Micro/Nano‐Pores and Anti‐Fingerprint Coating by Vacancy‐Capture and Interface Intelligent Control

In this work, a simple anti‐fingerprint strategy (reduced ≈40–60%) to rearrange fingerprint droplets by patterned micro/nano‐pore/spoons texture, air cushion and low surface energy is reported. Superior to previous precise and complex preparation methods, the micro/nano‐texture are fabricated through conventional process of spray and dip. The pore and spoon can be freely switched by changing the process. The number (N) and size (r) of pores and spoons are controlled intelligently by solvent volatilization and interfacial tension ( γFP/PU−H2O${{\bm{\gamma }}_{{\rm{FP/PU}}{\bm{ - }}{{\rm{H}}_2}{\rm{O}}}}$ and γFP/PU − Solvent), the key for pore/spoon's forming and expanding. A physical model of interface intelligent controlling the micro/nano‐pore/spoon surface texture induced by “Vacancy‐Capture” is proposed. The description is being reported, for the first time, to the knowledge, expecting to make it possible to prepare anti‐fingerprint engineering materials with newly potential industrial applications on optical device for large‐scale in the future.


Introduction
The pursuit of aesthetic and clean appearance has been arousing enormous requirements toward anti-fingerprint surfaces in various fields such as interactive systems, optical lenses, automotive interiors and packaging. [1][2][3][4][5][6] To avoid the formation of fingerprint smudges (epidermis, grease) on the glass surface, two major anti-fingerprint strategies have been developed. [7][8][9][10][11][12][13] The first method is the glass-nanostructure strategy by etching smooth glass surface into nanostructure has been used to reduce depositing area or adhesive force for fingerprint smudging. Wang et al fabricated the opposite wettability glass surface with nanopillars (oleophilic top surface and oleophobic sidewalls and bases), increasing anti-fingerprint effectiveness by 74.6% and fingerprint removal ease by 59.2%. [14] The method applied to glass by integrating RIE technology with standard www.advmatinterfaces.de an issue accompanied by the coating application process with mechanical stability remains a critical challenge. [29,30] Based on our previous study of bionic PTFE/PPS superhydrophobic coating surfaces with both the porous network and micro-nanoscale binary structure (MNBS), [31] static analysis of anti-icing, [32,33] the dynamics of droplet impact on inclined surfaces, [34,35] and anticorrosive/inhibiting scaling performances, [36,37] nonetheless the fabrication of above polymer coating with hierarchical micro/nanostructures requires high temperature to remove polar functional groups, rendering the coatings' transparency reduced still for the light scattering effect. Therefore, fabricating a low surface energy and nanostructured polymer coating by simple method at room temperature may be a unique direction to develop an application oriented anti-fingerprint coating technology and seldom reported.
The common test method for anti-fingerprint properties is the evaluation of the surface hydrophobicity and oleophobicity achieved by measuring the static contact angles against water and organic liquids (e.g., n-hexadecane), respectively. However, real contamination is a complex mixture of sweat and mainly sebum; thus, the model compounds are not able to accurately model the behavior of real contamination. [38] It is necessary to consider a range of deposition parameters in order to make a statement about a surface's anti-fingerprint performance. [39] Specifically, the spatial arrangement of macroscopic features within the ridge pattern of a fingerprint could be considered emphatically.
In this communication, we synthesized polytrifluoroethyl methacrylate homopolymer (FP) with low molecular weight based on previous research, [40][41][42][43][44][45][46][47][48] and prepared FP/Polyurethane (PU) composite polymer coating with transparency and anti-fingerprint performance by a simple, inexpensive and conventional curing process at room temperature. The antifingerprint surface with micro/nano-pore or micro-spoon texture roughness, and the lowest surface energy hydrophobic groups (CF) was fabricated by using innovatively synthesized FP and commercially available PU on glass. Fabricating a micro/nano-porous and anti-fingerprint coating surface by "vacancy-capture" and interface intelligent control mechanisms is employed in this communication. The description is being reported, for the first time, to our knowledge, expecting that this technique will make it possible to prepare anti-fingerprint engineering materials with newly potential industrial applications on optical device for large-scale in the future. Figure 1a shows the scanning electron microscope (SEM) diagrams of the sprayed and dipped FP/PU Coatings using acetone solvent (denoted as FP/PU-AC-S and FP/PU-AC-S respectively), which present quite different surface topographies. For the FP/PU-AC-S Coating (WCA = 96°) as shown in Figure 1ai-iii, there are micro-pores with a diameter (D) of about 3.76 µm and mean area (S) of ≈13.32 µm 2 . The porosity of the FP/PU-AC-S Coating is ≈1.1%. For the FP/PU-AC-D Coating (WCA = 98°) as shown in Figure 1aiv-vi, micro-spoon texture with D ≈ 5.16 µm and S ≈ 26.45 µm 2 is observed. The total coverage rate of the spoons (71.4%) via dip process is nearly 70 times higher than that of pores (1.1%) via spray process. As shown in Figure 1b, the two coatings fabricated above show certain transparency of ≈60-70% in the full visible light region (300−800 nm), indicating their potential application in optical device surfaces. Thus, an in-depth exploration on anti-fingerprint performance is evaluated in Figure 1c-g.

