Interpenetrating Janus Membrane for High Rectification Ratio Liquid

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An Interpenetrating Janus Membrane for High Rectification Ratio Liquid Unidirectional Penetration Lanlan Hou, Nü Wang, Xingkun Man, Zhimin Cui, Jing Wu, Jingchong Liu, Shuai Li, Yuan Gao, Dianming Li, Lei Jiang, and Yong Zhao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08753 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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An Interpenetrating Janus Membrane for High Rectification Ratio Liquid Unidirectional Penetration Lanlan Hou,1, ‡ NüWang,1, ‡ Xingkun Man,2 Zhimin Cui,1 Jing Wu,3 Jingchong Liu,1 Shuai Li,1 Yuan Gao,1 Dianming Li,1 Lei Jiang,1, 4 Yong Zhao1, *

1

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, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, P. R. China. 2

Center of Soft Matter Physics and its Applications, School of Physics and Nuclear Energy

Engineering, Beihang University, Beijing 100191, P. R. China 3

Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing Engineering

Research Center of Textile Nanofiber, School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, P. R. China. 4

Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and

Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

These two authors contributed equally to this work.

Corresponding *E-mail: [email protected] 1 ACS Paragon Plus Environment

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ABSTRACT: Anisotropic interface with opposite properties bestows numerous unusual physical chemical properties have played irreplaceable roles in broad domains. Here we rationally designed an anisotropic Janus membrane with opposite wettability and special interpenetrating interface microstructure, which shows unidirectional liquid penetration “diode” performance. Liquid is allowed to penetrate from lyophobic to lyophilic direction, while blocked in the reverse direction. While conventional works suggested the liquid unidirectional penetration is driven by anisotropic wettability in heterogeneous interface, here we theoretically and experimentally reveal that special interpenetrating topology plays another important role in liquid unidirectional penetration. This insight gives a general guide to build a series of Janus membranes for liquid unidirectional penetration with high hydraulic pressure rectification ratio. The liquid diode Janus membrane implies great promising for liquid manipulation, smart separation membrane, functional textiles and other fields.

KEYWORDS: unidirectional penetration, Laplace pressure, wettability, electrospinning, nanofibers

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Manipulation of liquid transportation in a specific direction has been a critical research area from natural biological to industrial processes.1-5 Different from accustomed active transport manners by using energy consuming external forces, many nature creatures adopt a smarter liquid regulation approach through various specific anisotropic interfaces.6-10 For example, the nepenthes peristome needs a permanent wetting state in order to keep its surface slippery, it thereby evolves a directional water transportation ability by asymmetric wedge-shape microgrooves.11 The wings of butterfly allow water fluently rolling outward of its body while block inward flow so that it could keep its body dry.12 Similar anisotropic structural or chemical strategies are also widely discovered in spider silk,13 cactus spine,14 and desert beetles15 for various intriguing biofunctions. To date, nature has given us numerous inspiration to develop diverse anisotropic wettability materials, which have been widely used in liquid collection, condensation, anti-fogging/icing surfaces, lubrication, and other fields.16-19

Besides directional liquid transportation on outer surfaces with chemical or structural anisotropic,20 controllable manipulate liquid across the porous bulk anisotropic media is also significant investigated for diverse applications.21-23 Recently, Janus membranes with opposite wettability have drawn attentions due to their unique liquid unidirectional penetration property. This kind of Janus membrane allows liquid penetration from lyophobic (LO) to lyophilic (LI) direction, while penetration in the opposite LI to LO direction is prohibited.24 Currently, wettability anisotropic membranes are mainly achieved through two strategies. One approach is chemical modification of a porous membrane to LO completely, and then selectively degrade one side into LI through such as ultraviolet light irradiation or LI modification, while keeps the 3 ACS Paragon Plus Environment

