Bioinspired Controllable Liquid Manipulation by ... - ACS Publications

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Bio-inspired Controllable Liquid Manipulation by Fibrous Array Driven by Elasticity Qingan Meng, Bojie Xu, Meijin He, Ruixin Bian, Lili Meng, Pengwei Wang, Lei Jiang, and Huan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09846 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Materials & Interfaces

Bio-inspired Controllable Liquid Manipulation by Fibrous Array Driven by Elasticity Qing’an Meng+, Bojie Xu+, Meijin He, Ruixin Bian, Lili Meng, Pengwei Wang, Lei Jiang, and Huan Liu* Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, International Research Institute for Multidisciplinary Science, Beihang University, No. 37 Xueyuan Road, Haidian district, Beijing 100191, P. R. China KEYWORDS: bio-inspired, liquid manipulation, fiber, elasticity, dynamic +

These two authors contributed equally

ABSTRACT: Fibers exhibit excellent performances in liquid manipulation, normally aroused by either the structural or chemical gradient. Here, we developed radially arranged fibers array with different fibrous elasticities, which exhibited distinct different performances in liquid manipulation as the fibrous elastocapillary coalescence the high-efficient water encapsulating and the inability in liquid manipulation. It is proposed that the fiber elasticity acts as a driving force when interacting with liquid, equivalent with the structural and chemical gradient. We

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revealed the fundamental of how fiber elasticity affects its dynamic wetting behaviors, which will shed new light on designing fibers system with different liquid manipulation abilities.

Fibrous media, as an open system that enjoys advantages of lower hydrodynamic resistance and easy fabrication, have shown versatile attractive performances in liquid manipulation both in natural systems and practical applications, including highly efficient liquid collection,1-6 controllable liquid transfer,7-9 directional liquid pumping,10 robust water repellence,11,12 water-oil separation,13 biomimetic adhesives,14 and self-assembly.15,16 Central to these functions is the liquid moving on/in fibrous system, which generally can be driven by either structural or chemical gradient of the fibers.17-19 For the structural gradient, the conical fibers with asymmetric axial radius have been commonly used because of its ability in generating the Laplace pressure difference, which can propel the liquid moving to the low curvature area along the fiber;17,18 while for the chemical gradient, fibers with asymmetric surface chemical composition make the liquid move to the more wettable area.19 For example, the natural spider silk and cactus spine enable the efficient water collection by taking advantages of the conical structure;1,4 the leg of water strider is repellent to microscale water droplets by the virtue of its unique flexible conical fibers array;12 various artificial conical fiber arrays made of both organic and inorganic materials are capable of water-oil separation even downsize to micrometer scale.13 When the chemical gradient is incorporated, the liquid transfer along fibers shows various behaviors (accelerated/retarded/kept still) depending on the distribution of the surface chemical gradient.20,21 For one-dimensional fibers, the elasticity is one of the most important inherent property, especially for those with big aspect ratio, which normally leads to the bending deformation when the fiber interacts with liquid.22,23 However, the elasticity of fibers was little


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considered as a parameter for controlling the dynamic wetting behavior on fibers, and remains largely unexplored so far. In nature, biologic fibers are mostly flexible as a synergistic effect of organic chemical nature and the high aspect ratio,24 which have shown various strategies in liquid manipulation in order to benefit the inhabiting to the local environment. As we recently reported, the dandelion pappi arranged in an open radial geometry enabled highly efficient liquid transfer, and the Chinese brush made of freshly emergent animal hairs showed the ability in controllable liquid transfer, in which processes both biologic fibers show clear bending deformation. Moreover, various artificial fibers show clear bending deformation when interacting with liquid (Figure 1a), which leads to either the self-assembly15,16 or the elastocapillary coalescence of fibers.25,26 Taken together, it clearly indicates that the elasticity of fibers plays an important role when fibers interact with liquid. However, how fibrous elasticity affects its interaction with liquid remains unexplored. Here, inspired by natural pappi fibrous system, we developed radially arranged fibers array by using fibers with different elasticities By varying the bending stiffness of the fiber, fiber arrays show significant different performances in liquid manipulation, including the fibrous elastocapillary coalescence the high-efficient water encapsulating and the inability in liquid manipulation. Here, we revealed the fundamental of how elasticity of fiber affects its dynamic wetting behaviors: the fiber elasticity serves as a driving force to balance the surface tension of the liquid when interacting with liquid. As a synergistic effect, dynamic balance of liquid within fibers is possible, which is essential for the controllable liquid manipulation. We envision that the result will shed new light on designing functional fibers system with different liquid manipulation abilities.