Results and Discussion
As shown in Figure 1c, there are clearly 19 red droplets of organic grease pigment left on the smooth inorganic glass surface after the fingerprint was pressed, presenting different sizes and irregular distribution. The total coverage rate of these droplets is ≈22% and the average area of each droplet is ≈4294 µm 2 using the graphic analysis software (Figure 1d-e). On the contrary, the organic grease pigment is observed to spread out on the PU coating surface and completely coverage the surface (78%) in Figure 1d. However, the anti-fingerprint performance presents changes, when they are pressed onto the surface of the FP/PU composite coatings. For the FP/PU-AC-S and FP/ PU-AC-D Coatings (Figure 1d,e), the coverage rates of their fingerprints are 10.4% and 14.6% respectively, and the mean area of the droplets is 722 and 16 235 µm 2 respectively. The fingerprint coverage rate of the FP/PU-AC-D and FP/PU-AC-S coatings are ≈47-66% of the bare glass, indicating that the FP/ PU coatings with low surface energy present anti-fingerprint properties (≈40-60%). To further validate the variations in the distribution of fingerprint droplets on the two coatings, the distance between adjacent fingerprint droplets was measured by Image-Pro Plus (Figure 1c) from tangent direction (X) and normal direction (Y). Figure 1f indicates that the average distance of the droplets on X direction decrease in the order of bare glass (0.6 mm), FP/PU-AC-D (0.44 mm), and FP/PU-AC-S (0.3 mm). The value on Y direction ( Figure 1g) for FP/PU-AC-D and FP/PU-AC-S are equal (0.38 mm), which are lower than that of the bare glass (0.53 mm). The array-like directional distribution of the fingerprint droplets may be attributed to the micro/nano-pore/spoon surface texture, "air cushion" stored in the texture, and low surface energy at the texture boundary. In order to reveal the forming mechanism of the micro-pore and micro-spoon surface structure, a "Vacancy-Capture" physical model is shown in Figure 2.
As shown in Figure 2a, when t = t 0 = 0, a mol acetone (AC) molecules in a liquid state escape from Wet Coating-Air Interface (expressed as γ wet−air ), which is an endothermic evaporation process (Q e > 0). Simultaneously, b mol H 2 O molecules in a gas state at the interface quickly seize the "vacancies" left by escaped acetone and condense into liquid molecules (Q c < 0). Since the system energy is conserved (Q e = Q c ), a and b depend on the molar evaporation enthalpy (expressed as AC ∆ H l g ) and the molar condensation enthalpy (expressed as H For the wet coating surface (γ wet−air ), when the liquid H 2 O molecules get into the wet coating, they tend to gather together for their poor compatibility with the polymer phase in the wet coating. When H 2 O molecules accumulate to a certain content in the wet coating, the initial pores are formed by phase separation ( FP/PU H O 2 γ − ). It implies that the number (expressed as N) of the "vacancies"/pores on the FP/ PU polymer coating is related to the volatilization of the solvent. Thus, due to the interface diffuse force (∑F) with its corresponding time (t) between H 2 O-FP/PU system ( FP/PU H O 2 γ − ) and FP/PU-AC system (γ FP/PU−AC ), the "vacancies" (t = t 0 = 0, r ≈ 0) expand to the pores (t = t n ≠0, r = r n ≠0) as shown in Figure 2a.

