11 Wings of Morph aega butterflies are natural surfaces that exhibit anisotropic liquid adhesion. The 12 direction -dependent arrangement of wing scales creates orientation -turnable microstructures with two 13 distinct contact modes for liquid droplets . Enabled by recent developments in additive manufacturing, 14 such natural surface design s coupled with hydrophobicity play a crucial role in applications like self- 15 cleaning, anti -icing, and fluidic manipulation . However, the interplay among resol ution, architecture, 16 and performance of bio-inspired structure s is barely achieved . Herein , inspired by the wing scales of 17 the Morpho aega butterfly , full-scale synthetic surfaces with anisotropic adhesion fabricated by two - 18 2 photon polymerization are reported . The quality of the artificial butterfly scale is improved by 19 optimizing the laser scanning strategy and objective lens movement path. The corresponding contact 20 angle s of water on the fabricated architecture with various design parameters are measured, and the 21 anisotropic fluidic adhesion is investigated . Results demonstrate that tuning the geometrical parameters 22 and spatial arrangement of the artificial wing scales enables anisotropic behaviors of the droplet’s 23 motion. The measured results also indicate a reverse phenomenon of the fabricated surfaces in contrast 24 to their natural counterparts, possibly attributed to the significant difference in equilibrium wettability 25 between the fabricated microstructures and the natural Morpho aega surface. These findings are utilized 26 to design next-generation fluid -controllable interfaces for manipulating liquid mobility on synthetic 27 surface s. 28 INTRODUCTION 29 Natural structures found in organisms have evolved over thousands of years to be functionally 30 efficient . Such structures in biosystem s can exhibit various properties , including high strength- to-mass 31 ratio, adhesion, fluid direction control lability , and hydrophobicity.1-5 Particularly , hydrophobic 32 surface s exist naturally in plants and insects like lotus leaves , rice leaves , beetle s, and butterfly wing s.5-33 8 These structures hold great promis e for applications in modern engineering designs and the 34 manufacturing sector . Inspired by natural hydrophobic surfaces, artificial surfaces often exhibit macro 35 to nanoscale hierarchical structures comprising a range of features organized in a structured geometric 36 pattern . The integrated structure composed of ordered features provides the means to manipulate the 37 interfaces and dynamics of liquid- air and liquid -solid interactions. Consequently, the presence of 38 hierarchical microstructures and nanostructure s can improve hydrophobicity by promoting high 39 contact angle of water .9 40 With the recent development in additive manufacturing (AM) , the fabrication of bio- inspired 41 hydrophobic structures is enabled .9-11 Many AM techniques, including fused deposition modeling 42 3 (FDM), selective laser melting (SLM), and digital light processing (DLP), have been used to rapidly 43 prototype and develop such hydrophobic surface s. Among them , FDM is an extrusion- based AM 44 method using polymer -based filaments . The f ilament is melted by passing through a heated nozzle and 45 deposited on a heated platform when the filament is in the semi- liquid state. The resolution of the 46 printed part produced by common FDM is approximately 0.1 mm.12 While FDM is widely recognized 47 as a cost-effective 3D printing method, its capability to fabricate small- scale bio -inspired features is 48 relatively limited due to its sub-par resolution in microscale manufacturing .13 Alternatively , SLM is a 49 powder -based AM technique, which fuses the uniform ly spread metallic powder with a high -power 50 laser to produce the metallic structure layer -by-layer .14 It enables the fabrication of metallic bio - 51 inspired hydrophobic structures with high strength, enhanced impact resistance, and high thermal 52 conductivity.15 For instance, Mekhiel et al. showcased a 3D printing bio- inspired structure utilizing 53 partially fused stainless steel microparticles to achieve hydrophobicity on a 3D printed metal 54 structure.16 However, t he resolution of SLM product s is based on the laser beam spot size, which is 55 typically about 70 to 200 μm.14 The spatial arrangement of hierarchical features may suffer from some 56 imprecision and inconsistency due to irregular particle size and thermal cycling. Conversely, DLP is a 57 projection- based AM technique utilizing liquid photosensitive resin, which is selectively cured by 58 controlled ultraviolet (UV) light exposure , to print a 3D structure in a layer -by-layer formation . It is 59 capable of produc ing polymer -based bio -inspired structures with a resolution ranging from 15 µm t o 60 100 µm.17 Davoudinejad et al. fabricated gecko -inspired adhesive surfaces, featuring two- or three - 61 leveled hierarchical micro -pillars , via UV polymerization , demonstrating high contact angle of water .18 62 However , the printed feature using DLP tends to be significantly larger than the natural counterparts , 63 and achieving complex geometries can be challenging . 