Letter www.acsami.org
Flexible Slippery Surface to Manipulate Droplet Coalescence and Sliding, and Its Practicability in Wind-Resistant Water Collection Yuanfeng Wang, Baitai Qian, Chuilin Lai, Xiaowen Wang, Kaikai Ma, Yujuan Guo, Xingli Zhu, Bin Fei, and John H. Xin* Nanotechnology Centre, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong SAR 999077, China S Supporting Information *
ABSTRACT: A flexible slippery membrane (FSM) with tunable morphology and high elastic deformability has been developed by infusing perfluoropolyether (PFPE) into a fluorinated-copolymer-modified thermoplastic polyurethane (TPU) nanofiberous membrane. To immobilize PFPE in TPU matrix, we synthesized a fluorinated-copolymer poly(DFMA-co-IBOA-co-LMA) with low surface energy, high chemical affinity to PFPE, adequate flexibility, and strong physical adhesion on TPU. Upon external tensile stress, the as-prepared FSM can realize a real-time manipulation of water sliding and coalescence on it. Furthermore, it exhibits the ability to preserve the captured water from being blown away by strong wind, which ensures the water collection efficiency in windy regions.
KEYWORDS: nanofibers, flexible, slippery surface, droplet coalescence, wind-resistant, water collection
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captured water because it is incompatible to simultaneously improve the water drainage/sliding and increase the water adhesion to resist wind force, especially for rigid solid surfaces on which drastic wind impact occurs. Herein we developed a flexible slippery surface by infusing lubricant perfluoropolyether (PFPE) into an electrospinning thermoplastic polyurethane (TPU) composite nanofiber membrane (NFM) (Figure 1a). To build the chemical affinity between TPU and PFPE, we introduced a layer of fluorinated polymeric coating (Figure 1c) on the surface of TPU nanofibers (Figure S1). By adjusting monomer proportion (1:2:1, weight ratio) among dodecafluoroheptyl methacrylate (DFMA), lauryl methacrylate (LMA) and isobornyl acrylate (IBOA), we synthesized a random copolymer poly(DFMA-co-IBOA-coLMA) (PDIL) with low surface energy, high chemical affinity to PFPE (with strong C−F stretching vibration appeared at 1100−1300 cm−1 in the FTIR spectrum of PDIL, Figure S2), adequate flexibility and strong physical adhesion, which is one of the critical innovations in this work. To be specific, the long carbon chains contained in LMA and their interaction21,22 can lower the surface energy and improve the molecular arrangement for a better exposure of the fluorine groups in DFMA, and the rigid side group of IBOA ensures the film-forming property and the strong physical adhesion.23,24 The stable adhesive property of PDIL on solid surfaces (Figures S3 and S4 show a good adhesive property on both metal surface and TPU)
he manipulation of droplet behaviors on solid surfaces is fundamental to many applications, such as self-cleaning,1,2 oily water treatment,3,4 water collection,5,6 desalination,7 microfluidics transportation,8 and anti-icing.9 For water collection, the mainstream design concepts were inspired by nature creatures living in arid regions and capturing fogwater for surviving. Pariticularly, Stenocara beetle’s back,10 cribellate spider’s silk,11 and Cactaceae species’ leave12 are the three most popular models that have been widely studied and imitated.5,6,13−15 Through sophisticated design of shape and wettability gradients, these bioinspired materials can accelerate the water condensation or the sliding/drainage of the captured water droplets.6,13−15 Recently, Aizenberg and co-workers designed a lubricant-infused surface with asymmetric bumps and reported its highly improved water condensation and drainage performance compared with conventional superhydrophobicitybased surfaces.16 In their work, the concept of the designed asymmetric bumps is derived from Namib desert beetles and cacti, and the slippery coating is inspired by Nepenthes pitcher plants17 with lubricated peristome on which insects aquaplane and are captured. Based on such a slippery surface with negligible friction, a new concept of manipulating the droplet motion has been pointed out, i.e., creating a “liquid” layer between solid surface and target liquids, which has been adopted in fabricating a wide range of omniphobic surfaces.18−20 Integrating such slippery surfaces, the efficiency of water collection can be highly improved due to the enhanced water condensation and shedding. However, water captured on such surfaces is easy to be blown away by nature wind. Few works have considered to eliminate the wind-caused loss of the © 2017 American Chemical Society
Received: May 14, 2017 Accepted: July 12, 2017 Published: July 12, 2017 24428
DOI: 10.1021/acsami.7b06775 ACS Appl. Mater. Interfaces 2017, 9, 24428−24432
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) Illustration of the fabrication process of the PFPE-PDIL-TPU composite membrane; (b−e) FE-SEM image showing the top view of the prepared membrane after each fabrication step; movement observation of water and n-hexane drops on (f) the pure TPU NM and (g) the PFPE-PDIL-TPU membrane. The scale bars in f and g are 5 mm.