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other side intact, then a LO/LI anisotropic membrane could be generated.25-27 Another method is integration of a LO layer with a LI layer to form a Janus membrane.28-30 The wettability anisotropic membranes fabricated by these two approaches show interesting liquid unidirectional penetration performances. While previous works have made significant progresses on liquid unidirectional penetration, two challenges still exist. On one hand, most of the current unidirectional penetration membranes are only valid for small amount of liquid. It means these materials show quite small hydraulic pressure difference in forward and reverse direction with low rectification ratio. In other words, these membranes are more like a quasiunidirectional penetration that largely limit their practical applications. On the other hand, although previous works have revealed that an anisotropic LO/LI interface is necessary for unidirectional penetration, the underlying mechanism is still elusive. It thereby largely hinders the rational designing of high performance unidirectional membranes.

Herein, we theoretically and experimentally demonstrate that a LO/LI interface as well as an interpenetrating topology interface play two important roles in liquid unidirectional penetration performance. We constructed a Janus membrane with an interpenetrating overlapping structure of a LI nanoneedles layer and a LO nanofibers layer. The special interpenetrating interface provides a significant directional Laplace pressure difference for liquid on the Janus membrane from its two directions, leading to a high hydraulic pressure rectification ratio. The Janus membrane exhibited outstanding liquid unidirectional penetration performance, that can be used in a wide range of liquid applications, including water and various organic liquids. This


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Janus interpenetrating strategy is promising for diverse on-demand fluid manipulation scenarios.

RESULTS AND DISCUSSION

Design Principles. The Janus membrane with anisotropic wettability and interpenetrating topology is presented in Figure 1. Firstly, a LI copper hydroxide (Cu(OH)2) nanoneedles structured mesh was prepared by chemical method.31-34 Then the Cu(OH)2 nanoneedles mesh was employed as a substrate, on which a layer of LO poly(vinylidene fluoridehexafluoropropylene) (PVDF-HFP) nanofibrous layer was deposited.35-38 Because there is numerous valley between nanoneedles, some nanofibers thereby fall between nanoneedles and form a Janus membrane with nanoneedles/nanofibers (nanoneedles interpenetrate with several fibers layers at the bottom) overlap interpenetrating topology (Figure 1a). Figure 1b showed the as-fabricated flexible Janus membrane with a blue Cu(OH)2 nanoneedles side and a white PVDF-HFP nanofibers side. The cross-sectional scanning electron microscope (SEM) image showed that two layers of the membrane were tightly inter-connected (Figure 1c). Figure1d, e revealed the randomly distributed PVDF-HFP nanofibers with average diameter of 405.4 nm (Figure S1a-c). Figure 1f, g showed the dense nanoneedles perpendicular growing on the Cu mesh surface. After the growth of nanoneedles, the pore size of nanoneedles coated mesh decreased to 20.58 ± 3.51 μm comparing to the original copper mesh size of 40.03 ± 3.37 μm. The average length of the nanoneedles is 9.83 ± 2.72 μm and the tip diameter is 180.00 ± 54.68 nm (Figure S2, Figure S3, and Figure S4). The XRD diffraction patterns confirmed that the


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nanoneedles are orthorhombic phase Cu(OH)2 (JCPDS Card, No. 13 - 420) (Figure 1h). It is noted that the strong (111) and (200) peaks are derived from the original copper mesh. A notable feature of as-fabricated Janus membrane is that it possesses a special nanoneedles/nanofibers interpenetrating interface, which is very important for the liquid unidirectional penetration performance. As we prepared an electrospun fiber layer with only a few of layers (electrospinning time for 15 s), Figure 1i confirms an interpenetrating structure in the PVDF-HFP/Cu(OH)2 Janus membrane for the several bottom layers (Figure S5).

Anisotropic

Wettability

Janus

Membrane.