64 Hence, the pursuit of a reliable method to accurately fabricate 3D microstructures has become 65 imperative . Two-photon polymerization ( TPP) emerges as a promising sub- microscale AM technique , 66 where a small spatial region within the liquid photosensitive resin is polymerized when two photons 67 4 are simultaneously absorbed by activating lasers .19 The polymerization process initiates when the 68 absor bed energy of the photosensitive resin surpasses a specific threshold. The photosensitive material 69 around the center of the laser’s focal point is solidified to form an ellipsoidal particle, namely a voxel, 70 because of the crosslinking of the polymer chains .20 The diameter of voxel gradually diminishes 71 vertically, forming a parabolic shape due to the non- linear reduction in energy intensity away from the 72 focal point , eventually dropping below the polymerization energy threshold. As a result, the printed 73 structures consist of periodically arranged voxels based on the laser scanning path according to the 74 programmed digital file . Additionally, the voxel size depends on factors such as type of objective lens , 75 as well as the optimal combination of laser scanning speed and power. Both horizontally and vertically, 76 multiple neighboring voxels overlap with optimal hatching and slicing distance s to form a stable 77 structure. The superior resolution of TPP enables the fabrication of numerous bio- inspired structures. 78 For instance, lotus -inspired topography and S alvinia -inspired structures , featuring arrays of papillae 79 and exhibiting hydrophobicity, have been manufactured .6, 21 Moreover, springtail -skin- inspired designs 80 have demonstrated liquid repellency in dry adhesive structures by adding the double re -entrant feature 81 to the T -shaped fibrillar .4, 22 82 However, t he aforementioned bio-inspired hydrophobic surfaces possess limited functionality due to 83 their isotropic design. Although the fabricated hierarchical structure may exhibit properties like self- 84 cleaning and anti -icing, directional controllability is lacking. By modifying the microstructures to 85 incorporate an anisotropic design, programmable surface hydrophobic properties can be introduced. 86 The different direction s of liquid movement can be granted by imitating the anisotropic texture of 87 biological surfaces . For example, the movement of the droplets can be directed by allowing the droplet s 88 to move towards the far edge of the butterfly wing effortlessly. As the wing is tilted upward and 89 downward, the area of the scales in contact with the droplet increases and decreases accordingly due 90 to the compliant scales rearrangement, which results in anisotropic wettability.1 Additionally, the 91 droplet sits above the hierarchical surface structures to prevent water accumulation on the body. They 92 5 can smoothly move along the roll -off direction and collect contaminants, reducing excess weight and 93 enhancing self -cleaning ability. The roll -off direction of the butterfly wing’s scale horizontally 94 orientates from the scale root to the tip . Each scale typically measures approximately 180 µm in length, 95 70 µm in width, and 3 µm in thickness.8 Smaller features known as lamella- stacking ridges can be 96 found on top of each scale. These lamella -stacking ridges play a n integral role in influencing the 97 hydrophobicity of the wing surface, and their dimensions are about 0.7 μm in width, 2 μm in height, 98 and 0.22 μm in stacking spacing.23 99 In this work, inspired by the anisotropic scale design of the Morpho aega butterfly wing , artificial 100 surface s for directional surface wetting are fabricated via the TPP process . The interplay among TPP 101 printing quality , architecture design, and anisotropic hydrophobic performance are investigated . 