic superhydrophobic materials with excellent hydrophobicity, oleophobicity and self-cleaning function are efficient protective coatings for heritage conservation. In the current review, we described the basic concepts of superhydrophobicity, the three basic models of wetting on surfaces: Young's model, Wenzel's model and Cassie's model. Based on the principles of surface hydrophobicity, two general strategies for preparing superhydrophobic/superoleophobic materials were proposed, namely the method of constructing microscopic roughness on hydrophobic surfaces and the method of adding low surface energy substances on the microscopically coarse surface. There are many physical and chemical methods derived, yet not all of them can be applied for stone conservation. Sol-gel, layer-by-layer assembly and spray methods have been approved and used for synthesizing materials for stone artworks. Recently, some superhydrophobic and superamphiphobic coating agents have already been proposed, and they showed good hydrophobicity, oleophobicity and self-cleaning property. Yet, they also showed some drawbacks which impede the practical application.
Overall, the application of superamphiphobic materials for stone protection is still in the preliminary stage, and few materials were used in the real case of stone restoration. The main existing problems are: (1) insufficient new compounds; (2) lack of in-depth structure and performance characterization of the modified stone surfaces; (3) lack of long-term performance monitoring of the modified surfaces. Some possible directions for future research in this field that we propose are as follows: -development of more kinds of superamphiphobic compounds, to meet the varied types of stone materials and varied conditions of application, e.g. high and low porous substrates, smooth-and rough-textured stones, dry and humid environment etc.
-in-depth evaluation of the properties of the newly synthesized products, as well as of the coated surfaces. Understanding the durability of the superamphiphobicity against chemical corrosion, water immersion, mechanical abrasion, UV exposure etc. is the first step, and then the evaluation of the stability and performance durability of the coated surfaces in harsh, open-air environmental conditions (UV, rain, pollution etc.) is also desired; -understanding the variation of the physicochemical properties of different stone materials after coating with the new products through the evaluation of some important parameters such as vapor diffusivity, porosity and surface color; -development of superamphiphobic, self-cleaning coatings with anti-biofouling property which can prevent micro-biological growth on outdoor stone surfaces. The interest in this topic is increasing, yet, few researches are found in literature.