Besides

structural

difference,

the

heterogeneous Janus membrane also showed significant wettability anisotropy (Figure 2a). The PVDF-HFP side is highly LO with a water contact angle (CA) of 140.6 ±1.0°(Figure 2b). On the opposite LI Cu(OH)2 nanoneedles side, however, the water can rapidly spread within 66 ms (Figure 2c-e). Moreover, the wettability of the PVDF-HFP nanofibrous layer to other liquids could be easily tuned by doping with low surface energy substances.35 For example, to enhance LO of nanofibrous layer, we systematically tuned its surface energy by blending with 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (PFDTMS). The pristine PVDF-HFP membrane (designated as M0), as well as the PVDF-HFP/PFDTMS blended membranes with weight ratio of 15:1 (M15/1) and 5:1 (M5/1) were fabricated with increased liquid repellency (Figure 2f). The SEM images of M15/1 and M5/1 and their diameter distribution were shown in Figure S1d-i. As shown in Figure 2g, the M0 membrane showed LO to water and formamide (136.4 ± 1.2°), while LI to ethylene glycol, propylene glycol and hexadecane. However, with the addition of PFDTMS into M15/1, it becomes more LO that formamide, ethylene glycol, 6 ACS Paragon Plus Environment

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propylene glycol and hexadecane exhibited large CAs of 135.8 ± 1.6°, 132.7 ± 0.6°, 128.3 ± 1.4°and 120.6 ± 2.8°(Figure 2h). The M5/1 with more PFDTMS showed higher LO to even hexadecane, dimethylbenzene and tetrachloromethane with CAs of 127.3 ± 1.3°, 125.7 ± 0.6° and 117.2 ± 0.8°(Figure 2i). These results demonstrated that Janus membranes with tunable wettability could be fabricated (Figure S6 and Figure S7).

Liquid Unidirectional Penetration. The anisotropic Janus membrane with LO nanofibers and LI nanoneedles interpenetrating structures showing a liquid unidirectional penetration performance with high hydraulic pressure difference (Figure 3). Take M0/Cu(OH)2 membrane as an example, water was blocked from LI Cu(OH)2 to LO M0 direction with critical water column height up to 141.4 ±14.2 mm (~1.4 kPa, Figure 3a), while on the opposite M0 (LO) to Cu(OH)2 (LI) direction, water can penetrate the membrane with tiny hydraulic pressure by few droplets (Figure 3b). Moreover, liquids with very low surface tension such as tetradecane and dimethylbenzene also exhibited unidirectional penetration with high hydraulic pressure difference on the M5/1/Cu(OH)2 membrane (Figure 3c-f). Besides evident unidirectional penetration performance, the Janus membrane showed excellent durability that could be reused over 25 cycles without any decay (Figure 3g), which ensures the Janus membrane promising long-term, repeated applicability. The nanofiber layer and Cu(OH)2 layer show a good bonding force, and it does not fall off each other during the application process, due to the strong electrostatic force during the electrospinning process. It is worth noting that liquid unidirectional penetration materials have been reported previously, but most of them were either focused on water29 or merely showing small hydraulic pressure difference.23 In this work, 7 ACS Paragon Plus Environment

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we rationally designed a series of Janus membranes that be used for a broad variety of liquids from water to low surface tension organic liquids with outstanding unidirectional penetration performance (Figure S8, Movie S1-S3).

Liquid Unidirectional Penetration Mechanism. Above results demonstrate that not only generally accepted wettability difference, but also special interpenetrating interface structures play important roles in unidirectional penetration. In fact, the essence of liquid unidirectional penetration is resulted from the Laplace pressure difference when a liquid penetrates from two different sides of a LO/LI anisotropic membrane. Here we illustrated several cases of liquid penetration to different porous membranes. For an isotropic LO membrane (Figure 4a), liquid will be blocked because it suffers a resistant Laplace pressure (PL) opposite to the hydraulic pressure (PH) direction. The Laplace pressure follows the relation PL = -2γlg/r, where γlg is the gas/liquid interface tension, and r represents the radius of liquid meniscus.39-42 Only when PH exceeds the maximum PL, then can the liquid break through the pores and penetrate LO membrane. While for an isotropic LI membrane (Figure 4b), the liquid could spontaneously wet and penetrate the membrane owing to the capillary effect. As for anisotropic LO/LI membrane, the liquid penetration behavior becomes quite different to ordinary isotropic counterparts. There are two scenarios for anisotropic wettability membrane, which are penetrant state from LO to LI direction and non-penetrant state from opposite direction. As to the former LO to LI case (Figure 4c), when a liquid is dropped onto the LO side, it should form meniscus, i.e. PL, at the lower surface of membrane pores if there is nothing underneath. However, there is a LI layer underneath the LO layer, which will eliminate PL once the 8 ACS Paragon Plus Environment

