Flexible Slippery Surface to Manipulate Droplet Coalescence and Sliding, and Its Practicability in Wind-Resistant Water Collection

Transcription

1 Supporting Information 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 , China. Corresponding Author * S-1

2 Experimental section Materials Thermoplastic polyurethane (TPU, with weight-average molecular weight, M w ~600,000 g mol -1 ) was purchased from Hong Kong Hi-Tech Enterprises Ltd. Dimethylacetamide (DMAc, Duksan Chemical 99.5%), dodecafluoroheptyl methacrylate (DFMA, XEOGIA 96%), lauryl methacrylate (LMA, Aldrich 96%), isobornyl acrylate (IBOA, Aldrich, technical grade), 1-dodecanethiol (DDET, Aldrich 98%), 2,2'-azobis(isobutyronitrile) (AIBN, IL, 99%), propylene glycol butyl ether (PGBE, Aldrich 99%), Perfluoropolyether (PFPE, Fomblin Y, Aldrich) were used as received. Fabrication of electrospun TPU nanofiber membrane (NM) The TPU electrospinning solution was made by dissolving 15 g of TPU chips in 100 ml of DMAC. A commercial electrospinning setup (TL-Pro, purchased from Micro & Technologies Expert, Shenzhen, China) was used to fabricate nanofibers at room temperature (23 C) with the ambient relative humidity around 55%. A positive voltage of 15 kv was applied on the needle and a negative voltage of 2 kv was applied on the collecting aluminum foil (anchored on a rotating drum with rotating speed of 10 rpm during the electrospinning process). The distance between the needle tip and the aluminum foil was 20 cm. The solution flow rate from the spinneret was controlled at 1 ml/h through a syringe pump. The as-spun TPU NM (~46 µm, Figure S2) was placed in an oven at 80 C for 10 min followed by a 12 h air-dry in dark S-2

3 room to remove the residual solvent. Finally, the TPU NM was peeled off the aluminum foil and flattened before the following treatment. Fabrication of poly(dodecafluoroheptyl methacrylate-co-isobornyl acrylate-co-lauryl methlacrylate) (PDIL) PDIL was synthesized by the random copolymerization of DFMA, IBOA and LMA with weight ratio of 1:2:1. First, 10 g of DFMA, 20 g of IBOA and 10 g of LMA were dissolved in 60 g of PGBE. The initiator AIBN (0.4 g) and polymerization regulator DDET (0.1 g) were then added into the solution. The reaction was kept at 80 C for 3 hours without sealing because of the generation of N 2 from the system. The resultant PDIL solution (40 wt %) was stored overnight at room temperature and then diluted into solutions with five concentration gradients: 1 wt %, 3 wt %, 5 wt %, 7 wt %, and 9 wt %. Fabrication of PFPE-PDIL-TPU flexible slippery membrane (FSM) The as-prepared TPU NM was firstly dipped into the PDIL solution for 5 seconds, then it was extracted and the excess solution was removed by tissue paper. Subsequently, the TPU NM was attached flatly onto a piece of TPU thin film (~50 µm, Figure S2) made by spin-coating method (15 wt % of TPU/DMAC solution with spin-coating rate of 1600 rpm for 10 s and 3000 rpm for 6 s in succession). The composite membrane was then dried at 120 C in an oven for 10 min. The liquid-infused FSM was prepared by infusing PFPE onto the as-prepared membrane and the infusing amount was controlled by the volume of liquid versus a known surface area. S-3