102 Particularly, the printing process is optimized to improve the geometrical accuracy of simplified 103 lamella -stacking ridges and the spatial arrangement of artificial wing scales . Furthermore, the 104 hydrophobicity of the printed structures is investigated thr ough the analysis of the contact angle of 105 water . Moreover, anisotropic adhesion is quantified by measuring the minimum required air pressure 106 to move a water droplet under various tilting conditions ranging from 0° to 90° and in two distinct 107 directions , namely roll -off and roll -against directions . Surprisingly, the results reveal a reverse 108 behavior of the artificial surfaces in contrast to their natural counterparts, possibly caused by the 109 significant difference in equilibrium wettability between the Parylene coating and the natural butterfly 110 wing scale surfaces. It is anticipate d that the findings will facilitate the design of advanced fluid - 111 controllable interfaces appli cable in future sub- microscale fluidic devices, directional coatings, and 112 advanced engineered synthetic scaled surfaces. 113 EXPERIMENTAL SECTION 114 2.1. Sample Preparation and Fabrication . To prepare for the TPP process, an indium tin oxide 115 (ITO) -coated square glass substrate (25 mm×25 mm×0.7 mm) is t horoughly cleaned sequentially in 116 6 deionized (DI) water bath, 99.5% acetone bath, and 99.9% isopropyl alcohol (IPA) bath in a sonicator 117 for 3, 3, 8 minutes , respectively. Next, the surface of the cleaned ITO glass is activated using an oxygen 118 plasma treatment with the Plasma Etch PE -50HF system (Plasma Etch, Caron City, USA) for 40 119 seconds. Then, a drop of IP -S photoresist (Nanoscribe, Eggenstein- Leopoldshafen, Germany) is 120 deposited at the center of the ITO glass, which is secured onto the substrate holder using polyimide 121 tape. The substrate holder is inserted into the TPP printer with the cleaned 25x objective lens installed . 122 The 25x objective lens is positioned in contact with the photoresist to initiate the polymerization of the 123 liquid photosensitive resin using a laser beam. The fabricated microstructures are then taken out from 124 the printer and developed in propylene glycol methyl ether acetate (PGMEA) (Merck K GaA, 125 Darmstadt, Germany) for 20 minutes to dissolve any uncured excess photosensitive resin. 126 Subsequently, the structures are bathed in IPA for 5 minutes to clean off any remaining PGMEA 127 residue. 128 2.2. Hydrophobic Parylene Coating. Based on previously reported work, m icrowave -assisted 129 chemical vapor deposition (CVD) is utilized to apply a Parylene -N hydrophobic coating onto the 130 artificial butterfly scale structures and a cleaned silicon wafer .24 The thickness of resulting coating is 131 approximately 55 nm as measured from the silicon wafer . 132 2.3. Scanning Electron Microscopy. The quality of the printed microstructures is inspected utilizing 133 an FEI Quanta 650 scanning electron microscopy (SEM). To enhance imaging quality, a thin layer of 134 gold is uniformly coated onto the samples using a 108 Manual sputter coater (Ted Pella, Redding, 135 USA). The SEM imaging is performed in high vacuum mode, at an acceleration voltage of 10 kV to 136 minimize the influence of atmospheric interference. 137 2.4. Equilibrium Contact Angle Measurement. The equilibrium contact angle measurements are 138 carried out using a digital microscope (AF4915ZTL, Dino- Lite) with a maximum magnification of 139 140x. All measurements are conducted under controlled environmental conditions . For the static 140 contact angle measurement, a 0.5 μL DI water droplet is transferred by a micropipette and dispensed 141 7 onto the microstructures within the designated 5×5 mm2 printed area. The contact angles are measured 142 using ImageJ software at a view perpendicular to the artificial scales’ roll -off direction. 143 2.5. Roll-off and Roll -against Pressures Measurement . A rotational stage is securely mounted 144 vertical ly on a flat wall surface, and a custom -made substrate holder is attached to the rotational stage . 145 Then, t he substrate containing the printed microstructures is placed horizontally on the holder, and the 146 roll-off direction of microstructures is perpendicular to the rotation axis of the stage. Following that, a 147 pneumatic nozzle, connecting to Ultimus II dispenser (Nordson EFD, USA ), is positioned 148 perpendicularly to the edge of the printed surface with 35 mm distance, and a 0.5 μL DI water droplet 149 is deposited on the edge close to the pneumatic nozzle. With this setup, a stable airflow is generated 150 via the pneumatic nozzle , and the minimum required pressure to initialize droplet motion is measured . 151 When the airflow direction is along the roll -off direction, the roll -off pressure (ROP) is determined 152 when the DI water droplet shifts along the airflow direction. Likewise, roll -against pressure (RAP) is 153 measured in the reverse roll -off direction. The experiment setup can be found in supporting information 154 (Figure S 1). 