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meniscus contacts the LI layer due to LI capillary force. The whole Janus membrane thereby exhibits more like a LI membrane penetration behavior rather than LO one. On the contrary, when a liquid is dropped onto the LI side of an anisotropic membrane, it firstly spread to a thin liquid film and then is blocked at the LI/LO interface. Further increase PH, the liquid will form meniscus underneath the lower LO membrane pores until it reaches the maximum PL then liquid could penetrate the membrane (Figure 4d). In simplest terms, the liquid penetration behavior of a Janus membrane is largely dominated by the wettability of the back layer toward the penetration direction (Figure S9, Figure S10). This understanding inspires us that two conditions are needed to achieve an ideal liquid unidirectional penetration membrane. One is that the two sides of anisotropic membrane need an opposite wettability as previously demonstrated.29, 43, 44 Another is that the distance between the LO layer and LI layer should be as short as possible in order to minimize PL. However, the latter distance effect of LO/LI interface is largely ignored and rarely discussed so far. In LO to LI direction, as the propelling of liquid meniscus, gas/liquid interface would disrupt when it contacts the LI layer. The rupture of the gas/liquid interface leads to elimination of PL, and liquid therefore can pass through the Janus membrane without having to exceed the maximum PL comparing to a single LO membrane (Figure 4e, Figure S11).

Based on this understanding, here we designed a LI/LO Janus membrane with special nanoneedles/nanofibers interpenetrating overlap structure that LI nanoneedles partially inserted into the gaps of LO fibers. In LO to LI direction (Figure 4e), it is seen that the curvature of liquid meniscus could be minimized comparing to routine contact model shown in Figure 9 ACS Paragon Plus Environment

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4c because of the overlap interpenetrating geometry. Once the gas/liquid interface contacts the LI nanoneedles, PL will be eliminated and liquid will penetrate the membrane under the capillary force. In the reverse LI to LO direction (Figure 4f), liquid has similar penetration model to LO membrane like Figure 4a. The more detailed penetration process was analysis in Figure S11. Here, the PL on the arch-shaped liquid surface follows Equation (1):

PL  

2 lg r



2 lg sin(   ) R(sin   D* )

(1)

where θ is contact angle, φ is local geometrical angle (the angle between horizontal line and tangent line of the cylindrical fiber) with a relationship of θ = φ + α, α is the extruded angle, D* is the interspace ratio D* = (D+R)/R, R is the fiber diameter and 2D is the distance between the two adjacent fibers. The maximum PL is thereby PL(max) = 2γlg/R(1 - D*) (because of hemisphere-shaped meniscus of α = 90°, and rmin equals to D).

It is seen that the interpenetrating structure results in the considerable PL difference of the Janus membrane. Here we define a parameter of PH rectification ratio k, what refers to divide PH in LI to LO direction by reverse direction in Equation (2):

k

PH ( LI  LO ) PH ( LO  LI )



D* - sin  ' ( D* -1)  sin( '-  ')

(2)

Here θ', φ', and α' represent the geometric parameters in LO to LI direction. The higher k indicates large difference between the breakthrough pressure of two directions. Equation (2) infers the k is determined by the LO spacing and the meniscus curvature of gas/liquid interface 10 ACS Paragon Plus Environment

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(D* and α'). In a word, the interpenetrating structure greatly reduces the PL through eliminate curvature of liquid meniscus in LO to LI direction, while has negligible impact on LI to LO direction. It thereby results in the considerable hydrostatic pressure difference of the Janus membrane.

