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Uni-Directional Droplet Transport on Bio-fabricated Butterfly Wing Peiliu Li, Bo Zhang, Hongbin Zhao, Lunjia Zhang, Zhe Wang, xiangyu Xu, Tingting Fu, Xiaonan Wang, Yongping Hou, Yuzun Fan, and Lei Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02550 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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Uni-Directional Droplet Transport on Bio-fabricated Butterfly Wing #

#

Peiliu Li,2 Bo Zhang,1 Hongbin Zhao,3 Lunjia Zhang, 3 Zhe Wang,1 Xiangyu Xu2,Tingting Fu,1 Xiaonan Wang,1 Yongping Hou,1 Yuzun Fan,1* Lei Wang3* 1

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of

Education, School of Chemistry, Beihang University, Beijing, 100191, China 2

Biomechanics and Biomaterials Laboratory, Department of Applied Mechanics, School of

Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, PR China 3

Advanced Electronic Materials Institute, General Research Institute for Nonferrous Metals,

Beijing 100088, China. #

These authors contribute equally to this work and should be considered to be the first

co-authors. *Email: [email protected] (Yuzun Fan) *Email: [email protected] (Lei Wang)

1

ACS Paragon Plus Environment

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ABSTRACT Water droplet uni-directional transport on asymmetric superhydrophobic surface has attracted much interest in theory analysis and applications, such as self-cleaning, anti-fogging, anti-icing, heat transfer and so on. Different from the symmetrical performance on the uniform topographies, the droplets acting on asymmetric surface perform anisotropic state and easily roll off the surface along special direction. This phenomenon is indicated by natural butterfly wings. The flexible asymmetrically arranged micro-step induces the droplet to release along the outside radial (RO) direction and to pin against the RO direction. Here, inspired by butterfly wings, a kind of surface for superhydrophobic and uni-directional droplet transport is achieved by integrating the methods of soft lithography and enhanced crystal growth. The water droplet shows anisotropic state on the bio-fabricated surface, it rolls off easily along the step direction. The droplet is uni-directionally driven off the surface by asymmetric surface tension force generated by the micro-step topography. This experiment is significant for designing self-cleaning surfaces.

Keywords：：Uni-directional transport ；Superhydrophobic；Asymmetric；Spreading； Surface tension

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INTRODUCTION Droplets rebounding and detaching from the asymmetric solid surface with the anisotropic state has been reported for applications, such as self-cleaning, drug delivery and microfluidic devices.1-(8) Different from symmetrical topographies, the droplets suspending on asymmetric surfaces show selective behavior.9-11 For example, the droplets roll off the surface along the outside radial (RO) direction with a small tilt angle on natural butterfly wing surfaces.12-14 In the past, the droplet uni-direction transport has been reported in many passages due to a different length of three-phase contact line (TCL) or an additional force generated by different advancing contact and receding contact angles, 5,15-18but we have hardly read analysis of the surface tension of micro-step on asymmetric topographies. In this passage, we not only observe some attractive phenomena in the experiment, but also establish a mechanism to explain why these phenomena occurred on the asymmetric topographies. The theory established in the passage reveals that the resultant force subjected to the droplet on asymmetric superhydrophobic surface rather than the length of TCL that contributes to the phenomena. At last, the theory is not only an excellent explanation for the performance of the droplet on asymmetric topographies, but a good reference for anisotropy structures in the future. Another interesting phenomenon occurring on natural perennial ryegrass was also observed in our experiments.19 The condensed droplets suspending on the ryegrass leaf surface showed an anisotropic state and would roll off along the leaf axis with the help of horizontal or vertical vibration. The water skipper legs with unidirectionally arranged micro-arrays elbowed the water out and prompted itself to run ahead freely on water surface.20-21 Inspired by such amazing performances of natural surfaces, researchers have obtained bio-fabricated topographies and realized the functions close to the natural characterizations.22-25 Here, 3


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we take advantage of the natural butterfly wing to realize droplet uni-directional detachment on the bio-fabricated composite surface. The functional surface is achieved by integrating the methods of soft-lithography and crystal growth. In order to fabricate asymmetric topography, Polydimethylsiloxane (PDMS), a kind of flexible polymer, is used to duplicate the micro-steps of the butterfly wing. ZnO nano-rods obtained by crystal growth method are used to enhance the surface roughness and HeptadecaFluorodecyltri-propoxysilane (FAS-17) is used to decrease the surface free energy. After those treatments, the bio-fabricated butterfly wing is achieved, which performs robust superhydrophobicity and anisotropic hysteresis. Most of other bio-fabricated surfaces are also obtained with the same method.

