Fabrication of Self-healing Superhydrophobic Surfaces from Water-Soluble Polymer Suspensions Free of Inorganic Particles through Polymer Thermal Reconstruction

Self-healing superhydrophobic surfaces have been fabricated by casting and drying water-soluble amphiphilic polymer suspensions at room temperature through thermal reconstruction. When compared with previous methods, this approach exploits modified natural hierarchical microstructures from wood instead of artificially constructing them for superhydrophobic morphology, which involves neither organic solvent nor inorganic particles nor complex procedures. The obtained superhydrophobic surface has acceptable resistance to abrasion. The surface can recover superhydrophobicity spontaneously at room temperature upon damage, which can be accelerated at a higher temperature. After depleting healing agents, the polymer suspension can be sprayed or cast onto wood surfaces to replenish healing agents and to restore self-healing ability. The superhydrophobic surface greatly increases the mold inhibition and water resistance of wood, which would prolong the service life of wood based materials.


Introduction
The fabrication of superhydrophobic surfaces has attracted extensive interests over the past few years due to its great potential both in theoretical research and practical applications, such as durable antibacterial action [1][2][3][4], screen filter [5], antifouling [6], gas sensing and droplet manipulation [7], oil/water separation [8,9], self-cleaning [10], and antireflection [11].The superhydrophobic property of the surface is governed by its chemical compositions and geometric microstructures.The combinations of hierarchical microstructures with low-surface energy materials have been the main strategy for fabricating superhydrophobic surfaces.In general, superhydrophobic surfaces can be obtained by enhancing the roughness of hydrophobic surfaces.Therefore, many methods have been developed to improve the roughness of solid surfaces to fabricate superhydrophobic surfaces, such as plasma polymerization/etching [12,13], chemical vapor deposition [14,15], solvent-mediated phase separation [16], solvent-free or wet chemical free approaches [17,18], and polymer self-assembly [19][20][21].Although many physical and chemical methods have been developed to fabricate superhydrophobic surfaces, the lack of cheap fabrications, mechanical robustness, and self-replenishment upon damage once have remained as the main obstacles for the widespread applications of some superhydrophobic surfaces since an artificially fragile surface texture can be damaged by scratches, abrasions, or even brief contacts with fingers, which made them apt to lose superhydrophobicity.However, up to date, through extensive efforts, many achievements have been made to fabricate mechanical durable superhydrophobic surfaces [22][23][24][25][26], which helps in applying them in practical applications.
Polymers without solvent and inorganic matter through tribocharging [17,18] or with solvent and inorganic matter through high temperature melting for thermal reconstructions [27] have been used to fabricate robust superhydrophobic surfaces, which are highly efficient, rapid, and inexpensive.In the present study, the superhydrophobic surfaces from unitary water-soluble polymer suspensions were obtained through polymer thermal reconstruction by taking advantage of natural hierarchical microstructures just by sanding wood, instead of artificially constructing them for superhydrophobic morphology.The whole process did not involve toxic organic solvents, inorganic particles, or complex procedures.The water CA with 152 • was obtained at room temperature, which was smaller than the water CAs 164 • for some polymer supehydrophobic surfaces [17].When compared with the excellent abrasion resistance of some polymer superdhydrophbic surfaces, which could maintain its superhydrophobicity under the pressure of 17.5 KPa with 15 abrasion cycles [27] or mechanical stability after 100 abrasion cycles with sandpaper [17], the obtained superhydrophobic surfaces in the present had acceptable level of mechanical properties, which could maintain its superhydrophobicity under the pressure of 3.5 KPa with nine abrasion cycles.However, even the most durable superhydrophobic surface would eventually become damaged by the repeated mechanical abrasion or chemical etching, which would damage its low surface energy material or texture.Hence, a superhydrophobic surface that could recover superhydrophobicity upon damage would be highly desirable [7,[28][29][30][31][32][33][34][35][36][37][38][39][40].Based on the intrinsic porous morphology of wood surfaces, superhydrophobic coatings can preserve a large number of amphiphilic polymers as healing agents in wood pores, such as cell cavities and grooves.Once the primary top polymer layer is decomposed or scratched, the preserved healing agents in cell cavities and grooves can migrate to wood surfaces driven by minimum interfacial energy at ambient temperature [31].Furthermore, healing agents, which cooperate with robust porous structures that are not easily destroyed and are essential for the recovering of the superhydrophobicity, result in the self-healing of the superhydrophobicity.Furthermore, after depleting healing agents, the polymer suspensions could be sprayed or cast onto wood surfaces to replenish healing agents and restore its self-healing ability.These reported principles can be extended towards the self-healing of other surface-dependent functionalities, such as anti-mildew or anti-water, which will remain high performance levels all through their life-cycle with low cost and energy demand for maintenance and surface repairs.
As renewable resources, wood has great potential in furniture and decoration materials that can take the place of steel, stone, and glass, due to the forthcoming exhaustion of un-renewable resources, such as petroleum, coal, and minerals [41][42][43].However, as a natural hygroscopic polymer composite with many hydrophilic groups such as hydroxyl groups (-OH), wood is apt to absorb water, which would result in dimension instability and microorganism attacks from decay fungi or mold.This would greatly reduce its service life.Improvement in the water repelling of wood is an effective way to prevent the harmful effects from water.Many methods have been put forward to improve the hydrophobicity of wood [44][45][46][47], which would help to apply them in practice.The method free of inorganic particles in the present study greatly improves the hydrophobicity of wood without varying its surface textures.Therefore, this approach to robust superhydrophobic surfaces is not only important for technologies, which avoids organic solvents, surfactants, and the application of complex methods, but it also opens up a new way to modify wood with better water and mildew resistance without inorganic particles.

