Mimicking from Rose Petal to Lotus Leaf: Biomimetic Multiscale

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Applications of Polymer, Composite, and Coating Materials

Mimicking from Rose Petal to Lotus Leaf: Biomimetic Multiscale Hierarchical Particles with Tunable Water Adhesion Cheng Chen, Liu Mingming, Liping Zhang, Yuanyuan Hou, Mengnan Yu, and Shaohai Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21494 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Mimicking from Rose Petal to Lotus Leaf: Biomimetic Multiscale Hierarchical Particles with Tunable Water Adhesion Cheng Chen, Mingming Liu*, Liping Zhang, Yuanyuan Hou, Mengnan Yu and Shaohai Fu* Jiangsu Engineering Research Center For Digital Textile Inkjet Printing, Key Laboratory of Eco-Textile, Jiangnan University, Ministry of Education, Wuxi, Jiangsu 214122, China KEYWORDS: superhydrophobic, multiscale hierarchical particles, raspberry-like particles, adhesive force, controllable

ABSTRACT: Water-droplet adhesions of the coatings constructed by all-polymer multiscale hierarchical particles (MHPs) were finely adjusted within the range from highly adhesive to selfcleanable. The MHPs were synthesized via thermal-induced polymerization of the reactants absorbed into self-made hollow reactors and in-situ capping of nanocomplexes onto the reactors’ shell simultaneously. The dynamic wettability of the prepared MHPs was tuned between water droplet sliding and water droplet adhering by simply controlling the type of the capped nanocomplexes. Water adhesive force changed in the range from 31.28 to 89.34 μN. In addition, the raspberry-like particles (MHPs without nanocomplex capping) were used to construct superhydrophobic rose petal-like surface with high water adhesive force, which can be applied in microdroplet transportation without loss. The MHPs with appropriate nanocomplex capping

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were used to fabricate superhydrophobic lotus leaf-like fabric, exhibiting excellent anti-fouling property and superior mechanical stability. We believe that the prepared superhydrophobic MHPs with diverse water adhesive forces are promising in potential academic research and industrial applications.

1. INTRODUCTION Biomimetic functional surfaces with extreme wettability have been intensively explored and applied in numerous practical applications, which not only deepen the understanding of the wetting behavior in nature but also accelerate the development of the surface technologies.1-4 In particular, lotus leaves exhibit superhydrophobic property with low adhesion or so-called “lotus effect”, on which water droplet can form near spherical shape with water contact angles (WCAs) larger than 150° and roll off easily.5 Rose petals are also superhydrophobic with WCAs of near 152.4°, whereas water droplets tightly adhere onto the surfaces even when the petals are tilted or turned upside down. The high-adhesion wetting state is defined as “petal effect”.6-7 Wettability of the superhydrophobic surfaces is determined by interfacial contact areas and affinity to water droplets.8-10 Indeed, the roughened lotus leaf surface covered with wax can entrap massive airpockets to prevent water from intruding into the micro-papillae.11 In contrast, for the rose petal with larger scale of micropapillae and nanostructures, water droplets easily intrude into the large grooves while leaving the small grooves nonwetting, forming the Cassie impregnating wetting regime.6, 12-14 Thus, the impregnation degree of water into the microstructures differentiates the petal effect and lotus effect. The transition between the two wetting states can be achieved by altering the size of microstructure.


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According to the adhesion mechanisms underlying the two wetting behaviors, artificial superhydrophobic surfaces with low or high adhesive forces have been intensively investigated and utilized in a wide range of applications.15-16 Superhydrophobic surfaces with low adhesive force that mimic lotus leaves can be applied in self-cleaning, friction-reduction, oil-water separation, and anti-icing/fogging.17-20 Highly sticky superhydrophobic surfaces mimicking rose petals show promising potential in water/fog collection and microdroplet transportation.21-23 However, for the emerging intelligent areas such as microfluidic switches and biomedicine transporters, the vast majority of artificial superhydrophobic materials with given adhesion property could not fulfill the requirements.24-25 Therefore, controlling the adhesion of liquid droplets toward the target surfaces becomes a significant issue. The surface wettability and water adhesion of the underlying substrates are mainly governed by structure morphology and chemical composition. Recently, manipulation of liquid adhesion behaviors is mainly achieved by controlling surface roughness or generating heterogeneous chemical composition via top-down methods. For example, different adhesive properties can be obtained on the controllable surface structures including micro/nano CNT cones and hair-like architectures,26

