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Fabrication and Condensate Microdrop Self-Propelling of Biomimetic Nanostructured Polymer Surfaces without Chemical Modification Han-Xiong Huang, and Yue An ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00200 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Polymer Materials

Fabrication and Condensate Microdrop Self-Propelling of Biomimetic Nanostructured Polymer Surfaces without Chemical Modification Han-Xiong Huang* and Yue An Lab for Micro Molding and Polymer Rheology, Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou 510640, P. R. China ABSTRACT: Condensate microdrop self-propelling (CMDSP) can occur on the cicada wing surfaces due to very low solid–liquid adhesion of the closely packed nanopillars. A promising and industrial strategy is proposed for successively replicating the nanopillars on the cicada wing onto the polypropylene (PP) surfaces. Its process includes nickel replica fabrication and PP replica injection molding. Packed nanopillars with nano-scale interspaces endow the PP replicas with CMDSP functionality. Mechanism for the CMDSP on the PP replica is analyzed. This work should be the first report of achieving the CMDSP function on polymer surfaces without chemical modification, which are favorable for industrial applications, such as anti-icing/frosting and enhanced condensation heat transfer. KEYWORDS: biomimetic surface, condensate microdrop self-propelling, cicada wing,

nanostructure, polymer injection molding

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Condensate microdrop self-propelling (CMDSP) phenomenon was found in mosquito’s eyes,1 cicada’s wings,2,3 butterfly wings,4 water strider’s legs,5 and so on. Inspired by the natural phenomenon, much effort has been devoted to fabricating biomimetic surfaces with CMDSP function. Materials of the as-prepared nanostructures with CMDSP function include metal,6,7 inorganic oxide8–11 and silicon.12 The CMDSP surfaces can be employed for applications in various fields, such as enhanced condensation heat transfer, anti-icing/frosting, microfluidic device, and self-cleaning.13–16 Wang and co-workers9 first reported that 30% higher condensation heat transfer coefficient can be achieved on Cu surfaces with CMDSP blade-like CuO nanostructure compared to dropwise condensation on smooth Cu surfaces. The CMDSP function, unfortunately, can be achieved only after additional low surface-energy chemistry modification to the fabricated surfaces, such as fluorosilane coating.6–12 The chemistry modification is both complicated and time consuming. Moreover, the research on biomimetic CMDSP surfaces is an emerging multidisciplinary frontier in the superwettability field.13 Achieving the CMDSP function on polymer surfaces is more significant. However, this is difficult because of very low thermal conductivity of polymer, which is not beneficial for the occurrence of CMDSP phenomenon.17 This may be why the reports on polymer surfaces with CMDSP function are very sparse.18 Gao and co-workers18 proposed an approach for fabricating polycarbonate (PC) nanocone films with CMDSP function. First, nanonipples were formed on PC films by nanoimprinting and alumina template removal. Only after chemical etching to sharpen the nanonipples into nanocones and sputtering a layer of thin Au film followed by low-surface energy octadecanethiol modification for hydrophobization, CMDSP functionality can be achieved on the PC films. This approach is complicated and unscalable. Industrialization techniques for developing CMDSP surfaces are expected to increase possibility for their industrial applications.


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Injection molding, an industrialization technique, can fabricate nanostructures on polymer surfaces in large quantities.19–21 Herein, a promising strategy is proposed for facilely and successively replicating the natural nanostructure of cicada wing onto polypropylene (PP) surfaces, endowing them with CMDSP functionality without additional chemical modification or surface coating. The overall process of the strategy includes nickel replica fabrication (Figure 1(1–4)) and PP replica molding (Figure 1(5–8)). The details of the fabrication are available in Supporting Information, S1.

Figure 1. Schematics of fabrication process for biomimetic PP replica with nanopillar structure. (1) 3D model of natural cicada wing, (2) conductive layer deposition by electroless plating, (3) nickel deposition on conductive layer surface by electroplating, (4) nickel replica, (5–7) μ-ICM process, and (8) final PP replica. (a) Digital picture of cicada forewing, (b) representative SEM image on wing (tilted view), (c) SEM image of nickel replica, and SEM images of PP replica at