Materials
Butyl acrylate (BA), styrene (St), acrylic acid (AA), and ammonium persulfate (APS), were purchased from Shanghai Chemical Reagent Co. (Shanghai, China).Allyloxy hydroxypropyl sodium sulphonate (HAPS, 40 wt % of solid content in aqueous solution) was kindly donated by Guangzhou Shuangjian Trading Co., Ltd.(Guangzhou, China).Chinese-fir wood and hexafluorobutyl acrylate (FBA) were kindly donated by Zhejiang Longyou Wood Bond Co. (Quzhou, China).Wood blocks of 40 mm × 35 mm × 10 mm (longitudinal × radial × tangential) were obtained from the sapwood of Chinese fir (Cunninghamia lanceolata).The wood was sanded with 240 grit sandpaper before its coating, unless other specified.Deionized water was used in all of the polymerizations.All of the chemical reagents were used without further purification.

Synthesis of Colloidal Polymer Spheres
A typical surfactant-free emulsion polymerization procedure was carried out, as follows: deionized water (80 g), HAPS (1 g), AA (1g), FBA (1g) BA (5 g), St (5 g), and APS (0.05 g) were added sequentially into a four-necked flask equipped with an N 2 inlet, a reflux condenser, and a mechanical stirrer at a stirring speed of 150 rpm.The polymerization was initially performed at 75-80 • C for 2 h and added with a mixture of AA (1 g), BA (15 g), St (15 g), and APS (0.2 g dissolved in 15 g of water) over a period of 2 h, sequentially.The reaction was allowed to proceed for another 6 h to obtain water-soluble polymer latex with a solid content of 34 wt %.The glass transition temperature of polymer is 30 • C measured by differential scanning calorimeter (DSC), unless other specified.

Fabrication of Superhydrophobic Surfaces
The above polymer latexes were firstly stirred and then drop-cast onto sanded wood surfaces.The superhydrophobic surface was obtained when the latexes was dried at 30 • C.