micro-lens

and

micro-bowl

PDMS

arrays,27

hierarchically

structured

bSi/elastomer composite surface,28 ball-like structured Cu(OH),29 porous TiO2 nanostructured films,30 and aligned PS nanopillars,31 which can be fabricated by surface treatment techniques such as hydrothermal, lithography UV-irradiation, wet-chemical etching, and electrochemical anodization. In addition, inspired by the desert beetle’s back with hydrophilic-hydrophobic submillimeter patterned surfaces applied for droplet collections,32 constructing surfaces with heterogeneous chemical composition can be used to modulate the water adhesive force. Various biomimetic microscopic structured surfaces have been designed and prepared by introducing


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hydrophilic microdomain on the roughened hydrophobic surface via photocatalytic lithography,33 site-selective ink printing,34 selective plasma etching,35 selective chemical functionalization36 and so on. The controllable water adhesive force of the patterned surface is attributed to the conflicting effect of adhesion on hydrophilic microdomains and repulsion on hydrophobic area.37 Despite the progress in fabrication of superhydrophobic surfaces with tunable water adhesion, most of the current top-down building strategies have focused on the fabrication surfaces with different morphologies and chemical compositions, which are constrained by the certain underlying substrates, severe preparation conditions, and complex procedures. Up to now, successful control of water adhesion on superhydrophobic surface solely constructed by accumulated particles as building units via a facile and universal bottom-up method is extremely rare. Here, superhydrophobic surfaces with controllable water adhesive force were prepared by the assembling of all-polymer multiscale hierarchical particles (MHPs) via a bottom-up method on different substrates. The MHPs were synthesized by absorbing styrene, ethylene glycol dimethacrylate (EDGMA) and 2,2'-azobis(isobutyronitrile) (AIBN) into the cavity of self-made hollow P(styrene-divinylbenzene-trifluoroethyl methacrylate) [P(S-DVB-TFEMA)] reactors and depositing hydrophobic nanocomplexes onto the reactor shell followed by thermal-induced polymerization.38-40 Based on the prepared hierarchical particles with different morphology, the raspberry-like particles (RPs) without nanocomplexes were used to prepare superhydrophobic glass with high water adhesive force, which was applied in the water droplets transportation without loss. Significantly, the MHPs with nanocomplexes as the building units were utilized to endow fabric with superhydrophobicity and low water adhesive force, realizing the construction of the robust antifouling fabric.


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2. EXPERIMENTAL SECTION 2.1. Materials Styrene (99%) was purchased from Shanghai Chemical Reagent of China and distilled to remove the inhibitor in a vacuum. Divinylbenzene (DVB, 50%), EDGMA (98%), trifluoroethyl methacrylate (TFEMA, 98%), dipentaerythritol penta-/hexa-acrylate (5Acl, 98%), branched polyethylenimine

ethylenediamine

(BPEI,

98%),

octadecylamine

(OTCA,

98%)

and

tetrahydrofuran (THF, 98%) were supplied by Shanghai Macklin Biochemical (China). AIBN (98%) and poly(vinylpyrrolidone) (PVP, K30) with a molecular weight of 30000 were supplied by Sinopharm Chemical Reagent of China and used as received. Tetraethyl orthosilicate (TEOS, 98%), vinyl triethoxy silane (VTES, 98%), absolute ethanol, aqueous ammonia (NH3H2O, 28%) and sodium hydroxide (NaOH) were analytical grade and purchased from Shanghai Chemical Reagent of China. Methacrylic resin adhesive DM5128 was purchased from Chengdu Dymatic Jingying Chemicals of China. Deionized water was used throughout the experiment. 2.2 Fabrication of P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs The route for fabrication of P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs was shown in Scheme 1.


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Scheme 1. (a) The synthesis path of nanocomplexes, (b) formation process of hollow P(S-DVBTFEMA) reactors and (c) the route for fabrication of superhydrophobic P(S-DVBTFEMA)@P(S-EDGMA)@nanocomplexes MHPs. 2.2.1 Synthesis of nanocomplexes and hollow P(S-DVB-TFEMA) reactors 5Acl, BPEI and OTCA were added into 30 mL of THF to obtain a mixture, the mass ratio of 5Acl:OTCA was maintained at 1:1 (5Acl, 1.5 g), and the amounts of BPEI based on weight of 5Acl were 0%, 6.5%, 13.5%, 20.0% and 26.5%. Then the mixture was stirred at room temperature for 12 h. The five kinds of nanocomplexes including NCs 0# (BPEI, 0%), NCs 1# (BPEI, 6.5%), NCs 2# (BPEI, 13.5%), NCs 3# (BPEI, 20.0%) and NCs 4# (BPEI, 26.5%) were produced by 1,4-conjugated addition reaction between amine and acrylate groups (Scheme 1a).