MATERIALS AND METHODS Preparation of bio-fabricated Morphoaega butterfly wing. Butterfly wing specimens were pasted on the culture dish and covered by polydimethylsiloxane (PDMS, sylgard 184 dowcorning, Dow Corning, USA) pre-polymer. The pre-polymer was obtained by mixing the PDMS and curing agent with a mass ratio of 12:1. The bubbles inside the pre-polymer were removed by vacuuming. The pre-polymer were cured at 70 oC for 1 hour. The negative topography was achieved by peeling off the butterfly wing from the PDMS surface. To get the positive topography, the negative surface was treated with C4F8 by physical vapor deposition and then covered by PDMS pre-polymer. When the pre-polymer transferred from liquid to solid, the positive topography was formed following peeled off. After that, ZnO nano-rods were fabricated to roughen surfaces. The crystal seed solution was prepared as follows: 5g Zn(Ac)2·2H2O, 0.8 g monoethanolamine (purchased from Shanghai Zhenpin Chemical Co., Ltd., China) and 20 mL ethylene glycol monomethyl ether (purchased from ZhongshanXinxin Chemical Co., Ltd., China) were mixed and stirred with a 4


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magnetic stirrer until the liquid became transparent. The growth liquid was achieved as follows: 0.4 g hexamethylenetetramine (purchased from Beijing Lanyi Chemical Co., Ltd., China) and 0.82g Zn(NO3)2•6H2O (purchased from Beijing Lanyi Chemical Co., Ltd., China) were mixed into 100 mL deionized water and stirred to be transparent. The positive surface was covered by crystal seed by dip coating and treated at 320 oC for 2 min. ZnO nano-seed was achieved on the surface. The positive surface with growth liquid was placed into a reaction kettle (100 mL) at 95 oC for 12 hours. The ZnO nano-rods were planted on the surface. For obtaining the low hysteresis surface, FAS-17 was used to lower the surface free energy. Characterization:

Observation

of

the

topographies

were

obtained

by

Environmental Scanning Electron Microscopy (ESEM, Quanta FEG 250, FEI, USA) under a voltage of 10-15 kV. The contact angle and sliding contact angle were tested by Dataphysics OCA 20 system with tilting stage. Dynamic motion of the droplet was observed by high-speed CCD camera (Phanton v9.1. Vision Research, America).

Theoretical and Numerical Models Herein，many attracted phenomena are presented when the droplet impacts on the bio-fabricated wing surface. For example, the sliding contact angle is different between the RO direction and the anti-RO direction, the droplet impacting on the bio-fabricated wing surface will bounce along the RO direction, and the distance at which the droplet moves to the RO direction is bigger than that to the anti-RO direction when the droplet spreads on the bio-fabricated wing surface. In this passage, the mechanics of these phenomena are established by analyzing the surface tension force and the pressure on every micro-step of the asymmetric topographies. The droplet easily roll off along the RO direction due to the resultant surface tension force on the surface. And then, a simulation was designed to clearly demonstrate the droplet 5


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motion on the asymmetric topographies step by step. Two-phase flow simulation was performed using VOF (volume of fluid) model in ANSYS FLUENT software. Transient PISO algorithm was used in the simulation to observe the rolling of 5uL droplets in RO or anti-RO directions and the bouncing of the droplet when impacting on the bio-fabricated surface. In the initial definition of the flow field, air and water were set as the main phase and the second phase, respectively. The interface tension was set as 0.072N/m, and the wall contact angle was 150°C. Other parameters in the simulation was set in accordance with the experiment (shown in Table 1).