Characterization
The radial surface morphology of the wood was characterized by scanning electronic microscopy (SEM, SU-8010, Hitachi High-Technoligies Corporation, Tokyo, Japan).All of the samples were coated with gold by sputtering prior to observation.The size and size distribution of polymer spheres was measured by TEM images with Hitachi H-600 TEM (Hitachi, Tokyo, Japan).Digital pictures were captured using Cannon Power Shot A 95 (Cannon, Tokyo, Japan).The size distribution and zeta potential of the polymer spheres were measured by the Zeta PALS BI-90 plus (Brookhaven Instruments Corporation, Holtsville, NY, USA).The test was repeated three times.The T g of the as-prepared sample was analyzed using a Perkin-Elmer DSC 7 (Perkin-Elmer, Waltham, MA, USA).Three-dimensional VK-X optical laser microscope system (3D OPM, Oxford Performance Materials Inc., South Windsor, CT, USA) was used to observe the optical images of the wood surface.The roughness factor (R a ) was calculated according to JIS B0601:1994 [48].The wettability of the surface was performed by a dynamic contact angle (CA) testing instrument (OCA40, DataPhysics, Filderstadt, Germany).CA was recorded at 30 s after a droplet of liquids (5 µL) was placed on the surface.The sliding angle (SA) was measured by recording the tilt angle of the sample platform, at which a droplet of liquids (10 µL) starts to roll off the surface.The average water CA was obtained on cross sections by measuring the same wood sample at six different positions.Fourier Transform infrared spectroscopy (FTIR) measurements were carried out with a Nicolet Nexus 470 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a resolution of 0.5 cm −1 for 32 scans.The dried powder of the wood surface was blended with KBr to prepared pellets for examinations.The surface composition of the wood surface was measured by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ECSA, Perkin-Elmer, Waltham, MA, USA) using Al K radiation at a 90 take-off angle.All of the binding energy value was calibrated using the reference peak of C 1s at 284.7 eV.The abrasion resistance of the superhydrophobic wood surface was evaluated by dragging a piece of 240 grit sandpaper under 500 g weights in one direction with a speed of 1 cm•s −1 at a distance of 10 cm per cycle.Unless specified, the characterizations, such as SEM, OPM, Water CA, FTIR, XPS, and abrasion tests were carried out on the radial section of wood blocks.

Synthesis and Morphology of Water-Soluble Amphiphilic Polymer Spheres
Polystyrene-block-poly(butyl-acrylate)-block-poly(hexafluorobutyl-acrylate)-block-poly(acrylic acid) (PST-BA-HFB-AA) were synthesized by soap-free batch emulsion polymerization.The amphiphilic polymer spheres are of uniform shapes with a dark hydrophobic core representing the PS-FBA rich regions, which are surrounded by the lighter shells of the PAA-PBA-rich domains, as shown in Figure 1a.The diameter of the polymer spheres is approximately 381 nm, with 0.005 polydispersity, as shown in Figure 1b.The Zeta potential of the polymer sphere is −52.6 mV, which ensures the stability of the polymer suspensions.