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The silica particles were prepared by sol-gel method. 100 mL of absolute ethanol and 33 mL of deionized water were charged into a three-neck flask. NH3H2O (10 mL) and TEOS (10 mL) were added into the above solution under stirring at 30 °C for 12 h. Then VTES (3 mL) was added in the above system for introducing vinyl groups on the surface of the silica. The modification was carried out at 30 °C for 12 h. Afterward, 3 g of VTES-modified silica particles, 2 g of PVP, mass ratio of AIBN to styrene (5.0 wt%), styrene (4 g), DVB (based on the weight of styrene, 30.0 wt%), TFEMA (based on the weight of styrene, 75.0 wt%) and 5 mL of deionized water were added into 45 mL of absolute ethanol and stirred at high speed for some time, then heated to 80 C for 6 h to obtain the silica/P(S-DVB-TFEMA) particles. NaOH aqueous solution (0.5 M, 50 mL) was added into the suspension of silica/P(S-DVB-TFEMA) particles, where the cores were etched at 80 C for 4 h. Thus, hollow P(S-DVB-TFEMA) reactors were prepared (Scheme 1b). It could be seen from TEM results, the silica/P(S-DVB-TFEMA) particles had obvious core-shell structure and the silica cores were completely removed in NaOH solution for the formation of hollow P(S-DVB-TFEMA) reactors. 2.2.2 Preparation of MHPs and RPs The prepared hollow P(S-DVB-TFEMA) reactors, nanocomplexes, and reactants (styrene, EDGMA, AIBN) were added into 30 mL of deionized water. The resulting mixture was stirred at room temperature for 10 h. The mass ratio of reactors:styrene:EDGMA:nanocomplexes was kept constant at 1:2:1:4 (styrene, 0.8 g), with 12.5% of AIBN (based on the weight of styrene) throughout the experiments. After absorption of reactants and deposition of nanocomplexes (NCs 0#, 1#, 2#, 3# or 4#), these loaded particles were added into petri dish and then heated at 80 °C for 6 h to initiate polymerization in an oven. Finally, P(S-DVB-TFEMA)@P(SEDGMA)@nanocomplexes MHPs including MHPs 0#, 1#, 2#, 3# and 4# were obtained,


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respectively. The formation mechanism of the MHPs was reflected in TEM results (Scheme 1c). The hollow P(S-DVB-TFEMA) reactors could simultaneously absorbed and deposited reactants and nanocomplexes, which formative loaded particles presented rough core-shell structure. The P(S-EDGMA) polymers firstly filled with confined cavity and then massive P(S-EDGMA) protrusions were produced on reactors surface as well as fixing nanocomplexes deposits during thermal initiation, finally showed multiscale hierarchical structure. In addition, P(S-DVBTFEMA)@P(S-EDGMA) RPs was fabricated via the above-mentioned method in the absence of nanocomplexes. 2.2.3 Preparation of superhydrophobic coatings In order to enhance the binding force between particles and substrates (including glass and fabric, 25 mm  75 mm), methacrylic resin adhesive DM5128 was introduced to construct robust superhydrophobic coatings. In a typical synthesis, glass and cotton fabric were coated with methacrylic resin (1 mL) and then were heated to 80 °C for 30 min. In addition, 1 g of the prepared RPs and MHPs were dispersed into 10 mL of ethanol, forming uniform suspensions. To prepare superhydrophobic glass, 3 mL of the RPs and MHPs ethanol suspensions were dripped onto the sticky surface of the treated glass slides followed by heating in an oven (80 C, 2 h) for fixation of RPs and MHPs on the glass slides. However, MHPs could not be tightly fixed on the surface of the original glass slide without resin reinforcement, on which these particles were easily taken away by water droplets (Figure S1). To prepare superhydrophobic fabric, 3 mL of MHPs 2# ethanol suspension was used to coat the sticky cotton fabric via dropwise dripping followed by heating at 80 C for 2 h. 2.3 Characterizations