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(d) low and (e) high magnifications (tilted view). (f) Microscopic optical image of condensate microdrop with receding contact angle indicated on PP replica surface during evaporation. Except for the aforementioned CMDSP function,2,3 cicada wings possess other remarkable properties, such as optical trapping, hydrophobicity, and bacteria killing.19,22–24 In this work, the forewings of the cicada that died of natural causes (Figure 1a) were taken. As can be clearly seen from Figure 1b, closely packed nanopillars orderly arrange on the cicada wing. The nanopillars have mean diameters of ~147 nm at the base and ~70 nm at the tip, a mean pitch of ~187 nm, and a mean height of ~358 nm. Subsequently, condensation tests were performed to verify the CMDSP function of the nanostructure on the cicada wing (for details, see Supporting Information, S2). The cicada wing was vertically arranged to mimick its natural orientation, and was cooled below the dew point temperature to induce condensation under ambient conditions. As shown in the inset of Figure 2a, the condensate microdrops remain spherical on the vertical wing surface with a contact angle of ~163°, indicating that the wing surfaces are superhydrophobic to the condensate microdrops. The condensate microdrops on the vertical wing grow, and some of them merge. Figure 2a, which was taken from Video S1 (Supporting Information), illustrates that the merged condensate microdrops can self-jump from the vertical wing surface and then fall along an approximate parabolic trajectory. The jumping speeds can reach as high as 0.29 m/s. The selfpropelled jumping behavior can occur for the coalescence of the microdrops with the radii ranging from a few micrometers to hundreds of micrometers on the horizontally arranged wing surface (Figure 2b). Such CMDSP function may be ascribed to the very low solid–liquid adhesion (Cassie state) on the nanostructured wing surfaces,19,23,24 where excess surface energy released from the coalescence of microdrops can sufficiently drive the self-jumping of the merged microdrops.25


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Figure 2. (a) Overlapped optical side-view for trajectory of merged microdrop self-jumping from vertical cicada wing (inset shows microscopic optical image of spherical condensate microdrop with contact angle indicated on wing surface). (b) Typical optical top views of CMDSP instant on horizontal cicada wing. Using the natural cicada wing as original template, the nickel replica was fabricated via combining electroless plating with subsequent electroplating. The cicada wing was pretreated before used as the original template and then was covered with a conductive nickel layer in the electroless plating stage (Figure 1(2)). A nickel layer with ~400 μm thickness was deposited on the conductive nickel layer after electroplating (Figure 1(3)). Finally, the nickel replica (Figure 1(4)) was obtained by removing the wing. For details, see Supporting Information, S1. Figure 1c clearly shows that tapered nanopores orderly distribute in dense rows throughout the nickel replica. That is, the nanopillar on the natural wing is accurately replicated onto the nickel replica with an inverse pattern. The energy dispersive spectroscopy spectrum of the nickel replica (Supporting Information, S3) verifies the existence of the nickel layer in the nickel replica.



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The biomimetic PP replicas were molded by the μ-ICM technique. As a template, the previously prepared nickel replica was mounted on the mold cavity surface (Figure 1(5)). PP (grade CJS700, China Petrochemical Co.) was used as the material for molding the replicas. In the μ-ICM process, the mold cavity was first partially filled with PP melt when the mold was partially closed (Figure 1(6)). Then the mold was compressed in the thickness direction to finish melt filling (Figure 1(7)). Finally, the melt was packed under the compression force and simultaneously cooled down, and the replica was molded (Figure 1(8)). For comparison, PP counterparts (i.e., smooth surface) were molded per the same protocol without the nickel replica as template. For details, see Supporting Information, S1. Figure 3a shows the typical time-lapse microscopic images of the CMDSP process induced by mutual coalescence of the microdrops on the horizontally placed PP replica surface. The condensation tests were performed under a replica surface of ~1℃, an ambient temperature of ~25°C, and a relative humidity of ~79% (for details, see Supporting Information, S2). The recorded coalescence-induced CMDSP processes are illustrated in Videos S2 and S3 (Supporting Information), and the typical images taken from the videos are shown in Figure 3a. As can be seen, the condensate microdrops remain spherical. When adjacent microdrops grow large enough to coalesce, frequent out-of-plane jumping occurs and the merged microdrops can depart from the replica surface without any external forces. Furthermore, longer-term condensation behaviors are compared on the PP replica and counterpart surfaces, and the six images, at 1 min interval over the range of 10–15 min, are shown in Figure 3b. By analyzing the six images, the surface coverage of the residence drops with different diameter ranges are obtained and also shown in Figure 3b. As can be seen, on the former surface the condensate microdrops always maintain sizes less than 50


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μm and can dynamically self-jump, whereas on the latter surface the condensate drops may merge but cannot timely self-jump due to solid–liquid “line-contact”.

Figure 3. (a) Typical time-lapse optical images of coalescence-induced CMDSP on horizontal PP replica (solid and dotted circles highlight areas prior to coalescence and right after self-propelling, respectively). (b) Optical images of condensate drop evolution and surface coverage of residence drops with different diameter ranges on horizontal PP replica and counterpart (at 1 min interval over 10–15 min range).