The fabrication of Superhydrophobic Surfaces through Polymer Thermal Reconstruction and the Effect of Surface Roughness on Wettability
The raw wood without sanding (USR) has natural and uneven hierarchical structures (Figure 2a), with a maximum height of more than 400 µm (Figure 2b).The roughness factor (R a ) was 56 µm calculated according to JIS B0601:1994 [48], which was not applicable for superhydrophobic morphology.The natural and uniform hierarchical structures that were applicable for superhydrophobic morphology are obtained simply by sanding wood with 240 grit sandpapers (SR-240) (Figure 2c), with a maximum height of less than 100 µm (Figure 2d) and a roughness factor (R a ) of 6.46 µm.Then, the homogeneous water latex is cast onto SR-240 wood surfaces and is dried at ambient temperatures for a certain amount of time to form superhydrophobic surfaces, as shown in Scheme 1.Since the raw wood have hierarchical microstructures [49] and many hydrophilic groups, such as -OH and C=O groups (Figure 3a), the surface of SR-240 wood is hydrophilic.The surface turns from hydrophilicity into superhydrophobicity with 152 • water CAs after the water-soluble polymer suspensions are drop cast onto SR-240 wood surfaces and are dried at room temperature (Figure 3b).The wettability is relative to the effects of both surface chemical compositions and surface roughness.In order to demonstrate the change in the chemical component on SR-240 wood surfaces from hydrophilic to hydrophobic, which is responsible for its wettability transition, the FTIR spectra are examined on SR-240 wood surfaces (Figure 3c).The absorbance band at 1733 cm −1 was ascribed to the hydrophilic groups (C=O groups from wood lignin and polymers), and the absorbance band at 696 cm −1 was ascribed to the hydrophobic groups (benzene groups from the polymers) [21].For SR-240 wood, there was absorbance at 1733 cm −1 , but no absorbance at 696 cm −1 .For the coated SR-240 wood with 152 • water CAs, there was absorbance at 696 and 1733 cm −1 .The absorption at 696 cm −1 had proven the existence of benzene groups from the polymers on SR-240 wood surfaces.The absorption of CF 3 or CF 2 was not observed, probably because the group number was not enough for the examination.It was just the addition of the hydrophobic benzene group from the polymers onto SR-240 wood surface that made it hydrophobic.Therefore, the changes in the chemical components on SR-240 wood surfaces resulted in its wettability transition.A possible illustration of these chemical component changes on the coated SR-240 wood surface is that the change mainly results from the phase separation of polymer segments driven toward the minimum interfacial energy: the hydrophilic groups (1733 cm −1 , C=O groups) tend to shield from the apolar air and shrink toward the interior of the polymer spheres, while the hydrophobic groups (696 cm −1 , benzene groups) tend to extend toward the apolar air.Hence, the resultant surface was water repellant [20].
On the other hand, the natural and uniform roughness on wood surfaces was the other important factor that governed wettability.In order to illustrate the effect of surfaces roughness on wettability, the same polymer latex was drop cast and dried on USR wood and the raw wood sanded with 60 grit sandpapers (SR-60) and 320 grit sandpapers (SR-320) under the same conditions, respectively.The results in Figure 3d show that the water CAs on USR wood, SR-60 wood, and SR-320 wood were 136 • , 140 • , and 147 • , respectively, which were smaller than those (152 • ) on SR-240 wood.For SR-60 wood, although hierarchical microstructures were present, these microstructures were uneven with the roughness factors (R a ) 13 µm (Figure 4a,b).The pore size on USR and SR-60 wood surface greatly varied.Some pores (grooves) were so large that the comprehensive force on the contact lines could not support the weight of the water droplet, and the water droplet penetrated and filled the pores.Therefore, the wetting behavior was homogeneous, which was in the Wenzel regime.In the Wenzel regime, the coated surface was not a superhydrophobic surface with CA hysteresis that was larger than 10 • .For SR-240 wood, the roughness factors were 6.46 µm and the pores were small and uniform.The comprehensive force on the contact lines could support the weight of the water droplet, allowing for the water droplet to sit on the crests of the rough surfaces.Hence, the wetting behavior was in the Cassie-Baxter regime.Therefore, the coated surface on SR-240 wood approached the superhydrophobic state with CA hysteresis as 4 • .When the sandpaper mesh further increased to 320 grit, the wood surface was smoother with the roughness factors as 5.6 µm (Figure 4c).The pores was smaller and fewer (Figure 4d) that the air in contact lines reduced.Hence, the water CAs decreased to 147 • with CA hysteresis as 15 • .Therefore, the surface roughness on SR-240 wood was the most appropriate for superhydrophobic morphology.