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The morphology and surface element content of samples were characterized using a scanning electron microscope (SEM, Hitachi su1510, Hitachi Co., Japan) with energy dispersive spectrum (EDS). The internal structure of samples was observed on a transmission electron microscope (TEM, Hitachi JEM-2100, Hitachi Co., Japan). The chemical bonds and functional groups of samples were investigated using infrared spectroscopy (iS50FT-IR, Thermo Fisher Scientific Co., USA). Thermogravimetric analysis (TG) was carried out with a thermal analyzer (Q500, TA Co., USA). X-ray diffraction (XRD) was performed on a D2 ADVANCE X-ray diffractometer (Bruker Co., Germany) with monochromatic Cu Kα radiation. All calculations of density functional theory were carried out with Gaussian09 programs.41 Optimized ground state geometry of the lowest-energy conformer of 5Acl, BPEI and OTCA was calculated at the B3LYP/6-31G (d) level. Density distribution of electron cloud was calculated by DFT/B3LYP method with 6-31G (d) basis set. The contact angle (CA) of static water droplets (5 μL) was measured using the contact angle meter equipped with a CCD camera (Ramehart Instrument Co., USA) at ambient temperature to evaluate the wettability of the samples. The sliding angle of water droplets (10 μL) was characterized using an optical contact-angle meter system (Krüss DSA 100, Germany). The water adhesive force was measured using a high-sensitivity micro-electromechanical balance system (Dataphysics DCAT11, Germany). Water droplet (5 μL) suspending on a hydrophobic metal ring was approached and retracted from the sample surface at a constant speed of 0.01 mm s-1 at the ambient environment with relative humidity of about 50%-80%. The droplet started to move away from the sample surface once the equipment detected a force that was equivalent to gravity caused by 0.01 mg (nearly 100 μN). Subsequently, the balance force gradually increased and reached the maximum before the droplet broke away from the surface. The abrupt change in


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data that recorded in the data curve was taken as the water adhesive force. The average CA and SA values were obtained by measuring the same sample at several different positions. The bouncing process of a falling water droplet on the sample surface was shot by LC321S highspeed camera (Phantom, USA). For sandpaper abrasion test, the coated sample was faced down to the sandpaper (mesh number, 1000) and moved for 10 cm along a ruler under a 100 g or 200 g weight by external drawing force. The abrasion test was performed for 200 cycles and the mass loss ratios, contact angles, sliding angles were measured after each 20 cycles. For mechanical stability test, ultrasonication (20 min, 100 W) was implemented in KQ-100E ultrasonic cleaning machine (KunShan Ultrasonic Instruments, China). 3. RESULTS AND DISCUSSION 3.1. Characterization of hydrophobic nanocomplexes 3.1.1. Morphology and chemical information As shown in Figure 1a, the particle size and morphology of the prepared nanocomplexes were changed with the addition amount of BPEI ranged from 0% to 26.5%. Indeed, the crosslinker BPEI was conductive to the crosslinked polymerization and further enhanced the branching degree of nanocomplexes. The crystallinity condition of the samples is negatively correlated to the branching degree, which can be calculated by the ratio of crystallization peak area and sum of relative peak area in XRD spectra using Peakfit software.42 From XRD analysis (Figure 1b), it could be clearly seen that the diffraction peaks of nanocomplexes appeared at nearly 19°-22° and the peaks of NCs 0# to NCs 4# gradually shifted to right. The crystallinity of nanocomplexes was decreased from 24.66% for NCs 0# to 20.71% for NCs 4#, indicating that the branching degree of nanocomplexes increased. Figure S2 showed that the infrared absorption peaks of characteristic groups of nanocomplexes were all appeared on the FTIR-ATR spectra;


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e.g., the C-C bond (peaks at 2917.18 and 2849.79 cm-1, NCs 3#; peaks at 2920.22 and 2850.98 cm-1, NCs 0#), the C=O bond (peaks at 1732.28 cm-1, NCs 3#; peaks at 1732.27 cm-1, NCs 0#), the N-H bond (peaks at 1466.20 cm-1, NCs 3#; peaks at 1465.96 cm-1, NCs 0#) and the C-N bond (peaks at 1165.07 cm-1, NCs 3#; peaks at 1162.20 cm-1, NCs 0#). Furthermore, the thermal stability of the nanocomplexes was investigated by the DTG curve (Figure 1c). As shown in Figure S3, the temperatures of the maximum rate of weight loss (Tp) for 5Acl, OTCA and BPEI were 473.8 C, 266.3 C and 356.7 C, respectively. Correspondingly, the Tp for the NCs 0# had two weight loss peaks at 301.3 C and 469.6 C ascribed to 5Acl and OTCA. For the DTG curve of NCs 3#, there were three peaks at 279.7 C, 355.5 C and 421.1 C. It was indicated that NCs 3# was synthesized by exploiting reaction of three chemical compounds (5Acl, BPEI and OTCA) and NCs 0# was synthesized by exploiting reaction of two chemical compounds (5Acl and OTCA). In addition, the mass ratios of the elements of NCs 0# and NCs 3# were measured by EDS mapping (Figure S4). Both NCs 0# (88.85%) and NCs 3# (87.17%) had high mass ratio of C element. Compared with NCs 0#, the mass ratio of N element of NCs 3# increased from 1.22% to 2.67% which ascribed to BPEI participated in 1,4-conjugated addition reaction.