RESULTS AND DISCUSSION Characterization of the Patterns The Butterfly wing with asymmetric micro-steps induces the droplet in an unstable state and to easily roll off along its outside radial direction.14 Figure 1a shows the top view of the butterfly. The wing surface is covered by the asymmetrically arranged micro-steps (inset). The flexible asymmetric micro-steps can adjust the tilt angle and change the three-phase contact line of solid-liquid interface, leading the droplets to roll off or pin. To reappear the functions of natural butterfly wings, the biomimetic wing was achieved by integrating the methods of soft-lithography and crystal growth. Figure 1b-c show the top view and the side view of the wing. The micro-topographies, such as the high raised spine and the edge of the wing can be observed clearly. The droplets on its surface show a superhydrophobic state (Figure 1d). The bio-fabricated surface is characterized by flexible due to the substrate material, i.e., PDMS, a kind of soft polymer material.(24)

Wettability Test Surfaces that control liquid motions are significant in liquid transport and 6


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microfluidics.(26)(29)(31) They can induce liquid droplets to show different adhesion along various directions. The sliding contact angle (SCA) along different directions are tested on the bio-fabricated wing surface. Figure 2a shows the SCA of droplets with a volume of 5µL. The droplets roll off along two directions with different sliding contact angles. Along the RO direction, the droplet can be driven off with a tilt angle of 3.9° (Figure 2a). In the anti-RO direction, the droplets roll off with sliding contact angles of 6.9° (Figure 2b). Figure 2c shows the relationship between the volume of droplet and the SCA along two directions, i.e., RO and anti-RO. The SCA declines with the increase of the volume. When the volume is smaller than 5 µL, the SCA is greatly affected by the gravity. The SCA declines to 2° along RO direction when the volume is larger than 7 µL. To indicate why the droplet shows selective motions, the topography of the bio-fabricated surface is observed by Environmental Scanning Electron Microscopy (ESEM). The surface is composed with micro-steps (Figure 3a), which cocks along the RO direction (Figure 3b-c). The magnified observation shows that the surface is uniformly covered by ZnO nano-rods. The nano-rods with a length of 2 µm and an average diameter of 100 nm are observed (Figure 3d). According to the asymmetric micro-nano-structured topography, the droplet can hardly penetrate into the valley of the surface and shows the Cassie’s state.(32) Droplets with the volume of 10 µL free fall and roll off the surface along the RO direction (Figure 4a). According to figure 4b, θ1 is the inclined angle between the tangent line of left contact point that the droplet connects with the microstructure and horizon, θ2 is inclined angle between tangent line of right contact point that the droplet connects with the microstructure and horizon, T1 and T2 are the surface tension between the droplet and the microstructure, P is the droplet pressure and ∆H is the difference in height between the left and the right contact point. The relation between θ1 and θ2 can be 7


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easily derived as:

cos θ1＞cos θ 2

(1)

Young’s Equation can describe free energy in liquid-solid, liquid-vapor, solid-vapor condition. With the three constant free energy, surface tension is constant. The radius of a 10 µL droplet is approximately 1368 µm, the height of the microstructure is a constant (20 µm), and the interval of the microstructure is 75 µm. The linear pressure difference caused by the liquid pressure as well as the height difference can be described as:

∆P =

2∆H T R

(2)

Where R is the radius of the droplet. Apparently, H is many times ∆H. Hence, ∆P can be ignored. The number of the micro-steps covered by the droplet is given as n. The resultant force (f) therefore is described as: n

f = ∑ T = ∑ γ ( cos θ1 − cos θ 2 )=nγ ( cos θ1 − cos θ 2 )

(3)

1

Where γ represents the surface tension of the bio-fabricated surface. When the component of the droplet gravity is balanced with the resultant force (f), the droplet will roll off along the inclined direction. The sliding contact angle can be described as:

2R γ ( cos θ1 - cos θ 2 ) 2γ ( cos θ1 - cos θ 2 ) =arc θ = arc d G π Rgd

(4)

When the SCA equals to the glide angle in theory, the droplet will roll off. According to the theory, we can also explain the phenomenon on the asymmetric topography surfaces. For a micro-step, asymmetric direction depends on the direction, and the surface tension force is asymmetric, which changes the droplet behavior. 8