155 RESULT AND DISCUSSION 156 The scales on the natural wings of the Morpho aega butterfly are arranged in a radial outward 157 orientation from the body , overlapping in a manner similar to that of roof shingles , as shown in Figure 158 1a-c. The average length and width of the butterfly wing’s scale are a pproximately 180 µm and 72 µm, 159 respectively, which aligns with values presented in another research.8 Inspired by the scale of butterfly 160 wing, the curved microstructure featuring ridges is designed based on the dimensions of the natural 161 wing scale ( Figure S2, Supporting Information) . Arc length similar to the scale of Morpho aega 162 butterfly wing is achieved by setting the length to 150 μm and root tilted angle to 120 °. Considering 163 all the above dimensions, the overall height of the fabricated structure is about 98 μm. The width of 164 the curved scale is set to 72 μm to ensure even distribution of the ridge structure into grating pitches 165 8 (γ) of 4 μm, 8 μm, and 12 μm. The lamella -stacking ridge nanostructure is simplified to a single ridge 166 with 5 μm height. The thickness of the artificial scales is set to 10 μm to avoid degradation in structure 167 due to overhanging. The base with 12 μm thickness supports the artificial scale structures to avoid 168 detachment from the built plate. A gap with half the grating pitch is added between each scale structure 169 perpendicular to the direction of the scale. The base width is equivalent to the sum of the scale width 170 and the gap. An array group of artificial scales is designed with varying base lengths (L) of 70 μm, 80 171 μm, and 90 μm. A total of nine designs with varying grating pitches γ and base length L are fabricated 172 and further studied. 173 174 Figure 1. (a)-(c) SEM image of the natur al butt erfly wing . (d) Schematic diagram of the TPP 3D 175 printer . The microscope objective moves along the z -axis, and the substrate can move in the xy- plane. 176 Two mirrors control the laser beam scanning path in the xy-plane. (e) Schematic diagram of the 177 9 artificial butterfly scale microstructure s printing process . The microscope objective is immerse d in 178 photosensitive resin, and the liquid resin at the focal point of the laser beam is polymerized. (f) 179 Patterned artificial butterfly scales on ITO -coated glass substrate. (g)-(h) SEM images of the top and 180 side views of the printed microstructures . 181 182 As shown in Figure 1d, f or the specific TPP printer used in imitating the scale of butterfly wing, the 183 Nanoscribe Photonic Professional GT2 system (Nanoscribe, Eggenstein -Leopoldshafen, Germany) 184 employs an objective lens to fabricate microstructure s. In the process, laser pulses are generated by a 185 near-infrared laser beam in a TPP printer and used to excite the photoinitiator molecules in the 186 photosensitive material.25 To ensure structural stability, the smallest feature in the design, which is the 187 ridge structure with 2 μm width, requires at least three neighboring voxels to support the weight of a 188 water droplet. Therefore, the 25x objective lens, capabl e of fabricat ing microstructure s with a voxel 189 size of 600 nm in diameter and 3.6 μm in height, is necessary and more suitable for efficient printing 190 of the designed artificial scale surfaces . 191 The computer -aided design (CAD) model of the artificial scale surface is processed by the 3D slicer 192 DeScribe software with the default settings for the 25x objective lens using 1 μm slicing distance and 193 0.5 μm hatching distance. Then, the artificial butterfly scale is printed in a bottom -up matter, starting 194 from the scale root to the scale tip ( Figure 1e). During the process, voxel fusion occurs parallel to the 195 roll-off direction to prevent the formation of floating polymerized particles when dealing with 196 overhanging features. To establish a robust bond between the printed structures and the substrate, the 197 number of base layers is set to 6 with 80% laser power. As a result, t he anisotropic scale surface , 198 inspired by the scales of Morpho aega butterfly wing, with 5×5 mm2 is successfully fabricated on an 199 ITO glass (Figure 1f). The artificial scales are neatly arranged with specific offset and spacing with the 200 same orientat ion as designed (Figure 1g,h) . 