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Figure 1. (a) SEM, (b) XRD spectra and (c) TG analysis of nanocomplexes. 3.1.2. Gaussian simulation analysis Density distribution of electron cloud and charge numbers for 5Acl, BPEI and OTCA were calculated at the B3LYP/6-31G (d) level based on optimized molecular geometry configuration. As shown in Figure 2, Millikan of oxygen in acrylate group for 5Acl was -0.44 (negative electrostatic potential), Millikan of hydrogen in amine groups for BPEI and OTCA was 0.3 (positive electrostatic potential). It was inferred that reaction between oxygen in acrylate group and hydrogen in amine group would happen, due to the attraction arising from the electrical property difference of electrostatic potential. The simulated calculation results were in agreement with characterization results of FTIR-ATR and TG, further confirming that nanocomplexes was synthesized successfully via 1,4-conjugated addition reaction between acrylate and amine groups.


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Figure 2. Gaussian simulation for reaction of nanocomplexes. 3.1.3. Surface energy CA measurements were conventionally employed to evaluate the surface energy of various materials, which played a significant role in the wetting states. Hence, the related surface energy (γ) of NA 3# and NA 0# was calculated by the measured CA (θ) according to Owens and Wendt equations.43 𝛾𝑙(1 + cos 𝜃) =

1 𝑑 𝑑 2 2(𝛾𝑠 𝛾𝑙 )

+

1 𝑝 𝑝 2 2(𝛾𝑠 𝛾𝑙 )

Eq.(1)

where the subscripts l and s denote liquid and solid, respectively. Fowkes44 suggested that the surface energy of a solid (γs) consists of two components including the dispersive component (γsd) and the polar component (γsp): 𝛾𝑠 = 𝛾𝑑𝑠 + 𝛾𝑝𝑠

Eq.(2)

Consequently, by measuring the CAs of the test liquids and the polar and dispersive components of the test liquids, the surface energy can be calculated. Here, the test liquids were water and CH2I2, and the corresponding parameters γd = 21.8 mJ m-2 and γp = 51.0 mJ m-2 for water, γd = 48.5 mJ m-2 and γp = 2.3 mJ m-2 for CH2I2.45 The CAs of water and CH2I2 on the surfaces were 138° and 35° for NCs 0#, 142° and 58° for NCs 1#, 144° and 74° for NCs 2#, 148° and 88° for NCs 3#, 142° and 79° for NCs 4#, respectively (Figure S5). Based on the abovementioned parameters, the surface energy of NCs 0#, 1#, 2#, 3# and 4# were calculated and listed in Table S1. Surface energy of nanocomplexes gradually decreased with increasing of the amount of BPEI, indicating that nanocomplexes synthesized by 5Acl, BPEI and OTCA could be applied as low surface energy substances for hydrophobic treating. The occurrence of the decline in surface energy of nanocomplexes was caused by the increase of reactive sites for long chain alkyl amine OTCA in 1,4-conjugated addition reaction during improving the amount of


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crosslinker BPEI. The growing structure scale of nanocomplexes contained long chain alkyl was also the cause of this downtrend in surface energy. 3.2. Morphology and wettability of P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs To investigate the importance of nanocomplexes capping on the construction of superhydrophobic multiscale hierarchical structure via an in-situ seeded polymerization localized in hollow P(S-DVB-TFEMA) reactors (Figure 3a and 3b), structure morphology and wettability of prepared P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs and P(S-DVBTFEMA)@P(S-EDGMA) RPs were analyzed comparatively. As shown in Figure 3c and 3d, P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes

MHPs

possessed

three-scale

rough

structure including the reactors P(S-DVB-TFEMA) as primary structure, the reactively formed corona P(S-EDGMA) as secondary structure and the nanocomplexes deposits as tertiary structure, which presented more complicated hierarchical structure than P(S-DVBTFEMA)@P(S-EDGMA) RPs with dual-scale structure. The static water CA of the MHPs (167) was higher than that of the RPs (154). And water droplet rolled down along the MHPs coating surface with tilt angle near 25, indicating the MHPs coated glass exhibited low adhesion (Figure 3e). On the contrary, water droplet tightly adhered on the RPs coated surface rotated at 90. Moreover, when the RPs coated glass got in touch with a water droplet that was placed on the silanes modified SiO2 self-cleaning surfaces, the water droplet was completely transferred to the RPs coated glass attributing to its high water adhesion (Figure 3f).It was revealed that nanocomplexes capping could promote surface roughening of hierarchical particles and enhance capacity in holding metastable trapped air, realizing controlled variation for water adhesive force of the particles.


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Figure 3. (a) Formation and wettability of P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs and P(S-DVB-TFEMA)@P(S-EDGMA) RPs, SEM (left) and TEM (right) images of (b) hollow P(S-DVB-TFEMA) reactors, (c) P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs, (d) P(S-DVB-TFEMA)@P(S-EDGMA) RPs, (e) low water adhesion and sliding of the MHPs and (f) high water adhesion and lateral adhesion of the RPs (horizontal substrate was hydrophobically modified SiO2). 3.3. Morphology and adhesion control of MHPs Nanocomplexes including NCs 0#, 1#, 2#, 3# and 4# could be introduced to form various tertiary structure of P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs, obtaining MHPs 0#, 1#, 2#, 3# and 4#, respectively. As shown in Figure 4a, morphology control of P(SDVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs could be realized. All the as-prepared


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MHPs could reach superhydrophobic with CAs larger than150 (Figure 4b). Particularly, the static CA of MHPs 3# was 169, which was higher than that of other MHPs. Figure 4c presented that water droplet could slide off from the surfaces constructed by MHPs 1#, 2#, 3# and 4# under different sliding angles in the range from 14 to 45. For MHPs 0# with the similar structure of RPs, water droplet could suspend on its surfaces even totally inverted. Furthermore, the water adhesive forces of the MHPs coated glass were shown in Figure 4d and Figure S6. It was found that the water adhesive force decreased first and then increased with improving the amount of BPEI in the synthesis of nanocomplexes, being in accordance with the variation of sliding angle. The water adhesive force could be controlled from 31.28 to 89.34 μN. Clearly, the MHPs 2# performed superhydrophobicity with sliding angle of 14 and water adhesive force of 31.28 μN. It could be concluded that with the increase of added BPEI amount, the branching degree and particles size of the nanocomplexes gradually became larger. Thus, the hierarchical scale differences of the MHPs resulted by different types of nanocomplexes as tertiary structure directly changed their contact behavior and surface adhesion. In the Cassie-Baxter model, the rough structure can store air between the water droplets and the underlying substrates. Because air is a hydrophobic medium, the numerous entrapped air pockets in the rough structures can effective decrease the liquid/solid interfacial contact area and raise the repellency to water. In other words, surface wettability and adhesive force can be modulated by liquid-solid contact areas and the air pocket ratio at the nanoscale roughness. For the MHPs 2# with appropriate structural hierarchy and suitable surface roughness, dense and tiny nanoscale gaps generated among the well-sized nanocomplexes could trap massive air to form metastable air pocket layer, achieving the Lotus wetting state (Figure 4e). Nevertheless, with the further reduction or enlargement of nanocomplexes deposits sizes, the gaps between the


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undersized or oversized nanocomplexes became larger, which results in a large liquid-solid contact area and a thin air pocket layer. Thus, water droplets were inclined to impregnate into these apertures and the adhesive forces of MHPs coated glass were increased (Figure 4f and 4g). Furthermore, when surface nanoscale roughness on the hierarchical particles was continuously decreased until disappeared, the intensive enhancement of wicking effect caused by interparticle gaps endows MHPs 0# and RPs coated glass with high adhesive property, forming the Cassie impregnating wetting state (Figure 4h). Nonwetting over the minuscule grooves can enhance advancing contact angle while the impregnation of water on the large grooves will bring down the receding contact angle. Thus, the contact angle hysteresis defined as the difference value between advancing angle and receding angle can characterize the pinning effect of water on a surface.46 To further investigate dynamic wettability for MHPs with different morphologies, advancing angle and receding angle were measured to calculate contact angle hysteresis values of various MHPs (Figure S7 and Table S2). Clearly, the variation of contact angle hysteresis values with sliding angle was in good agreement, especially the contact angle hysteresis value of MHPs 2# attained minimum point (15). Actually, contact angle hysteresis was relevant to the contact area between the solid surface and the liquid droplet. The massive nanocomplexes on the protrusions of MHPs 2# could decrease the liquid-solid interfacial area, which could lead to the reduction in the intermolecular interactions and hence a reduction in the contact angle hysteresis. These findings were in accord with the calculation of contact angle hysteresis for rough particles containing fluorine reported by W. Hess.47