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As shown in Fig. 1a, the elasticity FElasticity is an important property of the flexible fiber that enables the bending deformation under the liquid surface tension when interacting with liquid.22,23 Thus, the interaction between liquid and fibers can be largely affected, indicating the crucial role of FElasticity in liquid manipulation. As has been revealed, liquid on fibers was commonly driven by either structural or chemical gradient of the fiber. For the structural gradient (Fig. 1b), the conical fiber with asymmetric axial radius was mostly used because it enables the generating of the Laplace pressure difference to the liquid droplet on the fiber, which can be

described as

R2

2!

R1

( R + R0 )

FLaplace = # $

2

sin " dz

(R is the local radius of the fiber, R0 is the radius of the

droplet, ! is the liquid surface tension, R1 and R2 are the local radii of the fiber of the two opposite sides of the droplet, ! is the half apex angle of the cone fiber, and dz is the minute incremental radius along the cone fiber). When a droplet was deposited on a conical fiber, the local curvature on the two sides of the droplet was different, which lead to a gradient in Laplace pressure,17,18 then the driving force FLaplace propelled liquid moving from high curvature to low curvature area. For the chemical gradient (Fig. 1c), fibers with asymmetric surface chemical composition make the liquid move to the more wettable area along the fibers. The driving force can be described as FChem = ! R0" ( cos#B $ cos# A ), where "A and "B are the contact angles at less wettable and more wettable sides of the droplet, respectively.19 Here, we firstly proposed that the elasticity of the fiber acted as a driving force when interacting with liquid, playing an equivalent role with the structural and chemical gradient. The natural fibers of the dandelion pappi exhibited the prominent bending deformation when interacting with liquid (Fig. 2a, 2b), by which large amount of liquid was manipulated steadily.9 Drawn inspirations, we developed various planar radial fibers arrays by using fibers with same


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length, similar hydrophilicity (by O2 plasma treatment), but different bending stiffness. As shown in Fig. 2c, by dipping into and withdrawing out of the water bath with a quasi-static velocity of 10 µm s-1, the behavior of the fibers array interacting with the liquid was investigated. As a result, three distinctive water manipulation behaviors were observed: the fibers array with bending stiffness (B) of 1.2 ! 10-13 N m2 deformed inwards leading to the fibers coalescence, and only a very limited water (ca. 4.3 mg) was encapsulated (Fig. 2d); the fibers array with B of 1.1 ! 10-10 N m2 enabled the capture of a large amount of water (ca. 126.9 mg) with the proper bending (Fig. 2e); the fibers array with B of 1.4 ! 10-9 N m2 showed no observable bending with very limited water (ca. 3.2 mg) hanging on the fibers (Fig. 2f). The results clearly show that the fiber elasticity has a significant effect on the liquid manipulation. In order to investigate how the bending stiffness affects the deformation of the fibers, a high speed camera was used to in-situ record the whole process, and the results were summarized in Fig. 3. Although fiber arrays showed the essential same topology at the beginning of contacting water (Fig. 3a1, 3b1 and 3c1), drastically different liquid manipulation behaviors were observed by varying the bending stiffness of fibers. Detached from water surface, the fibers bent downwards from the initially horizontal radial arrangement because of the high adhesion to water (as indicated by arrows in Fig. 3a2, 3b2 and 3c2). With moving upwards, the extent of bending deformation increased, a water column was adhered to the fibers array hanging under the fibers. For the fibers with different bending stiffness, the smaller value of B, the larger deformation extent of the fiber. These fibers arrays exhibited different dynamic wetting behaviors when they were drawing upwards to a certain height of h (the distance between water surface and anchoring site A of the fibers, see Fig. 3 and Fig. 4a). The fibers array with smallest bending stiffness (B = 1.2 ! 10-13 N m2) was liable to bend from downwards to interior (Fig. 3a3)


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shortly after it was away from the water surface (at the position with a height, h, of 0.51 mm), making the most part of the fibers coalescence into bundles, i.e. very limited distance between the aggregating site B to the anchoring site A (Fig. 3a4). Therefore, the liquid encapsulating ability was very limited. With increasing the bending stiffness to a value of B = 1.1 ! 10-10 N m2, the fibers started to bend interior at the h of 2.06 mm, leaving a much larger concave-shaped curvature on the fibers (Fig. 3b). Thus, the fibers aggregated together at a position far from the anchoring site, namely the distance of A and B was enlarged dramatically, leaving a huge space within curved fibers for manipulating the liquid.