into the pores.27 As a consequence, the free sliding of water droplet on the surface will be decelerated and even halted after a sufficient tensile strain is applied. Figure 2e (and Movie S1) demonstrates the sliding-control of a water droplet (∼30 μL) on the tilted FSM. Specifically, the water droplet slides freely on the undeformed FSM, but slows down and nearly stops when a tensile stress is applied (with tensile strain of ∼50%). Meanwhile, the droplet shape changes apparently from a regular hemisphere to an irregular one (Figure 2f) with advancing angle much larger than receding angle, indicating an increased sliding resistance encountered. After the stress is removed, the high-speed sliding behavior starts again and the droplet shape recovers, indicating the recovery to the smooth topography of the FSM (Figure 2d). The same tensile stress shows higher controllability to smaller droplet (Figure S10b shows a similar result that larger tensile strain is required to pin the droplet with bigger size). Movie S2 demonstrates a sliding control process toward a smaller droplet (∼20 μL) on the FSM, wherein the droplet is stopped totally after the FMS is stretched and slides again after the FSM is released. In agreement with this performance, the recorded CAH and the sliding angle (SA) change along with different tensile stain (0−100%) shows that the difficulty of water sliding increases with enhanced deformation (Figure 2g, h). A more detailed topography change along with varied tensile strain is shown in Figures S11 and S12. Namely, the roughness and the surface pore ratio increases gradually with the increasing tensile strain, which is responsible for the gradually raised water sliding resistance. In addition to the tunable morphology, the prepared FSM can afford a high tensile strain (the elongation at break is ∼220%, Figure S13), by which a new function can be developed on the prepared FSM: enhanced droplet coalescence triggered by the relaxation of the imposed strain.
broadens the selection of matrixes with varied mechanical properties to immobolize PFPE. The as-spun TPU NFM was dip-coated with the PDIL solution and then attached to a spin-coated TPU base without porous texture before drying (Figure S5). The coated PDIL sublayer ensures a stable immobilization (Figure S6) and even distribution (Figure S7) of the lubricant PFPE. The coating amount of the PDIL should be well controlled, as the insufficient coating is incapable of lowering the surface energy (Figure s8b) and the excessive coating would block the porous texture for lubricant wicking as well as weaken the elastic recovery of the membrane (Figures S8a and S9c−e). The PDIL solution with concentration of 3 wt % was used because the as-coated TPU NM exhibited the best water repellency and an undiminished elastic recovery. Following the over infusion of PFPE, the membrane possesses a slippery surface on which water and n-hexane drops can slide easily (Figure 1g), whereas a pure TPU NM would pin the water drop and absorb the n-hexane (Figure 1f). The smooth liquid surface (Figure 1e) formed by the over infused PFPE (2.8 μL cm−2) with low surface energy is the key to the pin-free sliding of the testing water (with contact angle hysteresis, CAH < 5°) and even oil.25,26 As a comparison, a rough and porous surface forms (Figure 1d and Figure S8a vii−ix) on the membrane with insufficient lubricant infusion (1.6 and 2.2 μL cm−2), causing a higher contact angle hysteresis (CAH > 10°) (Figure 2f). The smooth and flawless topography on the PFPE-PDIL-TPU flexible slippery membrane (FSM) is changeable due to the easy deformation of the TPU-PDIL substrate and the fluidity of the lubricant. As shown by Figure 2a−d, the flat surface turns into rough and porous upon an external tensile stress. The reason is that the stretch-triggered deformation reduces the pressure in the porous matrix and drives the lubricant to retreat 24429
DOI: 10.1021/acsami.7b06775 ACS Appl. Mater. Interfaces 2017, 9, 24428−24432
Letter
ACS Applied Materials & Interfaces
Figure 2. Stain/release triggered topography change and water sliding control on the as-prepared PFPE-PDIL-TPU FSM. (a) The topography transition between smooth and rough along with the relaxing and straining (100%) of the FSM, respectively, shown by b−d) FE-SEM images; (e) sliding control of a water droplet (∼30 μL) on the surface by straining and relaxing (with maximum tensile strain of ∼50%); (f) magnified images of the droplet shapes corresponding to the different sliding states in e; the scale bars in e and f are 5 and 1 mm, respectively; (g, h) CAH and SA change along with increasing tensile strain (the inset in f shows the measurement method of the advancing/receding CAs).
Figure 3. Demonstration of the enhanced coalescence of pinned water drops along with the relaxation of the FSM from (a, b) in-plane stretching and (c, d) out-plane stretching (the right column of d illustrates the coalescence of two adjacent droplets around the dragging point).
