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Controlling of Water Collection Ability by an Elasticity-Regulated Bioinspired Fiber Sijie Wang, Shile Feng, Yongping Hou,* Yongmei Zheng* A special artificial spider silk is presented which is fabricated by using both an elastic polymer and a fiber, and the water collection behavior is investigated. Through exerting tension in varying degree, the length of the three-phase contact line (TCL) and the area of spindle knot can be regulated readily, which makes a great contribution to the improvement of collecting efficiency and water-hanging ability. The water-hanging ability can be predicted at a given stretching ratio according to the given expression of the TCL. As a result, liquid capture or release of distinct measure can be achieved via exerting tension. This research is helpful to design smart materials for developing applications in fogwater collection, dehumidification, high-efficiency humidity control, and controllable adhesion.

1. Introduction Creatures in nature exhibit almost perfect structures and properties after evolution over a long period of time.[1] Surfaces with special wettability in some animal and plant surfaces attract more and more interests due to extensive applying prospect,[2] including digital labon-a-chip,[3] painting,[4] printing,[5] DNA microarrays,[6] inkjet printing,[7] thin-film lubrication,[8] and so on.[9–20] For example, surfaces with super-hydrophobic properties could enlarge the application in the domain of self-cleaning,[17] icephobicity,[18] antifogging,[19] anticorrosion,[20] etc. Learning from nature generally gives us some important inspiration to develop new methods and approaches to realize structural optimization and functional integration.[9] Recently, our group has found that the Dr. S. Wang, Dr. S. Feng, Dr. Y. Hou, Prof. Y. Zheng Key Laboratory of Bioinspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bioinspired Energy Materials and Devices, School of Chemistry and Environment, Beihang University, Beijing 100191, P.R. China E-mail: [email protected]; [email protected]

water-collecting ability of the capture silk of the cribellate spider is attributed to a unique fiber structure that results in a surface energy gradient cooperation with a difference in Laplace pressure, which opens a new field for the development of functional fibers materials for water collection application.[2d] Based on this work, series of these special spider silk structures are reproduced and the relation between the maximal volume of a hanging drop and the length of the three-phase contact line (TCL) at threshold conditions is revealed.[10] According to the obtained relation, a kind of bioinspired fiber with multi-gradient and multi-scale spindle knots is fabricated to enhance hanging ability of fibers.[10] Lee and co-workers[11] has developed the application of microfluidic technology to fabricate bioinspired spider silk and investigated the water-collecting behavior. All of the results indicate that the collecting efficiency and water-hanging ability are related to the structure of special bioinspired fiber, i.e., Laplace pressure, wettability gradient force, and the length of TCL.[10] However, as for different conditions, the requirement may be distinct. For example, during the water collection process, a larger droplet-hanging ability of fibers is considered to be significant to avoid the loss of drop by wind.[9] Conversely,

Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400695

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a lower hanging ability is advantage for removal of collected water. To our knowledge, dynamic regulation of the collecting efficiency and water-hanging ability of fiber has not been attempted. Here, we use elastic polymer and fiber to fabricate artificial spider silk and investigate the water collection behavior. The results indicate that, compared to previous two-style directional water collection, the special bioinspired fiber could adjust collecting efficiency and water-hanging ability easily by exerting tension, which is resulted from the change of the length of TCL and the area of spindle knot. We could predict the water-hanging ability at a given stretching ratio according to the given expression of TCL. Therefore, the liquid capture or release could be achieved through exerting tension. This research is helpful to design smart materials for developing the application in the fogwater collection, droplet condensation, dehumidification, high-efficiency humidity control, and controllable adhesion, etc.[2c,12]

2. Experimental Section 2.1. Fabrication of Elastic Bioinspired Artificial Spider Silk As described elsewhere,[2d,10] elastic bioinspired fibers with a polydimethylsiloxane (PDMS) spindle knot were prepared by immersing elastic nylon fiber specimen in the PDMS/curing agent/methylbenzene solution (10:1:2, PDMS:curing agent: methylbenzene, mass proportion), then drawing it out horizontally at velocity of 150 mm min−1. A thin-polymer solution film coated on the original nylon fiber broke into a series of polymer drops as a result of Rayleigh instability.[12] Thus, the periodic spindle knots were formed after the polymer drops were solidified at the temperature of 70 °C for 12 h. Finally, the samples at different stretching ratios were made by applying certain

tensile force. Stretching ratio X could be calculated as follow: X = (m/m0)-1 (denoting m as periodicity length of spindle knots after stretching, m0 as original periodicity length of spindle knots).

