Article pubs.acs.org/Langmuir
Facile Fabrication of Binary Nanoscale Interface for No-Loss Microdroplet Transportation Weitao Liang, Liqun Zhu, Weiping Li, Chang Xu, and Huicong Liu* Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China S Supporting Information *
ABSTRACT: Binary nanoscale interfacial materials are fundamental issues in many applications for smart surfaces. A binary nanoscale interface with binary surface morphology and binary wetting behaviors has been prepared by a facile wetchemical method. The prepared surface presents superhydrophobicity and high adhesion with the droplet at the same time. The composition, surface morphology, and wetting behaviors of the prepared surface have been systematic studied. The special wetting behaviors can be contributed to the binary nanoscale effect. The stability of the prepared surface was also investigated. As a primary application, a facile device based on the prepared binary nanoscale interface with superhydrophobicity and high adhesion was constructed for microdroplet transportation. fields. Li15 fabricated multifunctional Au-coated Ni nanocone arrays (Au@Ni NAs) mimicking the cicada wing by a simple and cheap electrodeposition method. They also found that the high-adhesion porous surface can be used as a mechanical hand to transport precious microdroplets. Chu16 demonstrated a rapid method to fabricate large-area microcone arrays by onestep femtosecond laser irradiation on nickel targets in sucrose solutions. The prepared surfaces are superhydrophilic in air and superoleophobic in water. These processed surfaces covered with microcone arrays exhibit multiple functions, including microfluidic transportation. However, most of the methods above are complex and expensive, greatly limiting their application range. Moreover, droplet loss is an inevitable consequence of using solid substrates because of droplet wetting and contact angle (CA) hysteresis. Therefore, a simple and more effective route to generate a surface for no-loss microdroplet transportation is highly desirable. When an interface is formed by a mixture of nanostructures with mutually complementary properties, unusual interfacial properties can be created under certain cooperative conditions.2,18,19 This concept can be extended to the design of new interfacial materials for microdroplet transportation. In this work, a binary cooperative interface with binary nanoscale surface morphology and special wetting behaviors has been prepared by a facile wet-chemical method (Figure 1). The prepared surface presents superhydrophobicity and high
1. INTRODUCTION In recent years, binary cooperative nanoscale interfacial materials, i.e., materials with two cooperative properties on the nanoscale, have attracted much attention for wide applications in self-cleaning, antifogging, antifreezing, water− oil separation, wax prevention, microdroplet transportation, etc.1−4 By applying the binary coordinating concept on a solid surface, human beings expect to generate nanostructures with mutually compensating properties, e.g., hydrophilic and hydrophobic; conducting and insulating; convex and concave; p-type and n-type; oxidizing and reducing; ferromagnetic and antiferromagnetic; etc. Among these special surfaces listed above, smart surfaces with binary special wetting behaviors are always one of the hot issues.5−8 Many interfacial materials with special wetting properties have been developed to get new functions. Inspired by the self-cleaning property of lotus leaves, a superhydrophobic surface was invented;9 inspired by geckos’ feet, a climbing chuck with huge power was developed;10 and inspired by the scales of fish, oil-prevented materials were designed.11 More and more people have focused on developing high-level functional materials with special wetting behaviors. The transport of tiny volumes of liquid has attracted considerable attention because of its great promising applications, such as many localized chemical or biological reactions, trace analysis, and in situ detection.12,13 Persistent efforts have been made to develop new methods and materials to achieve microdroplet transportation.14−17 Tricoli14 developed a superhydrophobic polystyrene nanotube layer that was applied to perform no-loss reversible transport of microlitersized superparamagnetic liquid droplets by alternating magnetic © XXXX American Chemical Society
Received: April 15, 2016 Revised: May 18, 2016
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DOI: 10.1021/acs.langmuir.6b01455 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Schematic illustration of the superhydrophobic substrate with high adhesion for microdroplet transportation.
