Drag Reduction of Anisotropic Superhydrophobic Surface Prepared by

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Drag Reduction of Anisotropic Superhydrophobic Surface Prepared by Laser Etching Yanjing Tuo, Haifeng Zhang, Wanting Rong, Shuyue Jiang, Weiping Chen, and Xiaowei Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01040 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 6, 2019

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Drag Reduction of Anisotropic Superhydrophobic Surface Prepared by Laser Etching Yanjing Tuo†, Haifeng Zhang＊†,‡ ,§, Wanting Rong†, Shuyue Jiang†, Weiping Chen† ,§ , Xiaowei Liu†,‡ ,§ † MEMS Center, Harbin Institute of Technology, Harbin, 150001, China. ‡ State Key Laboratory of Urban Water Resource & Environment (Harbin Institute of Technology), Harbin, 150001, China. §Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Ministry of Education, Harbin, 150001, China Keywords: anisotropic, superhydrophobic, drag reduction

In this research, the anisotropic superhydrophobic surface is prepared on stainless steel by laser etching, and the drag reduction property of the anisotropic surface is studied by a self-designed solid-liquid interface friction test device. Periodic arrangement structures of quadrate scales with oblique grooves are obtained on stainless steel surface by laser. After modification by fluoride, the surface shows superhydrophobicity and anisotropic adhesive property. Here, the inclined direction of grooves and the inverse direction are defined as the RO and OR, respectively. By changing the inclination of the grooves, a surface is obtained with a contact angle of 160°, a rolling angle difference of 6° along RO and inverse RO direction. It is verified by numerical simulation and experiment that the subjected force of water droplets on the surface is different along RO and inverse RO direction. Furthermore, the as-prepared surface has different drag reduction effect along the two directions. With the increase of velocity, the drag reduction effect of superhydrophobic surface decreases against RO direction, while it remains a high ratio along RO direction. We believe the anisotropic surface will be helpful in novel microfluid devices and shipping transportation.

INTRODUCTION In the past decades, surfaces with superhydrophobicity have attracted widespread attention. In general, the superhydrophobic surfaces are divided into Wenzel and Cassie-Baxter modes according to the contact state between the water droplet and the solid surface.1,2 For the Cassie-Baxter mode, the most typical natural surface is lotus leaf with low adhesion and self-cleaning property.3,4 For the Wenzel mode, the famous natural surfaces are gecko foot and red rose petal, which have high adhesive forces for water.5-8 Inspired by these isotropic surfaces, various superhydrophobic surfaces with micro/nano structures have been prepared, which have great prospect in the application fields of self-cleaning, anti-corrosion, ant-icing, oil/water separation, drag reduction, etc.9-16 Recently, Jiang et al. have found directional adhesive properties on the wings of butterfly.17 A droplet can easily rolls along the radial outward direction but is tightly

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pinned in the opposite direction. The anisotropic superhydrophobic surface with special solid-liquid adhesion has aroused enormous interest, due to its advantages in both fundamental research and practical applications.18-22 The arrangement and orientation of micro/nano structures play a great role in the solid-liquid adhesion. By adjusting the micro/nano structures and composition of the surface, the adhesion force of the solid-liquid interface can be controlled. Inspired by the step-overlapping of butterfly wings, zhang et al. prepared the superhydrophobic surface on aluminum alloy with microscale ratchet structures.23 The ratchet structures lead to anisotropic behavior of superparamagnetic microdroplets under external magnetic field. Law et al. fabricated grooved structures on a silicon wafer, which showed directional self-cleaning property.24 Long et al. fabricated groove-like microstructures on copper surfaces by femtosecond laser micromachining and discussed the wetting state under different conditions.25 Li et al. made use of a soft lithography method to prepare oblique two-tier conical structures under an external magnetic field and realized the directional bouncing of droplets with low energy loss.26 The anisotropic surfaces can be potentially applied in the flied of biochemical separation, targeted drug deliver, novel microfluidic devices and directional easy-cleaning.27-30 There have been few studies on the drag reduction property of anisotropic superhydrophobic surface. Bixler and Bhushan have studied the fluid drag reduction with rice leaf and butterfly wing bioinspired surfaces.31, 32 But, it has not pointed out that the drag reduction effect of the prepared anisotropic surface is different in different direction. In this report, we use laser technique to fabricate periodic arrangement structures of quadrate scales with oblique grooves on stainless steel. Here, the inclined direction of grooves and the inverse direction are defined as the RO and OR, respectively. After modification with low surface energy material, the surface exhibits anisotropic superhydrophobicity. Its static contact angle (CA) is more than 150°, and the rolling angles (RAs) are different along RO and inverse RO direction. Through numerical simulation, it has found that the droplet velocity along RO direction is obviously faster than that against RO direction under the same external force. That is to say, the droplets moving along the two directions need to overcome different forces. In the experiment, when the superhydrophobic surface is tilted at the same angle, the moving speed of droplets is different along different directions. It is proved theoretically and experimentally that the adhesion of solid-liquid interface is different along RO and inverse RO direction. Then, we test the friction force between water flow and the anisotropic superhydrophobic surface. The results exhibit that when the water flows along different directions, the anisotropic superhydrophobic surface produces different drag forces and has different drag reduction effects. Finally, the durability of the superhydrophobic surface is test by water jet, and the results show that the as-prepared surface has good durability.