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In order to reveal the principle of pore expanding (t = t n , r = r n ), the process is decomposed into horizontal expanding procedure induced by interface tension ( Where G is the free energy of the FP/PU-solvent-H 2 O systems, γ is the interface tension, and r is the pore radius. For can be written as: Moreover, H 2 O is always subject to its own surface tension (P s ), which makes it retract toward the center of the pore. According to Laplace theory, P s is expressed as: Therefore, the resultant force for pore expanding (ΣF) can be expressed as: According to equation (1)(2)(3)(4), there are some pores with their radius as r 1 stop expanding at t 1 when F spread 1 + P s 1 = 0 (∑ F 1 = 0), and some other pores stop expanding at t 2 or t 3 as long as F spread 2 + P s 2 = 0 (∑ F 2 = 0) or F spread 3 + P s 3 = 0 (∑ F 3 = 0). The moment for pores to stop expanding (such as t 1 , t 2 , t 3 ) is influenced by surface roughness of the substrate, content   Schematic Diagram of micro/nano-pore surface structure fabrication by "vacancy capture" mechanism: a) "vacancy capture" mechanism, b-d) pore expanding process.

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When H 2 O molecules capture the "vacancies" left by the solvent molecules and accumulate in the wet coating, they are diffused into the solvent and prevented from contacting with pure FP polymers. Thus, there is no interface diffuse force (∑F) to form initial pores. The FP-AC-D and FP-THF-D pure polymer films without pores via dip process ( Figure S1, Supporting Information) demonstrates that the rationality and probability of the "Vacancy-Capture" mechanism (Figure 3a,e). Since n FP-D < n FP-S , it is easy to know η FP-D < η FP-S and V solvent-D > V solvent-S . The polymer content (n) of the wet film is a necessary factor for generating FP- Besides, it is found that the porosity as well as the total area of the pores of the two films is similar, indicating that the total amount of the "vacancies" occupied by H 2 O is almost the same no matter in FP-AC-S or FP-THF-S. which is accordance with the conserved system energy (Q e-AC = Q e-THF = Q c-H2O ).
In order to verify the proposed mechanism, the pores of FP-AC-S and FP-THF-S are classified according to r 1 (red), r 2 (blue), r 3 (yellow), and radius distribution is shown in Figure 3c,g. The results prove that the r of the pores is normally distributed. For the FP-THF-S film, the number of the pores with r 2 is much (71%) more than those with r 1 and r 3 . It is found that the distribution of the three different pore sizes present a certain rule: the pores with r 1 are concentrated in the middle, and the pores with r 2 are around the pores with r 1 and surrounded by the pores with r 3 distributed loosely. As all the pores are almost circular, it is implied that the pores are uniformly stressed during their expanding. For the FP-AC-S film, the pores with different r present disordered and most of the pores are elliptical, owing to asymmetry force during pore expanding process. Then the three adjacent pores with r 1 to r 3 or with the same r are selected randomly and their centers are connected to obtain triangles ( Figure S2a,b, Supporting Information). The results indicate that the triangles tend to be isosceles triangles ( Figure S2c-f, Supporting Information). The film-forming mechanism for FP system is a physical process of solvent volatilization for FP's poor deformability. Thus, the porosity and pore morphology of FP film are directly affected by the volatility of solvent.
Based on the proposed mechanism, the volatility of solvent is one of the factors controlling the number (N) and size (r) of pores. Therefore, THF with slower volatilization is selected as the solvent to fabricate the FP/PU-THF-S coating. It is easily found that N FP-AC-S > N FP-THF-S (Figure 4a,b). And the mean pore area of the FP/PU-THF-S coating is ≈0.23 µm 2 , 1.7% of the FP/PU-AC-S Coating (that is, r FP-AC-S < r FP-THF-S ). The result

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demonstrates that the relationship between the pore forming probability and pore expanding force is mutual restraint, consistent with the rule proven by the FP films which has been discussed in Figure 3. Due to crossing linking of the PU resin, the porosity of the FP/PU-THF-S coating (4.4%) is found different from the FP/PU-AC-S Coating (1.1%) which is different with the pure film (Figure 4c). Based on the proposed pore forming mechanism of "vacancy capture", the pore expansion of pure FP polymer film mainly depends on the volatilization of organic solvent, while that of FP/PU coating is mainly depends on the phase separation of FP/PU polymers and solvents ( FP/PU H O 2 γ − , γ FP/PU−AC ), which is affected by the crosslinking of PU. THF with slow volatilization gives PU molecular chains longer time to form cross-linking network. The PU wraps the FP to contact with the organic solvent, resulting in uniform phase separation. Due to the influence between adjacent pores, the number of pores is large, but the size is small. However, the AC with quick volatilization escapes from the wet coating, resulting in the movement of the PU chain segment limited, which inhibits the phase separation. Due to the lack of resistance of other pores around, the formed pores will continue to expand. If there is no forced termination by PU curing, the pore size will be larger. This is why the porosity of FP/PU-AC-S coating is smaller than that of FP/PU-THF-S coating. This is one of the applications using the proposed "vacancy capture" mechanism to achieve interface intelligent control on adjusting the number (N) and the size (r) of the micro/nano-pores on the FP/PU polymer coating surface by changing the volatilization rate of the solvent.