windy environment (an experimental setup simulating water collection in wind is shown in Figure S14). The wind in nature has pros and cons for water collecting. First, the accelerated fog droplets in light wind are more readily to be captured by a solid surface because of the increased impact force. Consistent with this point, our investigation also indicates the distinct improvement of the water collection efficiency on both the flexible and rigid slippery membrane (RSM)with increased wind speed from 0 to 2 m s−1 (Figure 4e). Therefore, even without wettability or shape gradients on the surface, water capturing can be highly enhanced by duly strengthening the contact and collision between the fog and collector. However, as the wind power increases, more captured water droplets are blown away because of the easy-sliding property of the slippery surface, especially the rigid one (gray columns in Figure 4e), on which water collection efficiency decreases sharply with the increasing wind power. This indicates that the smooth and slippery surface can hardly prevent the water splashing (Figure 4d). From Figure 4b (and Movie S5), water splashing can be observed along varied directions off the RSM and thus the captured droplets can hardly coalesce together and shed to the collecting container. Also in the rightmost image of Figure 4b, splashed water can be observed everywhere on the background wall near the RSM indicating a severe loss of the captured water. Compared to that, the FSM preserves the captured water significantly: as wind power increases, the collected water amount decreases much more moderately (Figure 4e, red columns). Meanwhile, an interesting water collection process can be observed on the FSM (Figure 4a and Movie S6): at early stage, the water
Upon relaxation of the FSM, some droplets may start to slide as the surface topography converts from rough to smooth. Simultaneously, those smaller droplets that are unable to slide can coalesce together owing to the significant distance decrease or even the position overlap among adjacent droplets. The sizes of those coalesced droplets are more likely to exceed the sliding threshold value and start to shed. The enhanced water coalescence and sliding along with releasing the FSM has been investigated in two situations. As shown in Figure 3a (and Movie S3), several adjacent water droplets were first added onto the top surface of the bidirectionally stretched FSM. During the decrease in the tensile stress, the membrane reverted gradually with adhered water droplets coalescing together along the reverting direction until they start sliding. Also, during the out-plane relaxation of the deformed circular FSM, some adhered fog droplets coalesced along the relaxing direction with a drastic size increase and shed (Figure 3c, d and Movie S4). It is easy to conclude that with a same droplet deposition density, the relaxation of the FSM from a higher degree of deformation means more coalescence among adjacent droplets and thus a more efficient liquid drainage. Comparing to the complicated design of wettability or shape gradients on rigid surfaces, which is commonly used to improve liquid coalescence and drainage, the method introduced here provides another solution with improved controllability and flexibility. The strain-responsive water sliding control and the remarkably high deformation ability make the prepared FSM a potential water collector that is more suitable to be used in a 24430
DOI: 10.1021/acsami.7b06775 ACS Appl. Mater. Interfaces 2017, 9, 24428−24432
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ACS Applied Materials & Interfaces
Figure 4. (a) Investigation of the water collection process on the prepared FSM (b) with the RSM as reference in a strong wind (12 m s−1) (the rightmost images of both showing the droplets amount collected on the background wall). (c, d) Illustration of the water droplets movement on the prepared FSM and the RSM, respectively, in strong wind. (e) Comparison of the amount of collected water within 5 min by the FSM and the RSM in different wind power.
PFPE. The strain/release induced manipulation of water pinning/ sliding, integrated with the high degree of deformation make the prepared FSM a potential material to enhance water/liquid coalescence and collection. Besides, because of the deformationsensitive sliding resistance change and the wind direction adjustment, the FSM exhibits the ability to preserve the captured water from being blown away by strong wind. This concept may provide solutions to develop water-collecting devices that are more suitable to be used in windy regions.
droplets deposited on the membrane are driven to the boundary and fixed; then, with more droplets coalescing at the boundary, larger drops forms and slide down along the boundary until they reach the bottom; finally, the accumulated drops drip down when they reach the size approaching the capillary length of water (≈ 2.7 mm).28,29 During the process, very few droplets splash out of the surface (the rightmost image in Figure 4a shows that almost no splashed water is collected on the background wall), which ensures the efficient collection of water. This can be mainly attributed to the sensitive deformation increment induced by the increasing wind power, which consequently generates increasing sliding resistance and thus effectively reduces the water splashing. In addition, splashing on soft substrates is harder than that on rigid substrates due to energy losses caused by the immediate deformations of soft substrates after impact.30 Moreover, through wind force analysis (Figure S15) at the boundary of the FSM, the composite wind force of the incoming and the backflow wind has the opposite direction to the droplet moving direction and thus impeding the water splashing off the boundary of the FSM. In conclusion, a flexible slippery nanofiber membrane has been fabricated by infusing PFPE into a PDIL-coated TPU nanofiber matrix. The innovative production of the coating agent PDIL with low surface energy, high chemical affinity to PFPE, sufficient flexibility, and strong physical adhesion broadens the selection of porous matrixes that can immobilize
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acsami.7b06775. Detailed experimental section, additional characterizations and performance tests, and captions for the six movies showing water droplet manipulation and water collection (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI) Movie S5 (AVI) Movie S6 (AVI) 24431
DOI: 10.1021/acsami.7b06775 ACS Appl. Mater. Interfaces 2017, 9, 24428−24432
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ACS Applied Materials & Interfaces
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yuanfeng Wang: 0000-0002-1167-804X Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from The Hong Kong Polytechnic University Internal Fund (PolyU G-YK50). Dr. Ngai Yui Chan, Dr. Hardy Liu, and Ms Mow Nin Sun are thanked for the FE-SEM tests and the mechanical tests. Mr. Chongyang Chuah is thanked for the assistance in experiments.
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