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Figure 4b and c indicate the resultant surface tension force along the anti-RO direction. The surface tension force in the anti-RO direction has a smaller angel with the surface, which increases the composition of the surface tension force along the anti-RO direction. According to equation 4, the droplet pins along the anti-RO direction and easily releases along the RO direction because the micro-steps have an inclined angle which induces the difference between θ1 and θ2. For a droplet, the resultant surface tension force is determined by the difference of the θ1 and θ2 (equation 1) and the number of the micro-steps. On the horizontal plane of the symmetric topography surfaces, the angles between the surface tension of the droplet and the bio-fabricated surfaces (left and right edge) are equal, which results in an isotropic state of the droplet. However, on the asymmetric topography surface or the inclined surface, the surface tension of the droplet (left and right edge) are not equal and have a resultant force (f). According to the theory, the resultant force (f) is along the anti-RO direction. The large SCA is caused by the high uplift topography on the wing surface. On the fixed surface, the gravity performs the most significant role in rolling off. When a droplet impacts on a superhydrophobic symmetrical topography, it could rebound vertically. Scanning electron microscope (SEM) image of the fabricated surface shows that its structure imitates the flake structure on the butterfly wing (Figure 5a, b). On the asymmetric surface, the droplet contacts the surface and rebounds in the anisotropic state. For example, when a droplet impacts on the bio-fabricated butterfly wing surface, it bounces and spreads along the RO direction (Figure 5c, d). Even under different Weber numbers (We), the droplet prefer spreading towards the RO direction (Figure S1). However, when the droplet impact on the symmetrical surface, the distance of the droplet spread to both sides is equal (Figure 9


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S2). Comparing the tension force on asymmetrical and symmetrical surfaces when the droplet impact on two surfaces, we find that once the droplet contacts with the solid surface, it causes an impact force (F) which is larger than the gravity (G) because of the free fall of the droplet. Hence, this condition is unbalanced and transient. And thus, a microstructure can be extracted to analyze its mechanical behavior. In this state, the droplet asymmetrically bounces and spreads because the resultant force received by the droplet is asymmetric. After that, three different situations are researched. The number of the microstructure is set as n+1. 1) The amount of T1 and T2 both are n; 2) The number of T1 is one less than T2, i.e., n-1 3) That amounts of T1 one more than T2 is n+1. We only examined the third example to determine the direction of the horizontal resultant forces. The horizontal resultant forces can be expressed as: n −1 n n   Fx = ∑T1 cos θ 3 − ∑T2 cos θ 4 = ( n − 1) γ  cos θ 3 − cos θ 4  n −1   1 1

(5)

Where γ represents the surface tension of liquid droplet, θ3 and θ4 represent the left and right angle between surface tension with the micro-step, respectively. The amounts of microstructure covered by the droplet are far more than 10 at the beginning of spreading. After the spreading process completes, the horizontal resultant force can be derived as:

cos θ3 −

π 11 11π n cos θ 4 >> cos − cos >0 n-1 4 10 36

（6）

In summary, the horizontal force of the third condition exerted by the droplets is rightward. Here, we find that the inclined angle of micro-step θ is bigger or the volume of the droplet is lager, the generated additional pressure directs towards to the apex of micro-steps due to the progressively smaller space between the two steps. 10


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To clearly understand the droplet motion on the asymmetric topographies surface, a simulation is designed and shown in Figure 6. Droplet falls, contacts, spreads and detaches from the surface step by step. With anisotropic force generated by the topography, the droplet unidirectionally shrinks and detaches along the right direction and the SCA along the RO direction is bigger than the anti-RO, whose performance is just like the motion on the bio-fabricated butterfly wing surface. Two-phase flow was performed and the transient PISO algorithm was used in the simulation by using VOF (volume of fluid) model in ANSYY FLUENT software. The roll angle of the droplet in the RO direction on the butterfly wing was 3.9°, and the roll angle in the anti-RO direction was 6.9°. According to the dichotomy, 3° RO, 5° RO, 6° anti-RO, and 8° anti-RO models were established. The microstructure size and the parameters in the simulation are shown in Table1. The simulation results show that the droplets don’t roll on the 3° RO and 6° anti-RO surfaces, but roll on the 5° RO and 8° anti-RO surfaces. According to the dichotomy method, the sliding contact angle in the RO direction is about 4°, and the sliding contact angle in the anti-RO direction is about 7°, which is in good agreement with the experimental results. In natural circumstances, the average drop speed of 1mm raindrops can reach 4m/s. 32 At this time, the dynamic effect of droplets is already very obvious. What’s more, we also need to consider whether the surface hydrophobic properties of butterfly wings will be changed by the impact of raindrops. The initial velocity of the droplet up to 1m/s was set in the simulation. From the simulation results, it can be seen that the droplets show obvious anisotropy during the contraction process, causing them to spring up in the RO direction. The droplet then contacts the wall again under gravity, and continue to deflect in the direction of RO after bouncing.