Owing to the lower η of the wet coating by dip process and excellent rheological property of the PU resin, the FP/PU polymers tend to stretch and move thermally in sufficient organic solvent to fill the "vacancies" once generated, leading to the probability of H 2 O molecules to seize the "vacancy" reduced. As a result, neither FP/PU-AC-D nor FP/PU-THF-D coating surface forms pores. Interestingly, the ordered and micro-spoon structure with obvious anti-fingerprint performance (Figure 5d) are innovatively fabricated on the two dipped polymer composite coatings (Figure 5a,b and Figure 1aiv-vi) induced by the asymmetrical forces. With the curing of the wet coating, the solvent molecules naturally escape from the wet coating surface driven by its saturated vapor pressure and a large amount of H 2 O (g) condenses to H 2 O (l) on the wet coating surface. Then the OH groups of H 2 O react with the NCO groups in PU leading to the curing rate of the coating surface much higher than that of the coating interior. Since moisture and oxygen are key ingredients for PU' crosslinking, different diffusion of moisture and oxygen caused varied curing rates in PU surface and bulk. Therefore, the wet coating is subjected to a force caused by crosslinking (F C ) pointing toward the wet coating interior (Figure 5d). At the same time, the PU with leveling characteristic, the main component of the wet coating, pulls the whole wet coating spread on the coating-substrate interface (F PU ) as shown in Figure 5d. The low surface energy FP, another component, tends to migrate to the wet coating surface (F FP ) to prevent the spoons from expanding (Figure 5d), which is induced by F C and F PU . Thus, a regular and orderly micro-spoon array structure is first formed under the synergistic effect of the above three asymmetric forces.
Based on the above analysis, if the boundary at the top of the spoon is regarded as an ellipse, the ellipsoidal model can be used to establish the shape function of the spoon. Take the spoon center (the deepest point of the spoon) as the coordinate origin (O 1 ), and take the directions of the minor axis, the major axis of the ellipse and the connecting line between O 1 and the center of the ellipse as the x, y, and z axes, respectively, to establish a three-dimensional coordinate system as shown in Figure 5c. Based on the ellipse size (the major axis ≈20 µm and minor axis ≈14 µm) measured from the SEM diagram, the coating thickness tested ≈5 µm and the distance between the ellipsoid center and O 1 assumed as 10 µm (50% of the major axis), The ellipsoidal function of the spoon, can be obtained and expressed as follows: When y = 0, the functional equation of profile Slice 1 is obtained as follows: When y = k, the functional equation of profile Slice 2 is obtained as follows: Selecting any point (expressed as O 2 ) on function (6) for force analysis, it shows that the component force of the F C on the x-axis and the F PU jointly determine the boundary size of the spoon, while the components of F C and F FP on the z-axis jointly determine the depth of the spoon. Based on the interface intelligent control mechanism of "vacancy capture", the freely switching of the coating surface structure between the micro/nano-pore to spoon with different sizes and numbers is realized. The anti-fingerprint property of the FP/PU polymer composite coating rely on the rearrangement of the fingerprint droplets which may be attributed to the low surface energy, micro/nano-texture and "air cushion".