Effect of ST/BA Mass Ratio on Wettability
With the constant fabrication temperature being set at 30 • C, the ST/BA mass ratio has great effects on wettability, as shown in Figure 5a.The corresponding water CAs were 152 • , 122 • , 109 • , and 60 • when the ST/BA mass ratio was 1.0, 1.5, 5.8, and 10.0, respectively.As shown in the above illustration, the critical feature of the transition of wood surfaces from hydrophilic to superhydrophobic was the change of the surface chemical component, which arose from the reorientation/phase separation of hydrophilic or hydrophobic polymer segments that were driven towards minimum interfacial energy.This process, called "thermal reconstruction", tends to become harder when the segmental mobility decreases.With the ST/BA mass ratio increasing, the polymer chain is composed of more rigid PS, which restricts chain movement and contributes to high T g .As demonstrated in Figure 5b, the polymer T g increases from 30, 40, 70, to 85 • C when the ST/BA mass ratio increases from 1.0, 1.5, and 5.8 to 10.0 in latex spheres.
The polymer T g determines the mobility of the polymer segments with the other conditions unvaried.When the polymer T g (85 • C) was much higher than the fabrication temperature (30 • C), the polymer chain or segment could not move, except for the vibrations of the atoms or groups that were near the balanced positions [21,27].The hydrophilic shells and the hydrophobic cores of the polymer spheres could not perform any movement.Eventually the hydrophilic groups that were exposed at the outside of the polymer spheres with the hydrophobic groups hidden inside the polymer spheres after the water evaporated, which resulted in surface hydrophilicity on the coated SR-240 wood with 60 • water CAs.When the polymer T g decreased to 70 • C, the polymer segments, except for the polymer chains, began to move under the hydroplastic effect of water [21].Some hydrophobic groups, such as CH 2 , moved towards the apolar air, extended, and exposed at the outside of the polymer spheres.In contrast, some hydrophilic groups were hidden inside the polymer spheres.Therefore, the surface was hydrophobic with 109 • water CAs.When the polymer T g further decreased to 40 • C, the polymer chain could move gradually in addition to the movement of polymer segments.Therefore, more hydrophobic groups moved and exposed at the outside of polymer spheres, and the surface became high hydrophobicity, with 122 • water CAs.When the polymer T g was 30 • C, the polymer chains could move or flow more freely, when combined with the plastic effect of water, which reached the viscous state.During coagulating, melting, and film-forming, more segments obtained enough energy for movement, namely, more hydrophobic groups, such as CH 2 and CF 2 , moved towards the apolar air, extended, and exposed at the outside of the polymer films.Furthermore, besides phase separation or inversion, which resulted from the segmental mobility of the polymer spheres, the polymer chains could move and cover the whole wood surface to form a continuous thin film [50].The sufficient hydrophobicity combined with the roughness from SR-240 wood surface made it superhydrophobic with 152 • water CAs.
The XPS analysis in Figure 5c-e demonstrates the change of the chemical component distribution on SR-240 wood surfaces with different water CAs corresponding to different polymer T g .There was a broad carbon peak with several kinds of binding energy of 284.7, 286.5, and 288.9 eV, which responded for the function groups of CH 2 , C-O, and C(O)O, respectively [20].According to the results in Figure 5c for the uncoated SR-240 wood, there is a strong peak for C-O, which holds for 60% of all C atoms.In addition, C(O)O and C-H peaks also existed, which held 6% and 34% of all C atoms, respectively.The main groups that existed on the wood surfaces were hydrophilic groups, in which hydrophilic C atoms held for 66% of all C atoms.For the coated S-240 wood with 60 • CAs (85 • C T g ), as shown in Figure 5d, there is a strong peak for C-H, which holds 60% of all C atoms.Furthermore, C-O and C(O)O peaks were also present, which held 32% and 8% of all C atoms, respectively.The hydrophobic C atoms held 60% of all C atoms.There was a strong peak for C-H groups on the coated SR-240 wood with 152 • CAs (30 • C T g ), which held 78% of all C atoms, as shown in Figure 5e.The peaks for the C-O and C(O)O groups held 12% and 10%, respectively.The hydrophobic groups of CF 3 , CF 2 , and CF were not observed, which was probably because the above groups were not enough to be examined, when compared with other C groups.However, the XPS survey in Figure 5f demonstrates that the main elements on the coated SR-240 wood with 152 • CAs were C (78%), oxygen (19%) and F (2%) as compared with those on the uncoated SR-240 wood, which were C (76%), oxygen (22%), and N (1%).This proved the existence of the F element on the superhydrophobic wood.