201 10 202 Figure 2. SEM of TPP printed artificial scales. (a)-(c) Side views of the designs with γ = 6 μm, and 203 L= 70 μm , 80 μm, and 90 μm . (d)-(f) Top view of the tip of the artificial scales with γ = 4 μm , 8 μm , 204 and 12 μm. 205 206 Figure 2a-c shows the side view s of the TPP fabricated microstructure s inspired by the scale of 207 butterfly wing. It can be observed that, when the base length is set to 70 μm, the tip of the printed scales 208 is in contact with the neighboring scales in the roll -off direction. The vertical gaps between two 209 adjacent scales for the 80 μm and 90 μm base length sets are about 5 μm and 10 μm, respectively. As 210 shown in Figure 2d-f, additive layers can be observed easily on the top of the scales. Smoother 211 curvature can be printed by reducing the slicing distance, but the required fabrication time increase s 212 drastically. The printed ridge structures on the artificial scales match the designed grating pitches 4 213 μm, 8 μm, and 12 μm. For all designs, the difference between the width of the peaks and valleys is 214 measured to be 2 μm . 215 An optimized printing strategy is crucial due to the effect of microstructure s’ quality on hydrophobic 216 performance.8 The quality of the printed artificial scales can be improved by modifying the objective 217 movement and laser scanning path s. Specifically, t he objectiv e lens move s along or against the ro ll-off 218 11 direction to fabricate once a row of scales is fabricated (Figure 3a). When the objective is positioned 219 at the desired location , the laser is activated and polymerize s the photoresist with a program med 220 scanning path (Figure 3b). During the printing process, voxel fusion occurs along the roll -off direction 221 to prevent the formation of floating polymerized particles when dealing with overhanging features. 222 As demonstrated in Figure 3c-e, two different laser scanning paths are tested , namely back -and-forth 223 and one -way paths . The resulting morphologies of the printed scales are shown separately in Figure 224 3c,e. In Figure 3c, two fused voxels are observed at the end of the hatching lines because the 225 photopolymer resin is exposed to the laser for a longer duration when reversing the scanning direction 226 to print the next hatching line. On the contrary, t he quality of the microstructures is improved by setting 227 the laser scanning path to one -way, and the protuberant features are precluded , as shown in Figure 3e . 228 However, compared with the back- and-forth laser scanning path, alternating laser scanning with a one - 229 way path significantly increases the fabrication time due to the discontinuous printing path. 230 Additionally, the objective lens moving path is a factor affecting the quality of the fabricated 231 microstructures. As shown in Figure 3f-g, the laser penetrates the completed structure if the objective 232 lens moves along the roll -off direction when fabricating a new row of artificial scales . The printed 233 structure underneath the adjacent fabricated scale mismatches the design due to the refraction of the 234 laser and shifted focal point. The differences between the printed microstructure s and the designs , such 235 as detachment from ITO glass substrate, misalignment between the base st ructure s, and discontinuous 236 curved surface of the artificial scale, can be precluded by moving the objective lens along the roll- 237 against path when printing the next row of artificial scales as shown in Figure 3h. 238 12 239 Figure 3. (a) Artificial scales are fabricat ed on an ITO -coated glass with a 25x objective lens moving 240 along the roll-against direction . (b) Voxels polymerization at the laser focal point and the laser scan 241 along the roll -off direction . (c)-(e) Schematic of a typical hatch line fabrication via voxel fusing with 242 back -and-forth and one-way laser scanning paths, and SEM of the artificial scales tip with γ = 12 μm 243 printed via the corresponding scanning paths . (f)-(h) Schematic of the objective moving path and the 244 corresponding laser focal point while fabricating adjacent microstructure s, and SEM of the side of the 245 artificial scales with γ = 12 μm printed via roll-off and roll -against objective moving path s. 246 247 To demonstrate water droplets can roll off from the surface, the printed microstructure s need to have 248 hydrophobicity, which can be determined based on the contact angle of water . The equilibrium contact 249 angles of water droplets on all the surfaces with different dimensions of artificial scales are measured 250 as previously described . The surfaces investigated include an ITO-coated glass, Parylene- N-coated 251 13 glass , as well as non- coated and Parylene -N coated micro structure s. As depicted in Figure 4a,b, the 252 contact angles of the ITO-coated glass and the Parylene- N coated glass are about 39° and 92°, 253 respectively. As illustrated in Figure 4c, while the IP -S photoresist microstructures elevate the contact 254 angle, nearing the effect seen with the Parylene -N coated glass, they fail to realize optimal 255 hydrophobicity. The liquid penetrates the gaps of the artificial scales , which agrees with the mechanism 256 for surface wetting on the hierarchical structure in Wenzel states.26 257 258 Figure 4. Water contact angle measurement. ( a)-(b) the contact angle of water of ITO-coated and 259 Parylene- N coated glass (c)-(d) The contact angle of water of non-coated and Parylene- N coated 260 microstructure design with γ = 4 μm and L = 70 μm design. (e) Left and right average contact angles 261 of ITO-coated glass, Parylene -N coated glass, non- coated artificial scale microstructure s, and the 262 coated microstructures with different design parameters . 