Figure 2. (A−D) SEM images of samples attained at different chemical replacement times: (A) 0 s; (B) 200 s; (C) 400 s; (D) 600 s. (E) AFM images of the samples in (A−D). 2.3. Characterizations. The crystallographic characterization was performed on an X-ray diffractometer (ARL XTRA, Thermo Electron SA, Écublens, Switzerland) with Cu Kα radiation at a scan speed of 6 deg/min over the 2θ range from 5° to 90°. Scanning electron microscopy (SEM) on a JSM-7500F microscope (JEOL Ltd., Tokyo, Japan) was used to observe the surface morphology of the obtained specimens. Before the observation, the specimens were sputter-coated with Pt under vacuum conditions for electrical conduction. The surface roughness was measured by atomic force microscopy (AFM) (DI, Veeco, Plainview, NY, USA). Besides, the chemical states of Zn and Co elements were examined by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific Co.) using monochromatic Al Kα radiation.The contact angles of bare water on the specimens were measured using a contact angle meter (DSA 20, Krüss Instruments GmbH) at five different positions for each surface. The volume of an individual droplet in all measurements was 5 μL. The dynamic microdroplet transportation process was also captured by the contact angle meter. The stability of the prepared surface was investigated by a flush test. The prepared sample was flushed with deionized water from 50 cm upon the surface at a speed of 5 mL/s. After a continuous test of several hours, the sample was moved away to test the CAs at the place where the water flush was applied to evaluate the flush resistance.
adhesion with the droplet at the same time. The composition, surface morphology and wetting behaviors of the prepared surfaces have been systematic studied. Hexagonal Zn particles with diameters of about 20 μm are uniformly distributed on the sample surface with 100 nm round pores on their surfaces. The special binary surface morphology endows the surface with special wetting behaviors. The stability of the prepared surface was also investigated. As a primary application, a device based on the special wetting behaviors of superhydrophobicity and high adhesion was constructed for microdroplet transportation.
2. EXPERIMENTAL SECTION 2.1. Materials. ZnSO4·7H2O, Al2(SO4)3·18H2O, and KAl(SO4)2· 12H2O were purchased from Xilong (AR, China). The surfactant and CoSO4 were purchased from Beijing Chemical Works (AR, China). A3 carbon steel sheets with a size of 20 mm × 40 mm × 2 mm were used as substrates. The A3 carbon steel substrates were ultrasonically cleaned in acetone and then rinsed with deionized water sequentially before use. All of the chemicals used in this work were of analytical grade without further treatment. 2.2. Preparation. The specimens were prepared on Zn-coated A3 carbon steel substrates by immersion in CoSO4 solutions with some surfactant for several seconds at 30 °C. After immersion, the specimens were thoroughly washed using running deionized water, dried at room temperature and then measured. The Zn coating was prepared by an electrodeposition method in a bath containing 2 M ZnSO4·7H2O, 0.04−0.05 M Al2(SO4)3·18H2O, 0.08−0.1 M KAl(SO4)2·12H2O, and 0.02 M surfactant.
3. RESULTS AND DISCUSSION 3.1. Morphology and Composition Analysis. SEM images of samples attained with different chemical replacement durations are shown in Figure 2. Figure 2A displays the Zn B
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min, the newly generated particles can be assigned as Zn (PDF 65-3358) on the basis of the well-matched peaks at 36.29, 38.99, 43.22, 54.32, 70.63, and 86.54°. After immersion in the chemical replacement solution for 200 s, peaks at 14.386 and 16.628° appear, which match well with the characteristic peaks of CoZn13 (PDF 09-0523). The results indicate that after immersion for 200 s, CoZn13 was produced by chemical replacement reaction. When the immersion time was increased to 600 s, no new peaks appeared, indicating that there were no new products. The obvious characteristic Zn peaks at 36.29, 38.99, 43.22, 54.32, 70.63, and 86.54° can also be observed, indicating that not all of the Zn on the surface was replaced. In addition, the intensity of the CoZn13 peaks further increased with increasing reaction time, indicating that continuing the replacement reaction results in a greater amount of product. To further identify the new products of the chemical replacement reaction, XPS spectra of the sample after a replacement time of 600 s were measured (Figure 4). The XPS survey spectrum in Figure 4a indicates that Zn and Co can be detected on the sample surface. Figure 4b shows the highresolution spectrum of the Zn 2p region, in which we can find a typical Zn 2p spectrum. The obvious peak at 1021.9 eV matches well with the characteristic peak of Zn. The spectrum also presents an obvious peak at 1045 eV, corresponding to Zn−X in zinc alloy, which can be attributed to Zn−Co in CoZn13. The high-resolution Co 2p spectrum in Figure 4c contains the Co main peak at 778.12 eV, indicating that Co appears after the replacement reaction. The Co 2p intensity is relatively low because the replacement time is short. The energy-dispersive spectroscopy (EDS) spectrum was also measured (Figure 4d), and the results indicate that Zn and Co can be detected on the sample surface. Overall, the replacement process can be described by the following equations:
coatings before the chemical replacement reaction, which present typical Zn particles with hexagonal shape. The diameter of the hexagonal plates is about 20 μm. The hexagonal Zn particles are distributed in a disorderly manner on the specimen surface and form a dense coating. The gaps between the hexagonal plates were significantly investigated, as shown in Figure 2. After chemical replacement reaction for 200 s, the surface morphology dramatically changes. Many uniform pores appear on the surface of the specimen. The pores appear on the surface of the hexagonal plates without changing the original hexagonal shape. The diameter of the pores is about 100 nm. In the high-resolution image we can see that not all of the surface is full of pores after a reaction time of 200 s. As the chemical replacement time increases to 400 s, the surface of the substrate becomes rougher. More pores can be seen on the surface of the Zn particles, and the pore-covered surface become larger. The gaps between the Zn plates can still be distinguished because the replacement reaction is not strong enough. With a further increase in the reaction time to 600 s, the hexagonal plates are totally covered by pores. The particle edges become smoother and the gaps are almost invisible after the replacement reaction. The wetting behavior of a solid is closely related to its surface microstructure, and a difference in the density of pores would result in a difference in wetting behavior.20 The surface roughnesses of the specimens attained with different replacement times were also measured using AFM, and the results are shown in Figure 2E, where panels (a−d) correspond to Figure 2A−D, respectively. It can be seen that the surface roughness decreases with increasing replacement time and that the particle size decreased from several micrometers to hundreds of nanometers. This decrease can be attributed to the chemical replacement reaction that breaks apart the large-sized Zn particles into small, evenly distributed pores, as shown in Figure 2D. (More detailed high-resolution SEM images are shown in Figure S1 in the Supporting Information). The X-ray diffraction (XRD) patterns of the samples attained at different preparation stages were also measured, and the results are shown in Figure 3. The XRD pattern in Figure 3a corresponds to the bare A3 carbon steel and shows obvious peaks at 45.07, 65.19, and 82.35°, indicating that it is mainly composed of Fe (PDF 01-1267). After electrodeposition for 20
Zn + Co2 + → Co + Zn 2 +
(1)
13Zn + Co → CoZn13
(2)
In general, it can be concluded that CoZn13 was produced on the sample surface after the chemical replacement reaction. 3.2. Analysis of Wetting Behaviors. Digital images of wetting behaviors on the surface are presented in Figure 5, and the contact angles of samples attained with different chemical replacement times are also given. Figure 5a shows the CA of the Zn-coated sample, indicating a hydrophilic property with a CA of about 65.6 ± 1.1°. After immersion in the replacement solution for 200 s, the surface property of the sample changed dramatically. The CA of the sample increased to 135.6 ± 2.5°, presenting hydrophobicity. The CA further increased with increasing immersion time, finally reaching 155.0 ± 2.2°, which can be considered to be superhydrophobic ( Figure 5d). It is noteworthy that the superhydrophobicity is attained directly on the Zn-coated carbon steel by chemical replacement without any low-surface-energy materials modification.21−23 The rolling angle (RA) was also measured, and the results are shown in Figure 5e,f. In contrast to most superhydrophobic surfaces,24 the wetting behavior of the surface prepared in this work is totally different: the water drop adheres firmly on the prepared surface when it is vertical (90°) or upside-down (180°), as shown in Figure 5e,f, indicating high adhesion between the water drop and the prepared surface. The droplet in Figure 5f presents an obpyriform shape as a result of gravity. The special
Figure 3. XRD patterns of (a) bare A3 carbon steel, (b) a Zn-coated sample, and (c, d) samples subjected to chemical replacement for (c) 200 s and (d) 600 s. C
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Figure 4. (a) XPS survey spectrum and (b, c) high-resolution XPS spectra of the (b) Zn 2p and (c) Co 2p regions of the prepared sample. (d) EDS spectrum of the prepared sample.