The Roubustness and Self-Replenishment of Superhydrophobic Surfaces
In practical applications, these superhydrophobic surfaces will be inevitably damaged by accidental scratches.Therefore, the surfaces mechanical resistance was firstly investigated.As shown in Figure 6a, the as-prepared superhydrophobic surface was submitted to 500 g of loading (a pressure of 3.5 KPa), while facing the 240 grit sandpaper and was moved at a distance of 10 cm per cycle.The water CA was measured after each cycle test.After nine cycles of mechanical abrasion tests, the water CAs remained unchanged with the CA hysteresis being less than 10 • (Figure 6b).This means that after nine cycles of tests, the surface continued to retain its superhydrophobicity.However, the surfaces lost superhydrophobicity with the CA hysteresis larger than 10 • after ten cycles of abrasion tests resulted from the damages of polymer films.These results demonstrated the acceptable resistance of the superhydrophobic surface to abrasion.The superhydrophobic wood will be inevitably damaged by prolonged contact with chemicals during practical applications, such as experimental furniture.The self-replenishment of the superhydrophobic surface was examined through the soaking test in alkali and acid solutions.The superhydrophobic surface lost superhydrophobicity after it was soaked in an alkali (pH = 12) or acid (pH = 1) solution for 2 h, with the water CAs decreasing from 152 • to 0 • .The original superhydrophobicity of the damaged surface was recovered after it was exposed in an ambient environment for 6 h with water CAs as 152 • and sliding angles less than 10 • , meaning that the surface of the damaged superhydrophobic polymer film on SR-240 wood was recovered again.Based on the intrinsic porous structures of wood surfaces, the superhydrophobic polymer film can preserve a large number of polymers as healing agents in wood pores, such as cell cavities, cell pits, and grooves.Once the primary top polymer layer is decomposed or scratched, the preserved agents in the cell cavities, cell pits, and grooves can migrate to the surface to heal the superhydrophobicity that is driven by the minimum interfacial energy [31].In this way, the damaged surface would recover its superhydrophobicity at ambient temperatures when the time is long enough for the hydrophobic segments from healing agents in the pores to migrate onto the surface.As shown in Figure 7a,b, the acid and alkali etching-healing process can be repeated nine times without decreasing the superhydrophobicity of the self-healed polymer film, because the multiple porous microstructures are too robust to be destroyed, as shown in Figure 7c,d, and these are essential for the recovery of superhydrophobicity.It was found that the self-replenishment was temperature-dependent, with a more accelerated self-healing process under a higher temperature, and vice versa.When the temperature increased to 50 and 100 • C, the time for superhydrophobicity recovery was 20 and 5 min, respectively.The higher the temperature, the faster the self-replenishment was.Therefore, the temperature participated in the self-healing process of the superhydrophobic films.The self-replenishment occurring at room temperature is important for the routine use of wood materials such as furniture and decoration materials, which are inconvenient to be heated or catalyzed by chemical agents or UV radiation.It was also found that after depleting healing agents, the self-healing ability could be restored just by spraying or casing the polymer suspensions onto wood surfaces to replenish the healing agent.