EXPERIMENTAL SECTION Materials. The substrate material is 304 stainless steel and the thickness is 2mm. The chemical reagents are fluoroalkylsilane (C13H13F17O3Si), oxalic acid and ethanol. These reagents are utilized without further purification and the deionized water is


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used throughout the experiment. Preparation procedures. The substrate (304 stainless steel) is ultrasonic cleaning with ethanol and deionized water. The quadrate scales with oblique grooves are prepared on the substrate using an ultraviolet laser. The laser is in the form of a pulse and the laser wavelength is 355nm. The textured procedure is schematically illustrated in Figure 1a-d. First, the laser beam is perpendicular to the horizontally placed substrate to etch transverse grooves with a spacing of 100μm. Second, the substrate is rotated horizontally by 90° and then one side is raised so that the trajectory of the laser beam is perpendicular to the transverse grooves and the substrate has a certain angle with the laser beam (the angle is less than 90°, such as 60°, 45° and 30°). In this method, the laser beam can prepare quadrate scales and oblique grooves on the substrate. During the whole process, the laser spot diameter is 12μm and the average power is 15W. Then, the saturated oxalic acid is prepared with deionized water. The textured substrate is ultrasonic cleaned in saturated oxalic acid for 10min to remove the floating steel powder on the surface. Finally, the as-prepared sample is immersed in fluoroalkylsilane solution (The fluoroalkylsilane solution is prepared by adding 0.2g C13H13F17O3Si into 40ml ethanol and stirring for 30min.) for one hour and heated at 120℃. The functionalization reagent is used to reduce the surface energy and achieve superhydrophobicity. Surface characterization and tests. A contact angle meter system (JC2000D2A, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) is used to measure contact angle and rolling angle of water droplet at room temperature. A field-emission scanning electron microscope (FE-SEM, TESCAN VEGA) is used to observe the surface morphology. A camera is used to monitor the motion of water droplet on the as-prepared surface along different directions. A self-designed device is used to measure the friction resistance at the solid-liquid interface and test the durability of the superhydrophobic surface.

RESULTS AND DISCUSSION Butterfly wing is a typical anisotropic wetting surface. When the wings swing, the droplets only roll along the radial outward direction. It is mainly due to the large number of quadrate scales covered on the wing. Along the radial outward direction, the quadrate scales are overlap each other and form a periodic hierarchy.33 Inspired by this overlapping structure, we use laser beam to texture periodically arranged quadrate scales with oblique grooves. As shown in Figure 1a and Figure 1b, all the transverse grooves are prepared by laser, and the laser beam is perpendicular to the substrate. As shown in Figure 1c and 1d, in the vertical direction, the laser beam etches the substrate at different incident angles to form oblique grooves. When the angle between the laser beam and substrate is 60°, the SEM images of substrate surface is shown in Figure 1e-h. The overall surface is a periodic arrangement of quadrate scales and oblique grooves. The width of scale is about 80μm and the width of oblique groove is about 200μm. When the magnification is enlarged to 10k, the surface is uneven and adhered by 0.5-2μm particulate melt protrusions as shown in the top right corner of Figure 1f.34 The groove has a slight inclination angle and the depth is about