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Evidently, the CMDSP function on the PP replicas is attributed to the nanostructure on them. The nanopillars are orderly and closely aligned on the PP replica with a mean tip diameter of ~68 nm, a mean base diameter of ~147 nm, a mean pitch of ~179 nm, and a mean height of ~358 nm (Figure 1d and e). Comparing the geometry and sizes of the nanopillars on the cicada wing and PP replica surfaces demonstrates that the nanopillars are successfully demolded from the nickel replica. Such packed nanopillars with nano-scale interspaces and tip diameters exhibit low solid−liquid interface adhesion and so greatly reduce the dissipation of coalescence-released weak surface energy. Combining the phenomena shown in Figure 3a with the researches performed by Boreyko and Chen26 and Chen et al.,27 the underlying mechanism for the CMDSP process on the PP replica surfaces is analyzed, which is schematically depicted in Figure 4. Initially, two microdrops approach each other (Figure 4a). Upon initiation of coalescence, a tiny liquid bridge forms to merge the two separate microdrops into one28 (Figure 4b). The liquid bridge diameter quickly grows to be larger than that of the original microdrops, and impinges upon the replica surface, exerting an upward force (F) to the merged microdrop (Figure 4c). So the merged microdrop self-jumps away from the replica surface in an oscillation mode (Figure 4d).

Figure 4.Schematics of underlying mechanism for coalescence-induced CMDSP process on PP replicas. (a) Approach of two microdrops; (b) formation of tiny liquid bridge; (c) impingement of liquid bridge upon replica surface; (d) self-jumping of merged microdrop. The condensate microdrop wetting state on the PP replica surface is predicted by the following equation29


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E* 

cos  aCB 1  W r cos  a cos  a

(1)

where E* represents the wetting-state energy ratio; θa and r represent the advancing contact angle on the PP counterpart surface (smooth surface) and the roughness factor on the PP replica surface, respectively. The θa was measured to be 107.3±0.9°, and r was calculated to be 4.54 (for details, see Supporting Information, S4). Using Eq. (1), the E* for the PP replica surface was calculated to be ~0.74 (< 1), which means that the condensate microdrops exhibit stable Cassie wetting state on the PP replica. It is noteworthy that some nanopillars on the PP replica are elongated with sharp tips (Figure 1d and e). The actual r value should be larger than the previously calculated value (4.54), which means more obvious Cassie wetting state appearing on the PP replica. Furthermore, the receding contact angle (θrec) of the condensed microdrops on the PP replica surfaces was recorded during their evaporation (Supporting Information, Video S4). During the evaporation of the condensed microdrops, their volumes gradually decrease, and their contact angles remain constant in later stage. The constant contact angle is determined to be the value of the θrec of the condensed microdrop on the PP replica,30 which is averaged to be ~155° (Figure 1f). According to the following equation30 Wad=γlg(1+cosθrec)

(2)

the work of adhesion of the condensed microdrops (Wad) on the PP replica is calculated to be ~7.1 mJ/m2 (at 1℃), which is low and can be overcame by the coalescence-released surface energy. In conclusion, the CMDSP behavior can occur for the coalescence of the microdrops with the radii ranging from a few to hundreds of micrometers on the cicada wing surfaces, which is ascribed to the very low solid–liquid adhesion of the closely packed nanopillars on the wing surfaces. The nanopillars on the wing surfaces are transferred onto the PP replica surfaces via the μ-ICM, thanks to the tapered nanopores in the nickel replica. Orderly and closely aligned nanopillars with nano-



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scale interspaces and tip diameters on the PP replicas endow them with the CMDSP behavior, the underlying mechanism of which is analyzed. To the knowledge of the authors, this is the first report for creating the CMDSP function on polymer surfaces without any low-surface-energy chemistry modification.


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ASSOCIATED CONTENT Supporting Information Descriptions of the fabrication for the biomimetic PP replica (including the nickel replica fabrication and PP replica molding), the condensation tests on the cicada wing and PP replica surfaces, the energy dispersive spectroscopy spectrum of the nickel replica, the advancing contact angle measurement on the PP counterpart surface, and the roughness factor calculation on the PP replica surface are shown in Figures S1−S4 and Tables S1 and S2 (PDF) Video S1 shows the self-jumping of the condensate microdrops from the vertically arranged cicada wing surface (AVI) Videos S2 and S3 show the coalescence-induced CMDSP processes on the horizontally placed PP replica surface (AVI) Video S4 shows the evaporation process of the condensate microdrops on the PP replica surface (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Han-Xiong Huang: 0000-0001-8805-3040 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support provided by the National Natural Science Foundation of China (Grant 51533003), the Natural Science Foundation of Guangdong Province (Grant 2016A030308018),



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and the Science and Technology Project of Guangdong Province (Grant 2017B090911009) is gratefully acknowledged.


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