2.2. Characterization of Microstructure The structures of fibers and spindle knots at different stretching ratios were observed by scanning electron microscope (SEM, Quanta FEG 250, FEI) at 25 kV with gold plating. The behavior of water drop was observed via the optical contact angle meter system (OCA 40, Dataphysics Instruments GmbH, Germany) with time scale.

3. Results and Discussion It is fact that the collecting efficiency and water-hanging ability of bioinspired fibers in a period of time could be regulated by driving forces to tiny droplets and the length of TCL to large droplets. In order to adjust dynamically the collecting efficiency and water-hanging ability, we employed the polymer (PDMS with low Young's modulus and high elastic structural flexibility) and nylon fiber (high elastic) to mimic the structure of spider silk, as shown in Figure 1. Clearly, periodic spindle knots are formed on the fiber surface as designed and the size of spindle knots could be easily tuned via tension. As shown in Figure 1 and S1 (Supporting Information), when the stretching ratio (X) reaches from 0%, 5%, 40%, 100%, 150% to 180%, the height, length, and periodicity of spindle knot change from ≈90, ≈240, ≈500 to ≈75, ≈510, ≈1380 μm, respectively. The half apex-angles change from 29o to 11o. The results indicate that only the periodicity is nearly proportional to stretching ratio, which

Figure 1. Optical microscopy and SEM images of spindle knot on fiber surface with different stretching ratios (X): a) 0%, b) 5%, c) 40%, d) 100%, e) 150%, and f) 180%. Optical images indicate the size of the spindle knot could be easily adjusted via stretching ratio. SEM images show that crack is appeared after the fiber is stretched and the obvious roughness gradient is formed from the center region of spindle knot to the side one.

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is due to the fact that the small length of spindle knot (compared to the periodicity) and the difference of elasticity between PDMS polymer and nylon fiber and the constant volume of spindle knot during the stretching process. The SEM observation shows the microstructure feature of spindle knot. As for original artificial spider silk, the surface is very smooth (Figure 1a). After tensile is applied, uniaxial wrinkles appear on the surface of all samples (Figure 1b–e, Figure S2, Supporting Information).[13] When the X is small (Figure 1b), few longitudinal cracks only appear on the side region of spindle knot. With the increase of the X, more longitudinal cracks generate and less ones occupy the center region (Figure 1c,d). Apparently, a uniaxial tensile force leads to the formation of roughness gradient. According to Wenzel’s law,[14] the roughness induces a more hydrophobic region on the hydrophobic surface (PDMS is hydrophobic). Therefore, the roughness gradient causes the formation of wettability gradient force (pointing to the center of spindle knot), which is useful for improving the collection efficiency. When the X reaches 180%, many microcracks appear on the whole surface and roughness gradient becomes unobvious. For a given tensile force (F), the tensile strength (σ) could be gotten as follow: σ = F/S, where S is the force bearing area. Due to the less crossarea on the side region and the connecting part between spindle knots, the longitudinal cracks initially appear on those areas when the tensile strength is above the critical value[13] and roughness gradient is formed. When the X reaches 180%, the tensile strength is very large so that the surface is totally destroyed. Many microcracks lead to the uniformity of surface.