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Figure 4. (a) Morphology, (b) static water CA, (c) sliding angle and (d) water adhesive force of MHPs, wetting state of (e) MHPs with well-sized nanocomplexes, (f) MHPs with undersized nanocomplexes, (g) MHPs with oversized nanocomplexes and (h) RPs without nanocomplexes. 3.4. Construction and performance evaluation of superhydrophobic fabric 3.4.1 Preparation of superhydrophobic fabric


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The as-prepared P(S-DVB-TFEMA)@P(S-EDGMA)@nanocomplexes MHPs with low water adhesive force (MHPs 2#) was adopted as the building units to fabricate lotus-like superhydrophobic fabric (Figure 5a). With the help of inherent high stickiness of thermo-curing methacrylic resin adhesive, the MHPs 2# could firmly adhered on the cleaned fabric (Figure 5b), forming a robust superhydrophobic coating. Surface element content of the prepared superhydrophobic fabric was measured by EDS test. Obviously, C element content reached to 81.72% and 2.22% of N element content occurred, whereas F element content was relatively low (1.90%), which could be ascribed to the coverture of the P(S-DVB-TFEMA) reactors by the corona P(S-EDGMA) protrusions and nanocomplexes (Figure 5c and 5d).

Figure 5. (a) schematic diagram for preparation of superhydrophobic fabric, (b) the SEM images the original fabric, (c) EDS and (d) SEM images of superhydrophobic fabric prepared by MHPs 2#.


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3.4.2 Anti-fouling and self-cleaning property The prepared superhydrophobic fabric (cutting area, 25 mm  30 mm) with low water adhesive force exhibited excellent anti-fouling and self-cleaning performance. As shown in Figure 6a, a falling water droplet touched onto the superhydrophobic fabric and then bounced to form a spherical shape, showing superior water-repellent property. Besides, black tea, cola and coffee droplets could easily slide from the superhydrophobic fabric and no residue adhered during the sliding process, which meant the superhydrophobic fabric had good anti-fouling ability (Figure 6b-d). Moreover, the self-cleaning properties of the original and coated cotton fabric were tested. For the untreated cotton fabric, carbon black stained on its surface was infiltrated into the fabric after water washing (Figure S8a). In contrast, the typical dirt including carbon black, yellow earth and gravel stained on the prepared superhydrophobic fabric were all cleaned totally without leaving any residues, showing outstanding self-cleaning property (Figure S8b-d).

Figure 6. (a) Bouncing process of a falling water droplet on the superhydrophobic fabric, sliding process of (b) black tea, (c) cola and (d) coffee on the tilted superhydrophobic fabric.


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3.4.3 Abrasion resistance and mechanical stability tests Mechanical stability of the as-prepared MHPs 2# was detected by ultrasonication test. As shown in Figure S9, MHPs 2# could maintain its multiscale hierarchical structure with nanocomplexes, high static water CA (163) and low sliding angle (9) even after ultrasonicated for 20 min, indicating the MHPs 2# exhibited stable mechanical stability. Moreover, to evaluate abrasion resistance of the prepared superhydrophobic fabric (cutting area, 25 mm  30 mm), static water CA, sliding angle, appearance and micromorphology of the superhydrophobic fabric during cyclical abrasion test using sandpaper (mesh number, 1000) (Figure 7a) were investigated respectively. During abrasion process with 100 g load-bearing or 200 g load-bearing, the CAs were on the decline and sliding angles were on the rise with extending the abrasion cycles (Figure 7b and 7c) due to the decrease of nano-scale roughness, which would give rise to the loss of air pockets (Figure S10). Furthermore, mass loss ratio of the coating after each 20 abrasion cycles had been measured as shown in Figure 7d. Mass loss value was the ratio of the mass of MHPs 2# coating after and before abrasion cycles. After 200 abrasion cycle times, the mass loss ratios of MHPs 2# coating for the superhydrophobic fabric facing down sandpaper under a 200 g and 100 g weight were 13.14% (mass loss rate 2) and 3.54% (mass loss rate 1), respectively. Seen from the SEM images of MHPs 2# coating during abrasion test (Figure S10), the coating thickness slightly decreased from 154 μm to 122 μm after 200 abrasion cycle times (loadbearing, 100 g weight), whereas the coating thickness treated by abrasion test with 200 g loadbearing was only 77 μm. However, the MHPs 2# remained integrity and no obvious damages appeared under high-intensity abrasions. Thus, by virtue of the organic resin, the prepared MHPs 2# coated superhydrophobic fabric had excellent stability and fastness even under high strength abrasion damages.