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50μm. After modified by fluoride, the surface exhibits superhydrophobicity and the CA (5μL water droplet) parallel to inclined grooves is 156°, as shown in Figure 1e. When the angle between the laser beam and substrate is 30°, the SEM images of substrate surface are shown in Figure 1m-p. The grooves become more inclines and the CA parallel to inclined grooves increases to 160°. As shown in Figure 1, with the angle between the incident laser beam and the substrate decrease, the groove becomes inclined and the CA of water increases. Figure 2 shows the RAs of 5μL water droplets on the as-prepared superhydrophobic surfaces. It is found that the RA of water droplet inverse the RO direction on each sample is larger than that the RA in the RO direction. As the angle between the laser beam and the substrate decreases, the difference of the RAs along RO and inverse RO direction increases. When the angle between the laser beam and the substrate is 30°, the RA is 12.5° inverse the RO direction and 6.5° in the RO direction. Therefore, it is considered that the adhesion of the superhydrophobic surface with oblique grooves is anisotropic, and the adhesion inverse the RO direction is larger than that in the RO direction. In the next experiments, the measured samples are prepared when the angle between the laser beam and the substrate is 30°. Then, the motion of water droplets along RO and inverse RO direction is studied when the superhydrophobic surface is tilted at the same angle. Under the action of gravity component, the droplet gets an acceleration at the moment leaving the syringe and begins to move on the superhydrophobic surface. In the experiment, the surface is tilted at 5°, and the volume of water droplets are 14μL. Figure 3 shows the motion of droplets on the anisotropic surface at different time. The distance between the two marking lines on the surface is 15mm. Figure 3a-d show the droplet moving inverse the RO direction, illustrating that the droplet moving 15mm requires 325ms. Under the same conditions, it takes only 180ms for the droplet to move 15mm along the RO direction, as shown in Figure 3e-h. The more details of droplets moving in the two directions are given in video S1. In this video, the motion of droplets rolling on the anisotropic surface is 8 times slower to make it easy to observe the difference between the droplets rolling along the OR and inverse the RO direction. The experimental phenomenon shows that the friction force of the droplet rolling against the RO direction is larger than that along the RO direction. It is demonstrated that the adhesion of the as-prepared surface is anisotropic. It is simulated that the motion of water droplet in different directions under the action of external forces. The radius of the water droplet is 1.5mm, and the magnitude of the external force F is 2000 volume force (The volume force is defined as the density multiplied by the acceleration). The simulation is in 2D, and Figure 4a and 4b show the local magnification of the simulation structures. The red area represents the water droplet, the blue area represents the air, and the white area represents the substrate. The change of color scale on the right represents the change of the volume fraction of water. The rough structure of the substrate is composed of inclined grooves with a spacing of 280μm. The simulation uses a reinitialized level set method to represent the fluid interface between the air and the water. The edge of the water droplet is used as the initial interface. The initial state of the simulation is that the


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water droplet is not contact with the substrate, and the spherical state is located at 35μm above the substrate. The edge of the rough structure of the substrate is used as the wetting wall. The contact angle of the wetting wall is 160°. The gravity factor is considered in the whole simulation. The transient results of water droplet moving in 100ms are calculated by the level set method and the calculated step size is 0.1ms. Under the equal force, the velocity of the droplet has a big difference. As shown in Figure 4d, when the external force F is inversed the RO direction, the droplet moves about 1.3mm to the left at 50ms. Correspondingly, when the external force F is along the RO direction, the droplet moves to the right by 3mm at 50ms, as shown in Figure 4h. Under the same external force, the velocity of the droplet along RO direction is obviously higher than that inverse the RO direction, indicating that the friction force at the solid-liquid interface is different in the two directions. As shown in Figure 4e and Figure 4i, when the simulation time reaches to 80ms, the droplet moves about 2.8mm to the left and about 5.7mm to the right, respectively. When the simulation time reaches to 100ms, the droplet moves about 3.6mm to the left and 7.4mm to the right. More results of simulation at different time are presented in video S2. There is a transient simulation result per millisecond, which is played at a rate of 10 frames per second. The simulation results show that the friction force at the solid-liquid interface along RO direction is less than that inverse the RO direction. When the substrate is tilted at 30°, the motion state of water droplet along the anisotropic surface is simulated. As shown in Figure S1, the water droplet moves slower against the RO direction than that along the RO direction. It is theoretically proved that the adhesion of the as-prepared surface is anisotropic. Next, the friction force between solid and liquid interface under continuous water flow is studied. Figure 5a shows the self-designed device that can measure micro friction force.35 It mainly consists of strain gauges, cantilever beam, constant current source, data collection system and water circulation system. The water circulation system provides different velocities of water flow, and friction force occurs when water flow jets the surface of the sample fixed on cantilever beam. Under the friction force, the cantilever beam produces a slight deformation and causes the strain gauges to produce electrical signals. Then the data collection system processes electrical signals and outputs them in the form of friction force. Figure 5b and Figure 5c show the magnification of water flowing through the as-prepared surface along RO and inverse RO direction, respectively. When the water flow is against the RO direction, it is easy for the water flow to shear the air layer at the interface and the trapped air in the oblique grooves is replaced by water. Conversely, as the water flow is along RO direction, it is difficult for the water to enter the grooves because of the existence of the scales and the inclined grooves. Then the friction force of the anisotropic and isotropic superhydrophobic surface is tested by the self-designed device. The isotropic superhydrophobic surface is shown in Figure S2. In the whole experiment, the flow is in a laminar state. The uniform size of all samples is 2cm×2.5cm. Figure 5d shows the friction force of untreated stainless steel surface, the isotropic surface, and the anisotropic surface. When the water flow velocity ranges from 1m/s to 4.5m/s, the solid-liquid interface friction force on the