The topology structure is important relevant in this work. A series of liquids unidirectional penetration quantitative control experiments verified the proposed vital roles played by the LO/LI interpenetrating designing (Figure 5). In Figure 5a, the critical breakthrough column height (Hc) of formamide on M0/Cu(OH)2 membrane increases with increasing of M0 thickness from 3.0 μm to 21.0 μm from both two directions. The M0 thickness was controlled through tuning electrospinning time. And the results reveal the formamide always more easily penetrates from LO to LI direction (yellow column) than in opposite LI to LO direction (blue column) in any M0 thickness. Here, the interpenetrating effectivity could be verified by a control result without the special nanoneedles topology structure, the LO layer of same chemical composition and structure was used. We constructed a Janus wettability membrane by combining M0 and smooth LI copper mesh without nanoneedles structures (Figure 5b). It can be seen the M0/Cu membrane without topology structure didn’t show evident unidirectional penetration property. As a result, Figure 5c showed that the penetrate pressure rectification ratio k of formamide on M0/Cu(OH)2 (violet line) was much greater than on M0/Cu (green line) at any M0 thickness, which demonstrated the effectivity of nanoneedles interpenetrating topology on liquid penetration performances. In addition, increasing of nanoneedles length also played a positive role in liquid unidirectional penetration on the Janus membrane (Figure S12, 11 ACS Paragon Plus Environment

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Figure S13). Another example of ethylene glycol on M15/1/Cu(OH)2 were shown in Figure 5d, which showed similar tendency of formamide on M0/Cu(OH)2 in Figure 5a. In fact, various liquids with broad surface tension range showed rather high k on three kinds of Janus membranes (for sample, tetradecane showed k ~17 on M5/1/Cu(OH)2) (Figure 5e, Figure S14). It is noted that these data were tested by ~6 μm LO layer thickness, further decrease the LO layer thickness of membrane could bring even higher rectification ratio k, but the absolute hydraulic pressure value become lower. Therefore, the trade-off of high hydraulic pressure and high rectification ratio could be balanced according to practical demands.

The above results are in good agreement with the analysis in Figure 4. Firstly, the interpenetrating nanoneedles makes the liquid easier to penetrate in LO to LI direction. We use the symbol n to indicate the nanoneedles effect. For a porous nanofibrous membrane, the pore size (d) has inverse function with the membrane thickness (T) due to the packing effect in electrospinning process (Figure S15). Then, we assume an exponentially decay function of pore size with membrane thickness, d ∝ e-βT, where β is a fitting parameter to indicate the decrease rate according to the experimental results. Experimental results show that both the nanoneedles length and the membrane thickness have critical effect in the rectification ratio. We assume that the effective pore size (dE) can be illustrated in form of dE = ne-βT. As Young’s equation described, there is relation in Equation (3):

Hc 

4 lg cos  n g

exp(  T )

(3)


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Here g is the gravity constant, based on Equation (3), Figure 5f draws the theoretical critical height curves (Hc) of formamide on M0/Cu(OH)2 membrane both of LO to LI direction (pink line) and LI to LO direction (green line) (Figure S15). These curves show a good consistency with the experimental values (pink rhombus and green triangle). This theoretical formula can be used to infer the critical liquid column pressure as a function of membrane’s thickness. Table 1 simply summarized the liquid unidirectional penetration results on three kinds of Janus membrane. ‘YES’ indicates liquid can realize unidirectional penetration, and ‘NO’ represents those cannot (Figure S16). Comparing to previous reported unidirectional membranes with quite small rectification ratio, our interpenetrating structured Janus membranes outperform on both high rectification ratio and broad liquids category, which are of great promising for broad practical applications.