Conclusion
In conclusion, a novel anti-fingerprint (reduced ≈40-60%) composite polymer coating (FP/PU) with transparency ≈60% is fabricated, using polytrifluoroethyl methacrylate homopolymer (FP) and Polyurethane (PU) resin. The number (N) and size (r) of the pores could be adjusted by changing volatilization of solvent. When fabricated by dipping, the FP/PU coating surface presents a patterned micro-spoon texture with r ≈ 5.16 µm. The FP/PU coatings' fingerprint coverage rate are ≈50% of the bare glass. The fingerprint droplets tend to show different directional distribution, indicating potential application for optical device.
A physical model of vacancy-capture and interface intelligent control is proposed. It describes that the "vacancy" is generated once the solvent molecules escape out of the wet coating and captured by H 2 O molecules (t = t 0 = 0, r ≈ 0). Then, the "vacancy" develops to pore under the interface force with corresponding time by FP/PU H O 2 γ − and γ FP/PU−AC (t = t n ≠ 0, r = r n ≠ 0). Finally, pore stops expanding as F spread + P s = 0. The force analysis including the curing force (F C ), the spreading force (F PU ) and the migrating force (F FP ) on the patterned micro-spoon structure of the dipped wet coating is discussed to reveal its forming mechanism.
Superior to previous precise and complex methods to prepare surface texture for anti-fingerprint property, switching freely between micro/nano pore and spoon of the composite polymer coating is realized by conventional spray and dip engineering painting process. The N and r of the pores and spoons could be adjusted intelligently based on the proposed "vacancy capture" mechanism. The secondary distribution of the fingerprint droplets (that is, intelligent distribution) may rely on the micro-pore/-spoon texture profile, "air cushion" stored and low surface energy at the edges of the pores and spoons. The antifingerprint composite polymer coating technique is expected to apply on optical device for large-scale in the future.
Synthesis of Poly (2,2,2-Trifluoroethyl Methacrylate) Homopolymer (Abbreviated as FP): According to the standard free radical polymerization process, 2,2,2-trifluoroethyl methacrylate (1.68 g, 10 mmol), AIBN (0.015 g, 0.1 mmol) and 5 ml methanol were added into a 100 mL modified Schlenk tube flask equipped with a magnetic bar. The flask content was degassed three time with nitrogen at room temperature, and then was immersed in a preheated water bath at 75 °C for 8 h. The synthesized FP was isolated by precipitation in deionized water, collected and repeatedly washed with ethanol and cyclohexane three times, and dried under vacuum at 40 °C for 12 h. The FT-IR, 1 H NMR, XPS, and TG spectrum of the synthesized FP homopolymer confirmed the structure ( Figure S3, Supporting Information).

Preparation of FP Film and FP/PU Composite Coating Process:
The fluoropolymer (FP) film samples were prepared by dispersing the resulting polymerization reaction product solution in an ultrasonic instrument for 5 min to obtain the film-forming solution. The FP film was constructed on aluminum and glass substrates by both dip and spray methods. Summary of the samples with processes, solvents, parameters, and labels were shown in Table S1 (Supporting Information). The above synthesized FP solution was added to PU adhesive in the ratio of fluoropolymer solution: PU adhesive = 5:2 by volume and dispersed in the ultrasonic apparatus for 5 min to obtain the mixed FP/PU composite coating. Coating process was the consistency with above FP film as Table S1 (Supporting Information). Fluoropolymer/polyurethane-acetone-spraying abbreviated as FP/ PU-AC-S, Fluoropolymer/polyurethane-acetone-dipping abbreviated as FP/PU-AC-D, and the details of the remaining abbreviations can be found in the supporting information (Table S1, Supporting Information).
Characterization: Microstructures of the FP film and FP/PU coating were observed by scanning electron microscopy (TESCAN MIRA3 FE-SEM, TESCAN, Czech). The WCA of FP/PU coating were tested by a contact angle apparatus (DSA100, KRÜSS GmbH, Germany) using 5 µL distilled water droplet, respectively. Each surface was tested five times for repeatability. The transmittance spectrum of bare glass, PU coating, and FP/PU composite coating were recorded by a Varian Cary 5000 UV-vis-NIR Spectrophotometer in a wavelength range of 300 to 800 nm in double-beam mode with air as the reference.
Anti-Fingerprint Property Evaluating of FP film and FP/PU Coating: The human fingerprints coated with slime were pressed onto the bare glass and coated glass surface and observed through an optical microscope (NV3000, Jiangnan, China), then the fingerprint droplet coverage, mean area, and distribution were calculated by software.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.