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superhydrophobic surface is always smaller than that on the stainless steel surface. The friction forces on the stainless steel surface and the superhydrophobic surface are defined as

f st

and

f su , respectively. The drag reduction ratio of the

superhydrophobic surface is D

f st  f su f st

(1)

As shown in Figure 5e, when the water flows against the RO direction and the velocity is less than 3.5m/s, the drag reduction of the superhydrophobic surface is about 35%±3%. As the velocity increases to more than 3.5m/s, the drag reduction effect begins to decline. When the velocity is 4.48m/s, the drag reduction ratio decreases to 18%. It is due to that the drag reduction effect mainly depends on the existence of air layer trapped by the rough substrate.36-38 The air layer can reduce the contact area of solid-liquid interface and lead to slip velocity of water flow on the superhydrophobic surface. But, the high speed water flow can destroy the air layer trapped by the rough substrate. When part of the air trapped in oblique grooves is replaced by water, the drag reduction will be obviously reduced. For the isotropic superhydrophobic surface, the drag reduction ratio also decreases with the increase of the velocity. When the velocity is less than 3.5m/s, the drag reduction of the isotropic superhydrophobic surface is about 40%. When the velocity is 4.48m/s, the drag reduction ratio decreases to 25%. However, there is no tendency to decrease when the water flows along the RO direction. During the whole measurement, the drag reduction ratio is kept at approximate 45%. This indicates that the scales and inclined grooves play an important role in protecting the trapped air layer when water jets along the RO direction. In short, the superhydrophobic surface has obvious drag reduction and the drag reduction effect along the RO direction is better than that inverse the RO direction. Finally, the durability of the superhydrophobic surface is tested. At present, the preparation of durable superhydrophobic surfaces is a major challenge. The main reasons for the instability of the superhydrophobic surfaces are as follows: the poor bonding force between the micro/nano structures and the substrate, and the weak bonding between the low surface energy materials and the substrate. Micro/nano structures and low surface energy materials on the substrate are easily destroyed under the presence of external forces. In this report, the rough structures of the as-prepared samples are obtained by laser etching. After ultrasonic cleaning, the floating powders on the surface are removed, so the remaining particles on the surface are firmly bonded to the substrate. In the durability test, the superhydrophobic surface is jetted by water flow. The velocity of water flow is about 2m/s, and the time of each water jet is 30min. Figure 6a shows the contact angles (parallel to the direction of inclined grooves) of the as-prepared surface after being jetted by water flow for different times. After each jetting by water flow, the CA of the sample is measured. Then, the sample is heated at 120℃ for10min, and the CA is measured again. The results show that as the number of water jets increases, the CA of the sample decreases. When the sample


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is jetted for five times, the CA reduces to 148°. After heating, the CA of the sample is recovered to more than 150°. The morphology of the superhydrophobic sample has not changed since the water flow was jetted on the surface, as shown in Figure 6b. It shows that the rough structures obtained by laser etch is firmly combined with the substrate. When water flow jets the sample, the fluoride molecular layer is locally destroyed, resulting in a decrease of the CA. When the sample is heated, the fluoride molecules in the rough structures automatically repair the destroyed molecular layer through thermal diffusion, and restore its original superhydrophobic properties. Hence, the as-prepared superhydrophobic surface has a good durability.