Potential Applications. The directional penetration Janus could be directly used as a liquid diode for controllable liquid manipulation (Figure 6). The diode symbol indicates the water penetration direction and faces orientation of the Janus membranes (Figure 6a-c). Moreover, unidirectional water penetration is also applicable for oil/water two-phase system (Figure S17). Owing to the conductive and non-conductive property of water and oil, the Janus membrane could be designed as an electronic liquid diode device for “1” (conductive) and “0” (nonconductive) ASCII code programming display (Figure 6d and Figure S17). This strategy offers a possibility of a liquid unidirectional membrane in fluidic detection applications.


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CONCLUSIONS

In conclusion, we have systematically revealed the mechanism of liquid unidirectional penetration on the Janus membrane with opposite wettability. Besides necessary wettability difference, we propose that LO/LI interpenetrating structures play another important role in promoting the unidirectional performance of Janus membrane. This concept is well demonstrated by a special designed LO nanofibers/LI nanoneedles interpenetrating membrane that realized liquid unidirectional penetration with high hydraulic rectification ratio for a variety of liquids. This work will ignite numerous designing options for Janus membrane of various materials. The Janus membranes with high unidirectional penetration performances will find broad applications such as waterproof sweat permeable coats, infant diapers, maternal goods and smart chemical separation membranes.


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MATERIALS AND METHODS

Materials: Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw = 400,000, Sigma-Aldrich), 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (PFDTMS, C13H13F17O3Si, TCI), copper mesh (Shanghai Qin Tong Hardware Wire Mesh Co., Ltd.), sodium hydroxide (NaOH, Beijing Chemical Works), potassium peroxydisulfate (K2S2O8), ethanol, acetone and dimethylacetamide (A.R., Xilong Chemical Co., Ltd.) were used without further treatment.

Preparation of the interpenetrating structured Janus membrane: The copper mesh was first rinsed in detergent, then soaked into acetone with ultrasonic processing and deionized water with ultrasonic processing for 30 minutes, respectively, to remove surface contaminants. After drying, the pre-cleaned copper mesh was immersed into 2.5 M NaOH and 0.1 M K2S2O8 aqueous solution for 30 minutes at room temperature. The copper mesh was then picked up and washed with deionized water, yielding blue Cu(OH)2 nanoneedles arrays covering the copper mesh. The chemical reaction equation can be described as follow:

Cu + 2NaOH + K2S2O8 ═ Cu(OH)2 + Na2SO4 + K2SO4

The LO PVDF-HFP nanofibers were fabricated by dissolving PVDF-HFP into acetone and dimethylacetamide solvent (weight ratio was 7 to 3) to obtain a homogeneous 15 wt% spinning solution. In additions, PFDTMS could be added to the spinning solution with appropriate proportion in order to adjust the wettability of membranes. By using Cu(OH)2 mesh as substrate, nanofibers were electrospun with 19 gauge needle, receiving distance of 20 cm, and working voltage of 18 kV. LO membranes layer with different thickness could be obtained by 15 ACS Paragon Plus Environment

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controlling electrospinning time. Finally, we obtained the PVDF-HFP/PFDTMS/Cu(OH)2 Janus membranes that allowed the unidirectional penetration of liquid.

Liquid unidirectional penetration performances: The as-prepared Janus PVDFHFP/PFDTMS/Cu(OH)2 membrane was tightly fixed onto the flange apparatus. The critical breakthrough column height (Hc), in a LO to LI direction and LI to LO direction, were tested by dropped the liquid onto the Janus membrane by a syringe pump at a slow flow rate of 40 mL h-1. Used films can be recycled after rinsing in ethanol to remove the liquid.