However, further increasing the bending

stiffness to B = 1.4 ! 10-9 N m2, the fibers showed only very limited deformation with a convex curve by the water column adhered, and fibers would immediately recover its original planar geometry after detaching from the water, showing severely impaired ability in manipulating liquid (Fig. 3c). It clearly shows that the larger the bending stiffness of the fiber, the smaller deformation when interacting with the certain liquid. The suitable elasticity of fibers enables maximizing the closed space within the deformed fibers, which is helpful for the highly efficient liquid manipulation. To explore the mechanism of how the elasticity of fiber affect its behavior in liquid manipulation, the forces involved were analyzed, as schematically shown by the cartoon in Fig. 4a. When fibers array interacts with the liquid, there are three main forces acting on the fibers: the adhesion force Fa to the liquid that keeps the liquid film intact; the elastic force Fe that makes the fibers bend to lower down the system energy (i.e. keep system stable); the hydrostatic pressure Fh that generated from the deformation of the fiber by adhering a water column 5 (Fig. 4a). In our experimental system, the Fa is equal to the water surface tension ! by the equation, Fa = F! = "2! ds (s is arc-length along the fiber spanning from 0 to the total length L); the Fh can be


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expressed as Fh = " ! g (Y + h ) n (where n = " sin ! ex + cos ! ex is the unitary vector normal to the upper face, h is the moving distance of the fiber array from the unperturbed water surface); the Fe is calculated by Fe = 2! B / l 2 (where " is the local angle between the X-axis and the fiber, l is the horizontal displacement of the deformed fiber, B is bending stiffness of the fiber). Particularly, the moving speed of 10 µm/s was used, making the detaching process to be quasi-static in the vertical direction, which means the fibers array kept stable during the whole process as a cooperative effect of these three forces. Considering the gravitational forces to the water within the fibers array, the Fh is smaller than the ambient pressure. Here, we denote the gravito-elastic 1

5 length as !g = $& BN fib %' (where Nfib is the number of the fiber array, "0 is the density of water, g

( 2"# 0 g )

is the gravitational acceleration)27 for cases in which Fh dominates Fa. Specifically, whether the fiber can be bent during interacting with liquid is dependent on the relationship between the parameter h and #g. When the height h is larger than the #g, the fibers are gradually deformed toward the interior after detached from the liquid bath (Fig. 4b, c). When the height h is smaller than the #g, the Fh is not large enough to bent the fibers (Fig. 4d). Here, in our system, the #g increases with increasing the B of fiber, as indicated by the arrow in figure 4a. Thus, the h by which fibers start to bend increases with the enlarging of the bending stiffness B (i.e. #g), as has been evidenced in figure 3. It suggests that fibers with suitable bending stiffness enable the maximized inner space to manipulate liquid, as schematically illustrated in figure 4b-4d. To further confirm the proposed mechanism, a model system of two horizontal arranged fibers was used to monitor the whole process, and the result was summarized in figure 4. Here, for fibers with B of 1.2 ! 10-13, 1.1 ! 10-10 and 1.4 ! 10-9 N m2, the #g was calculated as 0.48, 1.94 and 3.25 mm, respectively. For fibers with the smallest B of 1.2 ! 10-13 N m2, when the fibers