CONCLUSIONS The superhydrophobic steel surface is prepared by laser etching and chemical modification. When the angle between the laser beam and substrate is 30°, the rolling angle has 6° difference along RO and inverse RO direction. Under the same external force, the transient simulation results exhibit that the velocity of water droplet moving along the two directions is different. At the same tilted angle, the velocity of the droplets on the superhydrophobic surface moving along RO direction is faster than that inverse RO direction. It is verified by numerical simulation and experiment that the adhesion of the surface is anisotropic. Moreover, we have tested the friction force of solid-liquid interface and found that the superhydrophobic surface has drag reduction effect. With the increase of velocity, the drag reduction effect decreases against the RO direction, while it remains a high ratio along the RO direction. When the water flow velocity is 4.48m/s, the drag reduction ratio against the RO direction is only 18%, but it reaches to 47% along the RO direction. In addition, the water jetting test shows that the superhydrophobic surface has a good durability. We hope that the anisotropic superhydrophobic surface will be useful for practical applications.

ASSOCIATED CONTENT Supporting Information The motion of water droplet when the substrate is tilted at 5° (Video S1); The transient simulation results of water droplet motion under 2000 volume force (Video S2); Transient simulation results of droplet motion when the substrate is tilted at 30° (Figure S1); The contact angles parallel or vertical to the inclined grooves (Figure S2); The morphology and contact angle of the isotropic surface (Figure S3). AUTHOR INFORMATION Corresponding Author E-mail:[email protected] ORCID Haifeng Zhang: 0000-0002-4917-746x Notes


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work is supported by National Science Foundation of China (No. 61474034), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology ) (No .2016TS 06).

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Figure Caption

Figure 1. Preparation of anisotropic superhydrophobic surface. (a-d) Preparation of anisotropic superhydrophobic surface by laser, where (a) and (b) are the preparation of grooves in the lateral direction, and (c) and (d) are the preparation of the oblique grooves in the vertical direction. (e-f) The angle between laser beam and substrate is 60°. (e) The surface morphology with 50 magnification and the inset shows that the CA of water droplet is 156°. (f) The surface morphology with 200 magnification and the inset is 10k magnification. (g) The cross-section with 200 magnification. (h) The cross-section with 1k magnification. (i-l) The angle between laser beam and substrate is 45°. (i) The surface morphology with 50 magnification and the inset shows that the CA of water droplet is 158°. (j) The surface morphology with 200 magnification and the inset is 10k magnification. (k) The cross-section with 200 magnification. (l) The cross-section with 1k magnification. (m-p) The angle between laser beam and substrate is 30°. (m) The surface morphology with 50 magnification and the inset shows that the CA of water droplet is 160°. (n) The surface morphology with 200 magnification and the inset is 10k magnification. (o) The cross-section with 200 magnification. (p) The cross-section with 1k magnification.


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Figure 2. The rolling angles. (a) Schematic diagram of water droplet rolling inverse the RO direction. (b), (c) and (d) The rolling angles inverse the RO direction, when the angle between the laser beam and the substrate is 60°, 45° and 30°, respectively. (e) Schematic diagram of water droplet rolling along the RO direction. (f), (g) and (h) The rolling angles in the RO direction, when the angle between the laser beam and the substrate is 60°, 45° and 30°, respectively.

Figure 3. The motion of 14μL water droplets on the superhydrophobic surface, when the angle between the laser beam and substrate is 30°. (a-d) The position of the water droplet rolling inverse the RO direction at different time. (e-h) The position of the water droplet rolling along RO direction at different time.


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Figure 4. Transient simulation results of droplet motion under the equal external forces. (a, b) The

local magnification of the simulation structures. The substrate surface is periodically arranged with scales and oblique grooves with 280μm periodic length. The color scale on the right represents the change of the volume fraction of water. (c-f) The external force F is against the RO direction. (g-j) The external force F is along the RO direction.


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Figure 5. Friction force test of solid-liquid interface under continuous water flow. (a) Schematic diagram of the friction force test device. (b) The magnification diagram of water flowing through the anisotropic surface against the RO direction. (c) The magnification diagram of water flowing through the anisotropic surface along the RO direction. (d) The friction forces of stainless surface, isotropic surface, and anisotropic surface at different velocities. (e) The drag reduction ratio along the RO and inverse the RO direction.

Figure 6. The durability of the superhydrophobic surface. (a) The contact angles of the sample after being jetted by water flow for different times, where A represents the CA of the sample after being jetted by water flow, and B represents the CA of the sample after being heated in the case of A. (b) The surface morphology and CA of the superhydrophobic sample after being jetted by water flow for 5 times. The SEM image is 200 magnification and the inset is 10k magnification.


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Drag Reduction of Anisotropic Superhydrophobic Surface Prepared by

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