The droplet collection process on bioinspired fiber was observed via a charge-coupled device (CCD) camera. Similar to previous researches,[10] when the periodicity of the spindle knot is short (the X < 105%), droplets could be rooted on two spindle knots during coalescence process and finally detach from two spindle knots. Although the collection process is similar at different stretching ratios, the difference of collecting efficiency and waterhanging ability is quite obvious, as shown in Figure 2a,b. Compared with original bioinspired fiber, the maximal water-hanging volume of bioinspired fiber with X ≈ 105% exceeds 6.93 μL, i.e., increases by 78%. Figure 2c shows the relationship between times versus stretching ratios for an extreme hanging droplet in water collection. The insets are optical images of artificial fiber hanging water droplet at threshold conditions under different stretching ratios (e.g., 0, 30%, 50%, and 105%). Obviously, water-hanging volume of fiber could change from 3.89 to 6.93 μL, which could be controlled via stretching ratios simply. According to our previous studies,[10] we got the length of the TCL at threshold conditions. For original bioinspired fiber, We use L0 = 2m0 + 2πh0 (denoting m0 as original periodicity length of spindle knots, 2h0 as the original height of spindle knots) to estimate the length of TCL. For a given X, the length of TCL could be written as: Lx = 2m0(1 +X) + 2πh0(1 +X)−1/2 (as illustrated in Figure S3, Supporting Information). Due to the fact that water-hanging volume of fiber is proportional to the length of TCL for a given surface,[10] we could estimate the water-hanging ability (VX) at a given X as follow:

Figure 2. Optical images of directional water collection. Water is collected on the original artificial fiber a) X ≈ 0% and b) artificial fiber X ≈ 105%. Although the collection process is similar, the difference of collecting efficiency and water-hanging ability is quite obvious. c) The relationship of times versus stretching ratios for an extreme hanging droplet in water collection (before detaching off). Insets are optical images of artificial fiber hanging water droplet at threshold conditions under different stretching ratios. Apparently, we could control the water-hanging ability via stretching ratios. The scales are 500 μm.

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VX LX m0 (1 + X ) + π h0 (1 + X ) = = V0 L0 m0 + π h0



1 2

(1)

The values of V0, m0, h0 could be gotten from the optical microscopy easily. So, we could use the Equation (1) to calculate the water-hanging volume of fiber at different stretching ratios. For example, when X = 105%, we could obtain the water-hanging volume V105% = 6.82 μL, which is close to the actual measured value, 6.93 μL. We compare the actual measured values (the maximal water-hanging volume) at different stretching ratios with the calculated values, as shown in Figure 3a. Clearly, the actual measured values are very

close to the calculated values, which indicate the feasible method to estimate the TCL length. Therefore, we could use the Equation (1) to predict the water-hanging ability of bioinspired fiber at different stretching ratios. Furthermore, we investigated the water collection efficiency of original fiber and artificial fiber stretched by 105% (Figure 3b) and the small droplet collection behaviors of artificial fiber during the first 20 s (Figure 3c). All results indicate that during the same water collection time, the artificial fiber with larger stretching ratio captures a relatively bigger water volume (e.g., in the first 170 s, ≈3.89 μL for original fiber, ≈5.49 μL for stretching fiber). The reasons could

Figure 3. a) The relation between actual measured values and calculated values at different stretching ratios (X). We investigated the maximal water-hanging volumes of bioinspired fiber at different stretching ratios and compared them with the relevant values through theoretical calculation. The results indicate that the actual measured values are very close to the calculated values. b) Comparison water collection abilities between original artificial fiber (X = 0%) and artificial fiber (X = 105%). c) The water collection behavior of artificial fiber with different stretching ratios during the first 20s. The scale is 300 μm. Clearly, after imposing different stretching force, the fiber displays more efficient water collection and a larger hanging ability. d) Driving forces on original artificial fiber and stretched artificial fiber, respectively. On the original artificial fiber, there is single driving force resulted from Laplace pressure (FLaplace); on the stretched artificial fiber, there are double driving forces resulted from Laplace pressure (FLaplace) and gradient roughness (Froughness).

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Figure 4. a) The release process of hanging droplets with different volume: 4.0, 4.5, 5.5, and 6.5 μL. b) The relation between stretching ratio and the volume of water droplet at critical conditions. c) Model illustration of water droplet release and the change of the three-phase contact line (TCL). With the decrease of stretching ratio, the liquid film on the surface of the spindle knots and fiber breaks up gradually. When the liquid film breaks up completely, the value of L reaches a critical value Lc, i.e., Lc = 2mc + 2πhc (denoting mc as periodicity length of spindle knots, 2hc as the height of spindle knots at critical condition and the red lines represent TCL.). Further reduces the stretching ratio, water drop would detach from two spindle knots. Clearly, the release of droplets with different volume could be controlled via stretching ratio easily. The scale is 500 μm.