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array moved upwards away from the water surface (not totally left water surface), the value of h gradually increased from zero to the theoretical value of #g = 0.48 mm (consistent with the experimental data 0.51 mm), then the fibers began to bend interior under the Fa and Fh until the free sides of fibers aggregated together, by which process the most part of the fibers coalescence into bundles and only very limited water column was encapsulated by the closed fibers (Fig. 4b). When the B increases to 1.1 ! 10-10 N m2, the theoretical value of h enlarges to 1.94 mm, in consistent with the experimental data of 2.06 mm, and the fibers array deformed interior with a larger convex radian and as a result a large amount of water was encapsulated within the large space inside the fibers (Fig. 4c). These two fiber arrays were re-shaped with clear bending deformation when they just left water surface totally, as a result of water encapsulation. Hence, the liquid amount encapsulated was dependent on the bending deformation extent of fibers, and the volume of the liquid encapsulated by fibers array with larger B was bigger than that of fibers array with smaller B. Theoretically, with increasing the B to a value of 1.4 ! 10-9 N m2, the fibers would bend inwards and closed the free sides to encapsulate largest water droplet when the value of h reached 3.25 mm. However, as shown in figure 3c and 4d, the water column that adhering to the fibers array would suddenly collapse before reaching the #g of 3.25 mm, which is attributable to the small extent downwards deformation of fibers. In this case, the weight of water column adhered is much larger than the Fa to the fiber, and therefore the water column cannot be steadily held. With the separation of the fibers and water surface, the fibers returned to the no bending state with limited water adhesion (Fig. 4d4). Here, as a result of the capillary effect, the fibers aggregated together into three bundles via the horizontal direction, but no bending via the normal direction. Taken together, the fibrous elasticity plays a rather crucial role when interacting with liquid.


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In summary, we demonstrated that the elasticity of a fiber can serve as a main driving force when fibers interacting with liquid. By constructing radially arranged fibers array with different fibrous bending stiffness, we herein revealed that the fiber elasticity serves as a driving force to balance the surface tension of the liquid when interacting with liquid, and significant different behaviors in liquid manipulation were observed by simply varying the elasticities of fibers: including the elastocapillary coalescence of fibers, highly efficient water encapsulating by fibers, disabled liquid manipulation by fibers. Fibrous media have shown various attractive performances when interacting with liquid, where the interaction between fibers and liquids is the essence. Therefore, understanding how the fibrous elasticity affects its dynamic wetting behavior is rather meaningful for both fundamental and practical applications, as liquid capture9 and pumping10, controllable liquid transfer into various functional micro-patterns28,29 etc. We envision that the result would provide a guide for the designing and fabricating functional fibrous system with different liquid manipulation abilities,


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Figure 1. The cartoons show the driving forces for liquid on fibers. a) The elasticity of fiber can act as a driving force when fiber interacting with liquid by forming the obvious bending deformation. b) The curvature changes of the conical fiber generate gradient of Laplace pressure that can propel droplet moving from small curvature radius to large curvature radius. c) Liquid droplet moves from the less wettable side to the more wettable side along a cylinder fiber, because of the surface wettability gradient.


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Figure 2. Investigation the effect of B of fiber on manipulating liquid. a) The natural dandelion pappi. b) The natural dandelion pappi with typical open radial geometry show clearly bending deformation when interacting with the water bath, and as a result a big water droplet was encapsulated. c) Bio-inspired fibers arrays were constructed by using fibers with different B, and drastically different liquid manipulation behavior was observed. d) when the B is 1.2 ! 10-13 N m2, the elastocapillary coalescence of fibers was happened and as a result the limited water column of 4.3 mg was encapsulated; e) when the B is 1.1 ! 10-10 N m2, highly efficient water encapsulating of 126.9 mg by fibers was shown; f) when the B is 1.4 ! 10-9 N m2, no observable bending deformation was observed for fibers, leading to a disabled liquid manipulation.


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onto water surface. The fibers rebounded to the initial horizontal open geometry hanging little amount water.

Figure 4. The proposed mechanism of the effect of bending stiffness on liquid manipulation. a) After leaving water interface with a quasi-static velocity, the fiber adhering a water column is balanced by the adhesion force Fa, the hydrostatic pressure Fh and the elasticity Fe. b-d) With increasing bending stiffness, the deformation extent of fibers array decreased, causing the water amount captured by bending fibers first increased and then decreased; b1-b3, c1-c3, d1-d3) The experiments using two horizontal arrangement fibers to undergo the dip-withdraw process to


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confirm the proposed mechanism, where the bending stiffness was corresponding to that of b, c, d to visualize the process experimentally.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] ORCID Huan Liu: 0000-0001-9009-7122 Author Contributions +

Q. M. and B. X. contribute equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for financial support from the National Natural Science Foundation (21622302, 21574005), the Fundamental Research Funds for the Central Universities and the National Postdoctoral Program for Innovative Talents (BX201700261).

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