4 Characterization The morphology of the as-prepared TPU NM, TPU-PDIL NM, and TPU-PDIL-PFPE FSM were observed using a field emission scanning electron microscope (FE-SEM, JSM-6335F, Japan). To observe the morphologies of the FSM with different tensile strains, the tensions were applied to the FSM before the SEM observation. To be specific, prior to the gold sputtering, the FSM was stretched by certain tensile strain and anchored onto a rigid electroconductive double-faced adhesive tape. To prevent the resilience, we cut the stretched FSM with the length longer than the electroconductive tape and attached the extra part to the bottom of the tape. Then, the FSM-coiled electroconductive tape was attached onto the SEM holder through another piece of pristine electroconductive tape. Finally, several small pieces of electroconductive tapes were used to anchor the edges of the FSM-coiled electroconductive tape to further prevent the resilience of the FSM. Contact angle (CA), sliding angle (SA) and contact angle hysteresis (CAH) measurements were conducted using OCA20 (Data Physics, Germany). The CAH ( θ) was calculated by the difference between advancing CA (θ A ) and receding CA (θ R ) which were measured from the water droplet (4 µl) while the probe was added to and withdrawn from the drop: θ=θ A -θ R. The error bars in Figure 2 f and g were calculated from 5 repeated test runs on the same membrane. The chemical compositions of the samples were compared utilizing Fourier transform infrared (FT-IR, PerkinElmer Spectrum 100, USA). The tensile stress-strain curves and the breaking elongation curve of the samples were recorded using an INSTRON 5566 (USA) universal tensile tester. The S-4

5 water or oil sliding on the tilted (45 ) FSM was investigated (for Figure 1f, g) using deionized water (DI water, colored by methylene blue) and n-hexane (colored by solvent red). Investigation of the adhesion forces between TPU and PDIL, PFPE and PDIL To study the adhesion force between TPU and PDIL, we immerse one edge of a TPU NFM strip into the PDIL solution with concentration of 3 wt % for 5 s, and adhere this edge to the edge of another piece of TPU strip. After drying in 120 C for 10 min, tensile test was conducted on the composite strip using INSTRON Increasing tesile stress was applied on the membrane strip until it was broken. To study the better immobilization ability of TPU-PDIL NFM towards PFPE than the pristine TPU NFM, a repeated spray test was conducted on the TPU-PFPE and the TPU-PDIL-PFPE membranes using an AATCC Spray Tester (as shown in Figure S6a,b). First, the test sample adhered on a glass slide was fastened onto the inclined platform with the face of the membrane exposed to the water spray. Then, 250 ml of distilled water at room temperature was poured into the funnel of the tester. After 15 cycles of the spray test, the sample was dried in an over at 40 C. Finally, a drop of n-hexane was deposited on the membrane to observe if it can slide down or immerse into the membrane. Investigation of controllable water sliding and condensation The strain-release responsive water pinning and sliding was observed on the as-prepared TPU-PDIL-PFPE FSM. Videos (Movie 1 and 2) record the sliding cotrol of water droplets with volumes of ~30 µl and ~20 µl on the inclined FSM (each side S-5

6 was adhered between two glass slides using double faced adhesive tape, two clamps were utilized to further reinforce the adhesion). The tilt angle was controlled at 18 by a tilted metal plate through the whole experiment. The strain-release behavior was conducted by dragging or release the mobile side (right side) along the metal plate surface without any change of the tilt angle and the maximum elongation of the membrane was ~50 % for both water droplets. In addition to the observation of the sliding control of droplets at ~30 µl and ~20 µl with tensile strain of ~50 %, a series of water droplets (with volumes of 5, 10, 15, 20, 25, 30 µl) were also investigated with the tensile strain values required to pin them on the tilted (18 ) FSM. The enhanced water coalescing behavior on the FSM along with its release process from the strained status was investigated with both in-plane and out-plane deformation. The in-plane deformation (strain) was conducted by the same method as above, and the water droplets were placed on the horizontal and strained FSM, then the FSM was put vertically and gradually released. The out-plane deformation was conducted by attaching one terminal of a glass rod onto the midpoint of the back face of the rounded FSM (anchored on a hollow circle) using double faced adhesive tape and dragging the other terminal of the glass rod. Fog droplets generated from an ultrasonic humidifier were directed onto the vertically placed and strained FSM, after sufficient droplets were pinned, the FSM was released gradually. Investigation of water collection in strong wind The as-prepared TPU-PDIL-PFPE FSM was anchored onto a hollow circle with a diameter of 5 cm. An industrial air blower (1200 W, ZHIPU, China) with adjustable S-6