263 14 Consequently, an additional slender Parylene -N layer is incorporated to boost the hydrophobic 264 attributes. Figure 4d shows the addition of a 55 nm thick Parylene -N coating drastically increased the 265 water greater than 120° for the printed artificial scales. The water droplet is supported by the air that is 266 trapped in the gap, which matches the mechanism for surface wetting on the hierarchical structure in 267 Cassie -Baxter state.26 As Figure 4e shows, the microstructure surfaces with 4 μm and 8 μm grating 268 pitches had similar contact angles ranging from 121.5° to 125.5°. The 12 μm grating pitch design had 269 the highest level of hydrophobicity with contact angles ranging from 127.5° to 132.0°. A complete 270 measurement of contact angle of water of Parylene -N coated microstructure design s can be found in 271 supporting information ( Figure S3 and Table S1 ) 272 To test the anis otropic properties of the fabricated surfaces, air pressure is applied to the water droplet 273 to observe the effect of the microstructures on liquid movement in different directions. More 274 specifically, the anisotropic behavior of the microstructures is characterized by analyzing droplet ’s 275 mobility under various tilting conditions at 0°, 30°, 60°, and 90°. The anisotropic liquid transportability 276 is determined through the minimum required air pressure to initiate water droplet move ment in roll -off 277 and roll -against directions , nam ely ROP and RAP. 278 As shown in Figure 5a, the pressure required to move droplet s in roll -against direction is significantly 279 less than the roll -off direction, which is opposite to the natural butterfly wing scale. It should be noted 280 that previous research demonstrated that the water droplet on the artificial wing scale could naturally 281 move along the roll -off direction when the equilibrium contact angle was larger than 150 °.8 282 Unexpectedly, in this study, the water droplet is pinned on the surface without the assistance of airflow , 283 regardless of the variation in grating pitch and base length . Potential cause of this effect can be lack of 284 high enough hydrophobicity of the surfaces t o let the water droplet naturally roll down. 285 15 286 Figure 5. ROP and RAP measurements at various degrees and schematic of water droplet movement 287 under air pressure . (a) ROP and RAP for different γ and L at 0° of rotation. ( b) Schematic of liquid 288 transportation on artificial scale surface with applied air pressure. A water droplet sits on the 289 microstructures and presents hydrophobicity in a steady state. Detail views of the droplet’s deformation 290 under ROP and RAP at the left and right ends . (c) Average ROP and RAP for γ = 12 μm and L = 90 291 μm at 0°, 30°, 60°, and 90° of rotation. 292 293 Figure 5b shows a schematic of a droplet demonstrating Cassie -Baxter state surface wetting . RAP is 294 used to push the droplet to slide towards the scales’ decline direction and make the droplet in contact 295 with the nearby microstructures . In contrast, ROP is applied to support the droplet to move towards the 296 16 scales’ incline direction and cause the droplet to fall and sit onto the adjacent artificial scales, which 297 require s more energy to exceed the hydrostatic equilibrant at the tip as well as overcome the potential 298 energy of the droplet. As the base length increased from 70 μm to 80 μm, and lastly to 90 μm, the 299 contact angle of water had minor changes within 1° difference, but the ROP decreased significantly . 