Instruments and characterizations: SEM images were collected on a field-emission scanning electron microscope (JEOL, JSM-7500F, Japan). Liquid contact angles were obtained on an OCA20 instrument (DataPhysics, Germany) at room temperature. Each contact angle values for a liquid (3 μL) on the same samples was measured six times in different positions to get the average value. Optical photographs of the penetration process were taken on a SONY digital camera. The X-ray diffraction patterns were collected on an X-ray diffractometer at a scan speed of 5.0 degree per minute in scan range of 10.0 to 80.0 degree. (XRD-6000, Shimadzu).


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FIGURES

Figure 1. Sample preparation and morphological characterization. (a) Fabrication scheme of Janus membrane with LO/LI interpenetrating structures. The LI Cu(OH)2 nanoneedles mesh is prepared through chemical oxidation of copper mesh, then a layer of LO PVDF-HFP nanofibers is deposited onto the Cu(OH)2 nanoneedles mesh and then forms a Janus membrane. (b, c) Optical and SEM image of the PVDF-HFP nanofibers/Cu(OH)2 nanoneedles Janus membrane. The electrospinning time is 2.0 minutes (~14 μm) to achieve a good visual effect with a certain thickness in Figure 1b, and electrospinning time of 1.0 minute (~6 μm) in Figure 1c. (d, e) Low and high magnification of PVDF-HFP nanofibers with relatively uniform diameters. (f, g) Top and cross-sectional view of Cu(OH)2 nanoneedles mesh. (h) XRD spectra of original (black)


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and oxidized (red) copper mesh indicates nanoneedles are orthorhombic phase Cu(OH)2. (i) The nanoneedles (blue)/nanofibers (grey) interpenetrating structure of the Janus membrane. The pseudocolor is used here for better recognition, original image is shown in Figure S5c.


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Figure 2. Anisotropic wettability of Janus membrane. (a) PVDF-HFP nanofibers (M0) and Cu(OH)2 nanoneedles Janus membrane exhibited anisotropic wettability to water on its two sides (dyed by crystal violet). (b) M0 is highly lyophobic (LO) with water CA of 140.6 ±1.0o. (c-e) Cu(OH)2 nanoneedles mesh are lyophilic (LI) that water rapidly spread within 66 ms. (f) Liquids wettability of Cu(OH)2 nanoneedles and PVDF-HFP/PFDTMS nanofibers (M0, M15/1 and M5/1), respectively. Cu(OH)2 mesh is LI to all tested liquids, while nanofibers are of tunable LO to various liquids by PFDTMS doping. (g-i) Various liquids on three kinds of nanofibrous membranes (numbers in bracket are surface tension value of liquids, unit: mN m-1). The water and formamide are dyed by crystal violet and methylene blue in Figure 2g; the formamide, propylene glycol, and ethylene glycol are


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dyed by methylene blue, rose pink, and fast green in Figure 2h; the dimethylbenzene and tetrachloromethane are both dyed by Sudan Red.


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Figure 3. Liquids unidirectional penetration of Janus membrane. (a) Water is blocked on the Janus membrane from LI Cu(OH)2 to LO M0 direction. (b) Water could easily penetrate the Janus membrane from M0 to Cu(OH)2 direction. (c, d) Unidirectional penetration phenomenon of tetradecane and (e, f) dimethylbenzene on M 5/1/Cu(OH)2 Janus membrane. (g) Durable unidirectional penetration performance of Janus membrane by reusing test of formamide on M0/Cu(OH)2 for 25 cycles. The water is dyed by crystal violet, while tetradecane and dimethylbenzene are both dyed by Sudan Red.


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Figure 4. Mechanisms of liquid penetration on membranes with different wettability and microstructures. (a) Liquid on an isotropic LO fibrous membrane could be blocked within a certain hydraulic pressure (PH) due to the Laplace pressure (PL). (b) Liquid on an isotropic LI fibrous membrane could penetrate spontaneously owing to capillary effect. (c, d) Liquid penetration on a two-layered LO/LI Janus membrane without interpenetrating structure: (c) PL in LO to LI direction could be decreased in some extent by the underneath LI layer; (d) PL in LI to LO direction is similar to a normal LO membrane. (e, f) Liquid penetration on a LI/LO Janus membrane with interpenetrating structure: (e) PL in LO to LI direction could be dramatically decreased due to the


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interpenetrating nanoneedles; (f) PL in LI to LO direction is also similar to a normal LO membrane.