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Figure 7. (a) Demonstration of abrasion test, (b) static water CA and sliding angle of superhydrophobic fabric during abrasion test (load-bearing, 100 g of weight), (c) static water CA and sliding angle of superhydrophobic fabric during abrasion test (load-bearing, 200 g of weight), (d) mass loss rate 1 for MHPs 2# coatings of superhydrophobic fabric during abrasion test (load-bearing, 100 g of weight) and mass loss rate 2 for the MHPs 2# coatings (load-bearing, 200 g of weight). 4. CONCLUSION In conclusion, several types of nanocomplexes with different morphology and branching degree were synthesized via 1, 4-conjugated addition reaction by adjusting the mass ratios of BPEI. By absorption of reactants for thermal initiated polymerization and deposition of nanocomplexes, various MHPs with hierarchical rough structure were fabricated, which could be further used as building blocks to construct superhydrophobic surfaces with controllable water


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adhesive force via a facile bottom-up method on various substrates. We believed that these research findings would be conductive to revealing the influences of the structure morphology on the surface adhesion and providing an efficient fabrication principle for the superhydrophobic surface mimicked from rose petal to lotus leaf. More importantly, the prepared MHPs with tunable water adhesion could be used in many applications, such as lossless droplet transportation, micro-fluidic devices, and antifouling coatings.

ASSOCIATED CONTENT Supporting Information. Optical images of superhydrophobic glass; FTIR-ATR spectra, TG curves, Atomic EDS mapping of the nanocomplexes; contact angles of water and CH2I2 on nanocomplexes; water adhesive forces and contact angle hysteresis of the MHPs; anti-fouling property of the superhydrophobic fabric; SEM images of the MHPs and superhydrophobic coatings before and after mechanical treatments. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Shaohai Fu); E-mail: [email protected] (Mingming Liu). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by national first-class discipline program of Light Industry Technology and Engineering (LITE2018-21), the Fundamental Research Funds for the Central Universities (JUSRP51514) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_1827). We also thank Jiangnan University for supporting in the course of research. REFERENCES 1. Liu, M.; Wang, S.; Jiang, L. Nature-Inspired Superwettability Systems. Nat. Rev. Mater. 2017, 2, 17036. 2. Neinhuis, C.; Barthlott, W. Characterization and Distribution of Water-Repellent, SelfCleaning Plant Surfaces. Ann. Bot. London 1997, 79, 667-677. 3. Hansen, W. R.; Autumn, A. K. Evidence for Self-Cleaning in Gecko Setae. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 385–389. 4. Yang, S.; Ju, J.; Qiu, Y.; He, Y.; Wang, X.; Dou, S.; Liu, K.; Jiang, L. Peanut Leaf Inspired Multifunctional Surfaces. Small 2014, 10, 294-299. 5. W. Barthlott; Neinhuis, C. Purity of the Sacred Lotus, or Escape From Contamination in Biological Surfaces. Planta 1997, 202, 1-8. 6. Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal Effect: A Superhydrophobic State with High Adhesive Force. Langmuir 2008, 24, 4114-4119. 7. Bhushan, B.; Her, E. K. Fabrication of Superhydrophobic Surfaces with High and Low Adhesion Inspired from Rose Petal. Langmuir 2010, 26, 8207-8217. 8. Bhushan, B.; Nosonovsky, M. The Rose Petal Effect and the Modes of Superhydrophobicity. Philos. Trans. A Math. Phys. Eng. Sci. 2010, 368, 4713-4728.


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Table of Contents Graphic


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Mimicking from Rose Petal to Lotus Leaf: Biomimetic Multiscale

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