300 The decrease in ROP is possibly due to reducing the number of artificial scales in contact with the 301 droplet, resulting in lower resistance in movement. Specifically, for the 4 μm and 8 μm grating pitch 302 design, RAP is increased as the base length increases . This is possibly caused by the increasing gaps 303 between the artificial scales trapping the droplet. On the contrary, for the 12 μm grating pitches design, 304 RAP is decreased with a decrease in the base length. The higher contact angle of water presented in 305 the 12 μm grating pitch design perhaps lowers the required energy for the droplet to slide toward the 306 roll-off direction and jump onto the adjacent scales. In addition, ROP and RAP are decreased as the 307 rotary angle of the stage increased because the increased gravitational potential energy along the roll - 308 off direction reduced the need for pressure for water droplet shift to the adjacent artificial scales, as 309 shown in Figure 5c. Both ROP and RAP are measured for the surfaces with different printed 310 microstructures at 0°, 30°, 60°, and 90° , a complete measurement can be found in supporting 311 information ( Table S2 ). 312 To explore the possibility of the proposed bio- inspired microstructure s for innovative water -related 313 energy harvesting and droplet manipulating devices ,27, 28 as a test case, a functional hydrophobic 314 surface with multiple anisotropic liquid transportation directions is designed. The surface is divided 315 into three regions with different roll -against directions (Figure 6a). The r oll-against direction s of 316 Regions I, II, and III are oriented differently , and their geometrical designs (base length L and grating 317 pitches γ ) are b ased on the measurement of ROP and RAP. It should be noticed that all the design s 318 have the same base length , L = 70 μm. Specifically , Region I consists of the design with the highest 319 ROP ( γ = 8 μm ), providing strong resistance to water droplet motion and limiting the droplet rolling 320 across the boundary when airflow is applied from the front and air pressure is lower than 14.1 psi . 321 17 Regions II and III consist of the design with the lowest RAP (γ = 12 μm) with the same base length, 322 avoiding undesired gaps between the three regions . This design also enabl es droplets to roll from the 323 back to the front of Region II and right to left of Region III when air pressure is higher than 2 psi . 324 According to the aforementioned designs (γ = 8 μm and γ = 12 μm) , a 1×1 cm2 surface is fabricated 325 via TPP (Figure 6b,c). 326 To further demonstrate the anisotropic hydrophobicity behaviors , the water droplets, on top of the 327 horizontally placed fabricated artificial surface, are tested under airflow generated by two pneumatic 328 nozzles. Specifically, two nozzles are separately positioned 35 mm away from the front and right edges 329 of the fabricated surface. They are connect ed to an air dispenser which is set to provide a consistent 5 330 psi air pressure, point ing to the roll -against direction of Region II and III (Figure 6d). In addition, to 331 push all the water droplets distributed along the front edge of the printed surface, the front nozzle 332 moves perpendicular to roll -against direction of Region II and remain s the 35mm distance for the 333 fabricated surface. 334 In this study, to investigate the behavior of anisotropic transportability of the proposed design, six 335 droplets are initially deposited on left and right segments (“3+3”) in Region II ( Figure 6e). Under the 336 external airflow, we noticed that the deposited water droplets are moved and accumulated together as 337 the width of R egion II decreases (Figure 6f). Subsequently, t he majority portion of the accumulated 338 droplet moves within R egion II along its roll- against direction . The remaining portion of the water 339 droplets pinned by the microstructure s in Region I is dragged by the water surface tension and move s 340 along the boundary of Region II . Afterward, the motion of the accumulated droplet is pinned on Region 341 III due to the designed roll -against direction ( Figure 6g) . Finally, t he pinned water droplets are moved 342 by the stationary airflow generated by the side nozzle and removed from the fabricated surface (Figure 343 6h). 344 18 345 Figure 6. Demonstration of a functional hydrophobic surface for liquid transportation. (a) A printed 346 microstructure surface consisting of two designs of artificial scale in three orientations. The design 347 with γ = 12 μm and L = 70 μm are arranged in Regions II and III , and the design with γ = 8 μm and L 348 = 70 μm are patterned in the red regions. (b) Six drops of DI water are deposited on the printed 349 microstructures . (c)-(d) Droplets are moved and merged as the airflow is applied along the roll -against 350 direction of R egion II, and R egion I stops the droplets . (e) The droplets are moved and merged as the 351 airflow is applied along the roll -against direction of R egion III. 352 353 According to the observation, the synthetic bio- inspired hydrophobic surface demonstrates the 354 capability of transporting water droplets along the roll -against direction by pinning droplets in 355 undesired regions until supplied airflow aligns with favorable regions. In addition, the “1+1” and “2+2” 356 droplet patterns are also tested under the same conditions , and the results are included in supporting 357 information ( Figure S4- S5). From th ese findings , effective controllable fluid transportation can be 358 achieved by fabricating engineered hydrophobic surfaces, using multiple designs based on Morph aega 359 19 butterfly scale via TPP . Such engineered structures coupled with advanced AM can facilitate the 360 implementation of applications, such as energy harvesting and droplet manipulating devices . 