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Figure 5. Liquids unidirectional penetration performance. (a) The critical breakthrough column height (Hc) of formamide on the Janus interpenetrating M0/Cu(OH)2 nanoneedles membrane in LI to LO direction (blue columns) and in LO to LI direction (yellow columns). (b) Hc of formamide on the M0/copper mesh (without nanoneedles as a contrast experiment) membrane in LI to LO direction (blue columns) and in LO to LI direction (yellow columns). (c) The hydraulic pressure rectification ratio (k) of formamide on the M0/Cu(OH)2 membrane (purple curve) and M0/copper mesh membrane (green curve), respectively. (d) Hc of ethylene glycol on the M15/1/Cu(OH)2 membrane. The red line represents k. (e) k value for various liquids on M0/Cu(OH)2, M15/1/Cu(OH)2, and M5/1/Cu(OH)2 Janus membranes. (f) The theoretical (lines) and experimental (dots) values of Hc in LI to LO direction (green) and in LO to LI direction (pink).


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Figure 6. Directional penetration of Janus membrane for liquid diode. (a-c) Controllable directional penetration used as water (dyed by methylene blue) diode. (d) The Janus membrane used for electronic liquid diode in a water/oil (dyed by Sudan Red) two-phase liquid mixture system, penetration or non-penetration represent “1” and “0” state, which could be used for ASCII code displaying.


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TABLE Table 1. Liquids unidirectional penetration applicability of three Janus membranes. ‘YES’ represents unidirectional penetration, ‘NO’ represents not. Surface Tension Liquid

(mN m-1 25oC)

M0/Cu(OH)2

M15/1/Cu(OH)2

M5/1/Cu(OH)2

Water

72.7

YES

NO

NO

Formamide

57.5

YES

YES

NO

Ethylene glycol

48.4

NO

YES

NO

Propylene glycol

36.0

NO

YES

NO

Hexadecane

27.5

NO

YES

YES

Tetradecane

26.5

NO

YES

YES

Dodecane

25.4

NO

YES

YES

Benzene

29.0

NO

NO

YES

Methylbenzene

28.5

NO

NO

YES

Dimethylbenzene

28.5

NO

NO

YES

Dichloromethane

28.2

NO

NO

YES

Trichloromethane

27.1

NO

NO

YES

Tetrachloromethane

26.9

NO

NO

YES


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ASSOCIATED CONTENT

Supporting Information

The following files are available free of charge on the ACS Publications website at DOI:

Supplementary figures and extensional theoretical analysis (PDF)

Liquid unidirectional penetration process (Video)

The authors declare no competing financial interest.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

L. H. and N. W. contributed equally to this work.

ACKNOWLEDGEMENT

The authors acknowledge the National Natural Science Foundation of China (NSFC) (Grant Nos. 21774005, 21433012, 21374001, 21503005 and 51772010), National Natural Science 27 ACS Paragon Plus Environment

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Foundation for Outstanding Youth Foundation, the Fundamental Research Funds for the Central Universities, the National Program for Support of Top-notch Young Professionals, and the 111 project (Grant No. B14009).


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TABLE of CONTENT

Anisotropic Janus membrane with opposite interface wettability and special interpenetrating interface microstructure was prepared, which showing high rectification ratio liquid unidirectional penetration “diode” performance. Liquid is only allowed to penetrate from lyophobic to lyophilic direction. Besides of the anisotropic wettability, here we demonstrated that special interpenetrating topology in heterogeneous interface plays another important role for liquid unidirectional penetration.

ToC FIGURE


Interpenetrating Janus Membrane for High Rectification Ratio Liquid

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