7 wind speed generated was used as the artificial wind source. The distance between the blowing nozzle and the membrane was 20 cm. An ultrasonic humidifier (Midea S20U-A, China) was utilized to generate fog droplets and was placed between the air blowing nozzle and the membrane. A glass petri dish was placed under the membrane to receive the collected water. The experimental setup was demonstrated in Figure S5. As comparison, another piece of TPU-PDIL-PFPE FSM was anchored onto a top surface of a bottle cap with a diameter of 5 cm to form a rigid slippery membrane (RSM). The wind speed was measured by a digital anemometer (TA m/s, TASI, China). For the demonstration presented in Figure 4a-b, the wind speed was set at 12 m/s. To record the data in Figure 4e, each collection duration was set at 5 minutes, and the error bars were calculated from 5 repeated test runs on the same membrane. To observe the water splashing during water collection by both the FSM and the RSM, the membranes were placed near a background wall with a distance of 15 cm to collect part of the splashed water from the membranes. The experimental conditions in the investigations of both the water droplet mobility and water collection are the same with relative humidity of 50 % and temperature of 23. S-7

8 Figure S1. Fabrication procedure of the PFPE-PDIL-TPU FSM. S-8

9 Figure S2. (a) Separate FTIR spectrums of the as-spun TPU NM, as-synthesized PDIL, and the purchased PFPE; (b) composite FTIR spectrums of the TPU NM, PDIL-TPU NM, and the PFPE-PDIL-TPU FSM. Figure S3. The strong adhesion of two weights (450 g) with the assistance of PDIL. S-9

10 Figure S4. Evaluation of the adhesion force between PDIL and TPU. (a) Illustration of the adhesion method of the edges of two TPU NFM strips. (b-f) Adhesion force test by increasing tensile strain until the composite TPU strip broke. (g) The breaking location of the composite TPU strip. The blue dashed box indicates the adhesion area. The scale bars in b and g are 50 mm. As shown in (b-f), this membrane strip can afford ~200 % of tensile strain. Also, it can be observed from (g) that the breaking happened in the middle of one adhered strip instead of the adhesion area of the two strips indicating the strong adhesion force between the TPU and PDIL. S-10

11 Figure S5. FE-SEM images of the top view (left) and the section view (right) of the TPU-PDIL NM attached on a TPU thin film before PFPE infusion. S-11

12 Figure S6. Spray test conducted on the TPU-PFPE and the TPU-PDIL-PFPE membranes using an AATCC Spray Tester. (a) AATCC Spray Tester and a cylinder to measure 250 ml distilled water. (b) Demonstration of an on-going spray test process. (c) n-hexane repellent performance on the TPU-PFPE membrane after 15 cycles of spray test. (d) n-hexane repellent performance on the TPU-PDIL-PFPE membrane after 15 cycles of spray test. The scale bars in c-d are 50 mm. It is shown in (c-d) that even though the TPU-PFPE after spray test showed a better performance in repelling n-hexane than the prestine TPU which can be totally wetted by n-hexane shown in Figure 1f, spreading and penetration of n-hexane into the membrane can still be observed. This indicates a significant PFPE lost from the TPU NFM after 15 cycles of spray test. By contrast, in the presence of PDIL between TPU and PFPE, the S-12

13 FSM can exhibit an excellent n-hexane repellency even after 15 cycles of spray test. This result indicates a highly improved immobilization ability towards the PFPE after the TPU was pre-treated by a layer of PDIL. Figure S7. SEM images showing the cross-section images of TPU (a-b), TPU-PDIL (c-d), and TPU-PDIL-PFPE (e-f) membranes. S-13

14 Figure S8. (a) SEM images of TPU NM (i), TPU NMs coated with PDIL solutions of varied concentrations (ii-vi are 1, 3, 5, 7, and 9 wt %, respectively), PFPE-PDIL-TPU FSMs with varied PFPE infusion amount (vii-ix are 1.6, 2.2, and 2.8 µl/cm 2 ); (b) and (c) are their corresponding water contact angle (WCA) and contact angle hysteresis (CAH), respectively. The shaded region in (b) refers to the TPU NM coated with 3 wt % PDIL solution without infusion of PFPE, and the shaded region in (c) refers to TPU NM coated with 3 wt % PDIL solutions and infused with 2.8 µl/cm 2 PFPE. The error bars were calculated from 5 repeated test runs on the same membrane. S-14