361 CONCLUSIONS 362 In this article, inspired by scales of butterfly wing s, various 3D hydrophobic microstructure surfaces 363 are fabricated on ITO -coated glass slides using the TPP technique . The quality of the artificial butterfly 364 scale is improved by implementing a one-way laser scanning strategy and making adjustment s to the 365 objective lens movement in alignment with the roll-against direction during printing. The fabricated 366 microstructures enhance the hydrophobicity of the surfaces and significantly increase the contact angle 367 of water when coated with a thin layer of Parylene- N. The hydrophobicity is further i mproved by 368 increasing the grating pitch length of the ridge structure on the artificial scales . With the hydrophobic 369 coating, anisotropic drop adhesion is presented on the engineered 3D structured surface, which 370 showcases less favor to move along the roll-off direction than along the roll-against direction, opposite 371 to the natural counterparts . In terms of the geometrical parameters, the increase in base length of the 372 printed scales improve s the movability of the droplet . Still, it encounters more difficult y in moving 373 along the roll-against direction due to reduced hydrophobicity. The presented TPP printed scale 374 surface s with directional adhesion hold potential for various applications, such as tunable fluid 375 transport surface s, self-cleaning exteriors, and micro -channel filling. Future work may include 376 modeling the droplet movement on the artificial scale to determine the optimal dimensions for reducing 377 resistance in the roll-against direction . 378 ■ ASSOCIATED CONTENT 379 Supporting Information 380 Experiment al setup for roll -off and roll -against pressures measurement; CAD models of butterfly- 381 inspired artificial scales; C ontact angle of water on Parylene -N coated microstructure designs; The 382 20 “1+1” and “2+2” droplet patterns tested on functional hydrophobic surface; C ontact angles of water 383 measurements; Measurement of ROP and RAP of Parylene -N coated microstructures. 384 ■ AUTHOR INFORMATION 385 Correspond ing Author s: 386 *Meng Cheng - Key Laboratory of Metallurgical Equipment and Control Technology, Ministry of 387 Education, Wuhan University of Science and Technology, Wuhan, Hubei 430081, China 388 *Daewon Kim - Department of Aerospace Engineering, Embry- Riddle Aeronautical University, 389 Daytona Beach, FL 32114, United States; Email: daewon.kim@erau.edu 390 *Yizhou Jiang - Department of Aerospace Engineering, Embry- Riddle Aeronautical University, 391 Daytona Beach, FL 32114, United States; Email: yizhou.jiang@erau.edu 392 Authors 393 ∇Zefu Ren - Department of Aerospace Engineering, Embry- Riddle Aeronautical University, Daytona 394 Beach, FL 32114, United States 395 ∇Zhuoyuan Yang - Department of Aerospace Engineering, Embry- Riddle Aeronautical University, 396 Daytona Beach, FL 32114, United States 397 Rishikesh Srinivasaraghavan Govindarajan - Department of Aerospace Engineering, Embry- Riddle 398 Aeronautical University, Daytona Beach, FL 32114, United States 399 Foram Madiyar - Department of Physical Science, Embry- Riddle Aeronautical University, Daytona 400 Beach, FL 32114, United States 401 Author C ontributions 402 ∇These authors contributed equally to this work. The manuscript was written through contributions 403 from all authors. All authors have approved the final version of the manuscript. 404 21 Note s 405 The authors declare that the research was conducted in the absence of any commercial or financial 406 relationships that could be construed as a potential conflict of interest. 407 ■ ACKNOWLEDGMENTS 408 This material is based upon work supported partially by the National Science Foundation under 409 Grant s No. 2018853 and 2050887, and Natural and Science Foundation of China under Grant No. 410 52201272. The opinions, findings, and conclusions, or recommendations expressed are those of the 411 authors and do not necessarily reflect the views of the National Science Foundation and Natural and 412 Science Foundation of China . 413 ■