The Improved Water and Mildew Resistance of Wood with Superhydrophobic Surfaces
The raw wood is apt to absorb water when it is contacted with water, which will result in dimension swelling and mildew stain.The improvement in water repelling for wood could increase its water resistance and mildew inhibition greatly.After being soaked in water for 24 h, the control raw wood absorbs water quickly with the ratio of thickness expansion (C-TE), breadth expansion (C-BE), and weight growth (C-WG) as 5.4%, 2.4%, and 56%, respectively, where C stands for control raw wood (Figure 8a).The wood with superhydrophobic surfaces absorbs water much more slowly than the control raw wood with S-TE, S-BE, and S-WG as 2.9%, 2.1%, and 28.9% where S stands for superhydrophobic wood (Figure 8a).The wood with superhydrophobic surfaces absorbed water more slowly than the raw wood.The above results proved that the superhydrophobic surfaces greatly improved the water resistance of wood, which helped to inhibit the mold stains for wood, especially when it was under humid conditions for daily use.The raw wood begins to be stained by mildew at the 7th day and the whole surface is stained by mildew at the 14th days, as shown in Figure 8b.The wood with superhydrophobic surfaces is not stained by mildew at the 7th day, begins to be stained by mildew at the 14th day and approximately 75% of the surface is stained by mildew at 28th day, as shown in Figure 8b under conditions where thehumidity is 90% and temperature is 30 • C. It was just the superhydrophobic surfaces that made the moisture content of the wood increase more slowly than the raw wood for the mildew to grow, which improved its mildew resistance.The superhydrophobic treatment is an environmental friendly approach to improve the mildew inhibition of wood when compared with the general method, which uses bactericidal chemical agents [51,52].

Conclusions
In conclusion, superhydrophobic surfaces with good resistance to abrasion have been fabricated by simply casting and drying water-soluble polymer suspensions at room temperature through polymer thermal reconstruction.On the base of the robust hierarchical microstructures that were obtained merely by sanding wood, the commonly used inorganic particles for establishing geometric microstructures were neither needed and the coating that was produced from unitary polymer was transparent.The superhydrophobic surface could self-heal superhydrophobicity at ambient temperatures and the self-healing could be accelerated at a higher temperature.After depleting the healing agents, the polymer suspension could be sprayed or cast onto wood surfaces to replenish healing agents and restore the self-healing ability.The superhydrophobic treatments greatly improve both water resistance and mildew inhibition for wood, which extend the life expectancy and open up practical applications for wood based materials.

Figure 1 .
Figure 1.(a) TEM images and (b) size distributions of polymer spheres.

Figure 2 .
Figure 2. (a) Scanning electronic microscopy (SEM) and (b) optical laser microscope system (OPM) images of raw wood without sanding (USR) wood.(c) SEM and (d) OPM images of SR-240 wood.The insets in (a) and (c) are the magnified SEM images.

Figure 3 .
Figure 3. (a) FTIR spectra of the uncoated SR-240 wood; (b) Water contact angles (CAs) of the coated SR-240 wood; (c) FTIR spectra of the uncoated SR-240 wood and the coated SR-240 wood; (d) The effect of sandpaper mesh on water CAs.

Figure 5 .
Figure 5. (a) The effect of the styrene (ST)/butyl acrylate (BA) mass ratio on water CAs; (b) The effect of the ST/BA mass ratio on the T g of as-prepared polymers.X-ray photoelectron spectroscopy (XPS results of (c) SR-240 wood; (d) coated SR-240 wood with 60 • CAs and (e) coated SR-240 wood with 152 • CAs (red line: C-C/C-H; black line: C-O; blue line: C(O)O; green line: envelope); (f) XPS survey for SR-240 wood and coated SR-240 wood with 152 • CAs.

Figure 6 .
Figure 6.(a) Illustration of the abrasion test for the surface under 500 g of loading on sandpaper (240 grit); (b) Water CAs as a function of the number of abrasion cycles.

Figure 7 .
Figure 7. Self-replenishment of the superhydrophobic surface by (a) acid etching-healing and (b) alkali etching-healing tests.The SEM images of the etched wood surface after (c) acid etching and (d) alkali etching.

Figure 8 .
Figure 8.(a) The ratio of thickness expansion (TE), breadth expansion (BE), and weight growth (WG) for wood where S and C stand for superhydrophobic wood and control raw wood; (b) Mildew infection area percent for raw wood and superhydrophobic wood under humid conditions.