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CONCLUSIONS In this study, a bionic butterfly wing is achieved by integrating the methods of soft-lithography and crystal growth. The liquid droplet suspending on the bionic surface shows anisotropic state and rolls off the surface along outside radial direction easily. That performance is caused by the anisotropic additional force on the asymmetrically arranged micro-steps of the surface. The droplet inside the valley of the micro-steps generates unidirectional additional pressure and results in driving droplet out along the RO direction. The mechanism in this article is established to explain the anisotropic phenomenon on the asymmetric solid surface, which can also be a reference to judge and explain other performances on the asymmetric surface. This functional surface is expected to explore new theory for anti-fogging and self-cleaning performances.

Supporting Information The length of the droplet spread to both sides on asymmetric surface and symmetric surface in different Weber number.

Acknowledgments This work is financially supported by the National Science Foundation for Young Scientists of China (Grant No. 21805294). China Postdoctoral Science Foundation (2018M631303).

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Table 1: The related parameters in this work.

Surface tension

0.072N/m

Gravitational acceleration

9.8 m/s2

Liquid density

998.2 kg/m3

Liquid viscosity

0.001003 kg/m•s

Gas density

1.225 kg/m3

Gas viscosity

1.7894e-05 kg/m•s

Contact angle

150 deg

Initial velocity

1 m/s

Drop radius

1.0607 mm

Time step

Size 1e-06

Weber number，We

14.73

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Figure 1. Butterfly and its bio-fabricated wing. a) Optical view of the butterfly. The inset view is SEM image of the butterfly wing surface. It is composed with unidirectional arranged micro-steps. b-c) The top and side view of bio-fabricated butterfly wing. Here, two directions are defined, i.e., the direction along the step structure is outside radial (RO) direction, the direction cross the surface is cross direction. d) The contact angle of liquid droplet (with volume of 10 µL) on the bio-fabricated wing. The contact angle is ~150 o.

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Figure 2. The sliding contact angle (SCA) along different directions, i.e., RO and anti-RO. The SCAs on two directions are 3.9o a）and 6.9o b), respectively. The droplet volume in here is 5µL. c) The relationship between the volume of droplet and SCA along two directions.

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Figure 3. The topography analysis of the bio-fabricated wing. a) The surface is composed with micro-step structures. b-c) The magnified view of the step structures. It cocks along the RO direction. c) The enlarged view of the ZnO nano-rods on the micro-step surface. d) The nano-rods with diameter of ~100 nm and length of ~2 µm are observed.

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Figure 4. Droplet motion on bio-fabricated surface. a) Droplets with volume of 10 µL free fall and roll off from the horizontal surface along RO direction. b) The model of droplet along the Anti-RO direction on the surface. c) The force analysis of single micro-step on the surface.

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Figure 5. a, b) Scanning electron microscope (SEM) image of the fabricated surface mimicking to the flake structure on butterfly wing. c) The droplet impacts on the structure and uni-directional spreads on the surface, the distance of the droplet moves to the RO direction is longer than anti-RO direction. d) Isotropic bouncing is presented when the droplet impact on this surface and the droplet bounces along RO direction obviously.

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Figure 6. The stimulation of the droplet motion on the bio-fabricated surface. a) The simulation about the sliding contact angle 3o and 6o. b) Droplet impacts and spreads on the surface with isotropic state and shrinks with anisotropic state. With the help of unidirectional additional pressure, the droplet release easily along the micro-step.

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TOC Graphic

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300x237mm (150 x 150 DPI)


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251x129mm (150 x 150 DPI)


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327x264mm (150 x 150 DPI)


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383x228mm (150 x 150 DPI)


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257x182mm (150 x 150 DPI)


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202x191mm (150 x 150 DPI)


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296x167mm (150 x 150 DPI)


Unidirectional Droplet Transport on the ... - ACS Publications

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