be attributed to those: 1) After imposing stretching force, both of the contact length and the surface area S 3 2 1/2 of spindle knots ( SX = [(1 + X ) + tg β 0 ] cos β 0 /(1 + X ) , 0 as illustrated in Figure S3, Supporting Information) increase, which lead to more collecting sites for water drops collection. 2) Uniaxial wrinkles and longitudinal cracks developed via tensile force lead an extremely discontinuous TCL,[13,15] which could reduce the adhesion of the surface and accelerate the collection of water droplets. 3) As mentioned above, tensile force induces the formation of roughness gradient, generating a gradient of wettability. The direction of the wettability gradient force is same as that of another driving force, i.e., Laplace pressure arising from the conical spindle-knot geometry.[2d] Both of driving forces would act cooperatively to drive tiny droplets toward the center of spindle-knot, which improve the droplets collection efficiency (Figure 3d). Obviously, after imposing stretching force, the bioinspired fiber displays more efficient water collection and a larger hanging ability. More importantly, the water collection ability and hanging ability could be adjusted readily via regulating disparate stretching ratios, which is significant for designing microfluidic devices.

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In addition, the release of droplets with different volume could be controlled via stretching ratios easily (Figure 4). For example, a droplet with 4.0 μL would detach from artificial spider silk when the stretching ratio reduces to 44% (Figure 4a). After stretching and retracting for about 50 times, similar behaviors are observed. The results indicate droplets with different volumes correspond to different critical conditions (Figure 4b). The process could be described as follow: as well known, when the water drop hangs on a fiber, gravity can be balanced with capillary force,[10] i.e., G = γLcosθ sinα, where G is gravity, γ is the surface tension of the water, L is the length of TCL, θ is apparent contact angle between water droplet with the fiber surface, α is the off-axis angle. During the reducing process of stretching ratio, due to the fact that the values of G, θ and α change little, the value of L could be considered as a constant. With the decrease of stretching ratio, the liquid film on the surface of the spindle knots and fiber breaks up gradually to keep constant of the value of L. Finally, the liquid film breaks up completely and the value of L reaches a critical value (Lc), i.e., Lc = 2mc + 2πhc (denoting mc as periodicity length of spindle knots, 2hc as the height of spindle knots at critical

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condition) (Figure 4c).[10] If the stretching ratio reduces further, the capillary force could not provide enough force and water drop would detach from two spindle knots. Therefore, the release of droplets with different volume has been realized via stretching ratios easily.

4. Conclusion In order to adjust the water collection ability and waterhanging ability easily, we make use of both elastic polymer and fiber to fabricate artificial spider silk. The results indicate that, after regulating tensile force, the special bioinspired fiber could adjust collecting efficiency and water-hanging ability easily. The findings also manifest that tensile force could control water-hanging ability easily via adjusting the length of TCL. Tensile force induces the formation of wettability gradient, the reduction of the adhesion of the surface and the increase of surface area of spindle knots, which is much useful to improve the collecting efficiency. We could predict the water-hanging ability at a given stretching ratio. More importantly, the liquid capture or release could be achieved by changing stretching ratios. This study is helpful to design smart materials for developing the application in the microfluidic devices and fogwater collection, etc.[2c,12,16]

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This work was supported by the National Key Basic Research Program of China (2013CB933000), the National Natural Science Foundation of China (21234001, 51203006, 21204002, 21004002), the Doctoral Fund of Ministry of Education of China (20121102110035), the Specialized Research Fund for the Doctoral Program of Higher Education (20111102120049), and the Aeronautical Science Foundation of China (2012ZF51065).

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Received: December 5, 2014; Revised: December 23, 2014; Published online: ; DOI: 10.1002/marc.201400695 Keywords: elastic bioinspired fibers; regulation; water collection efficiency; water-hanging ability

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Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400695 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Controlling of water collection ability by an elasticity-regulated bioinspired fiber.

A special artificial spider silk is presented which is fabricated by using both an elastic polymer and a fiber, and the water collection behavior is i...
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