15 Figure S9. Stress-strain diagrams (including four strain-release cycles with a maximum tensile strain of 100 %) of the TPU NMs coated with PDIL solutions with different concentrations (a-e are 1, 3, 5, 7, 9 wt %, respectively). The red dotted line S-15

16 shows the status that the tensile stress of the membrane returns to 0, and the blue dotted line indicates the elastic recovery value. Figure S10. (a) WCAs of water droplets with different volumes on the FSM without tensile strain. (b) Tensile strains needed to pin the droplets with different volumes on the tilted (18 ) FSM. (c) Shapes of the droplets with different volumes at pinned state. The scale bar in c is 1 mm. As shown in the below (b), it needs a higher tensile strain to pin the droplet with bigger size. This can be attributed to the gravity effect that droplets with higher volumes are easier to slide down the surface due to the larger gravitational force. Droplet shapes with different volumes in (c) shows that bigger S-16

17 droplets have larger contact angle hysteresis and more apparent shape deformation. This can also be explained by the stronger sliding tendency of the bigger droplets. Figure S11. FE-SEM images showing the topographies of the as-prepared FSM with different tensile strain (0, 30, 60, 100 %) extracted from an integral strain-release cycle. S-17

18 Figure S12. Cross-section images of the FSM with different tensile strain. The scale bars are 10 µm. It can be observed that before the tensile stress was applied, the PFPE can also evenly cover the cross-section of the FSM forming a smooth surface. With increasing tensile stress was applied, the cross-section surface became rough with more nanofibers intruding out of the PFPE layer indicating the retreat of the PFPE into the expanded pores. When the tensile strain reached to 100 %, the morphology of the cross-section was mainly composed of pores the nanofibers showed high orientation along the strain direction. The morphology change of the cross-section along with increasing tensile strain was almost consistent to that of the top surface, which further helps to understand the droplet pinning upon external tensile stress. S-18

19 Figure S13. Stress-strain diagram showing the elongation at break of the prepared PFPE-PDIL-TPU FSM. Figure S14. Experimental setup for water collection test. S-19

20 Figure S15. The driven force analysis and the droplet movement at the boundary of the FSM and the RSM (all the images are presented in side view). (a-b) The analysis of the interaction between the incoming wind force and the backflow wind force, and whether the composite wind force is enhancing the splashing of water droplets off the surface at the boundary of the FSM and the RSM, respectively. (c-d) Illustration of the water droplets movement at the boundary of the FSM and the RSM, respectively. S-20

21 The wind directed to the membrane surface will change direction and form backflow wind which is the driven force leading the water droplets to move to the boundary and even splash out. At the boundary of the RSM, the direction of the backflow wind is vertical to that of the incoming wind, which would generate a force enhancing the splashing of the water droplets off the surface. In contrast, the backflow wind at the boundary of the FSM is in opposite direction with the incoming wind (with vector angle >90 ). Therefore, the generated composite force will hinder the water from splashing out of the surface. Supporting movies Movie 1 demonstrates the control of water droplet (30 µl) sliding by the strain/release of the FSM corresponding to Figure 2e. Movie 2 demonstrates the control of water droplet (20 µl) sliding by the strain/release of the FSM. Movie 3 shows the strain and release triggered pinning and condensation/sliding behavior of the water droplets on the FSM corresponding to Figure 3a. The deformation is in-plane by dragging the two side of a square FSM. Movie 4 shows the out-plane deformation (from strain to release) induced water condensation and sliding behavior on a rounded FSM with fog droplets sprayed on the surface. This video corresponds to Figure 3c. Movie 5 demonstrates the strong wind (12 m/s) caused severe water splashing on the rigid slippery surface corresponding to Figure 4b. The increasing amount of S-21

22 splashed water can be observed on the background wall and nearly no accumulated water drops can be observed at the bottom edge. The movie is presented in a speed three times of the original video. Movie 6 demonstrates the water-collection process on the flexible slippery membrane in a strong wind (12 m/s) corresponding to Figure 4a. During the whole process, nearly no splashed water can be observed on the background wall. The movie is also triple speeded. S-22