Figure 5. (a−d) Digital images of the contact angles on the samples attained at different replacement times: (a) 0 s; (b) 200 s; (c) 400 s; (d) 600 s. (e, f) Digital images of the droplet on the sample in (d) at rotation angles of (e) 90° and (f) 180°.
wetting behavior of surperhydrophobicity and high adhesion aroused our great interest. To further investigate the special wetting behaviors of the prepared surface, a superhydrophobic surface with low adhesion was employed to carry out a contrast experiment.25 The dynamic contact process was captured by the contact angle meter, and the results are shown in Figure 6. Figure 6A,B shows the dynamic contact processes of 5 μL droplets on the prepared and contrast surfaces, respectively. Figure 6A indicates that upon contact with the prepared surface, the droplet is attracted by the surface. As the syringe rises up, the high adhesion between the surface and the droplet draws the droplet into a spindle shape, as shown in Figure 6A(d). As the syringe further rises, the water droplet finally leaves the syringe and stays on the prepared surface. The CA of the droplet on the prepared surface is 155.0°, which can be considered to be superhydrophobic. As a contrast experiment, Figure 6B shows the dynamic contact process of a droplet on a superhydrophobic surface with low adhesiion. The droplet is also attracted by the surface and drawn into a spindle shape. In contrast to the
Figure 6. Contrast dynamic contact experiments on (A) the superhydrophibic surface with high adhesion and (B) a superhydrophobic surface with low adhesion.
prepared surface in Figure 6A, as the syringe further rises, the water droplet finally leaves the surface and is still attached to the syringe. The results indicate that in contrast to the normal superhydrophobic surface, the surface prepared in this work has both superhydrophobicity and high adhesion. The superhydrophobicity and the strong adhesion can be attributed to the binary cooperative nanoscale surface. It is known that for wetting liquids the contact angle decreases with increasing roughness, whereas for nonwetting liquids it increases.26 Many analytical models have been presented in the past to determine how roughness affects hydrophobicity. The first model27 was developed by Wenzel and is based on a consideration of the net energy decrease during the spreading of a droplet on a rough surface. For a larger solid−liquid D
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attach to various surfaces.31 This remarkable adhesive ability of the gecko arises from millions of nanoscale foot hairs on its feet. The detailed SEM images in Figure 2D and te schematic diagram in Figure 7B show that the prepared surface is composed of a dense layer of pores. It is assumed that the special surface structures can induce sealed air pockets trapped in the pores. The sealed air pockets in the pores are the key to the high adhesion, which can cause the negative pressure. The stack of the negative pressure and the adhesive force between the water and the surface arises from van der Waals interactions and results in the high adhesion. As a result, sealed air pockets in the pores play a dominant role in enhancing the adhesion behavior. 3.3. Application to Microdroplet Transportation. Because of the special wetting behaviors of superhydrophobicity and high adhesion, this surface is expected to be used in microdroplet transportation. The microdroplet transportation mechanism is illustrated in Figure 8A, in which surface (a)
interface area, a rough surface has a larger net energy, resulting in a contact angle increase for a hydrophobic surface and a contact angle decrease for a hydrophilic surface. On the basis of the above theory, an equation was developed that relates the contact angles of rough and smooth surfaces (θ and θ0, respectively), using the roughness factor Rf: cos θ = R f cos θ0
(3)
The roughness factor is defined as the ratio of the total surface area of the rough surface and the projected area of the rough surface or the footprint of the total surface area. The model indicates that roughness enhances the hydrophobicity when θ0 is larger than 90°. On the other hand, the contact angle for the rough surface decreases with increasing Rf when θ0 is less than 90°. However, eq 3 is valid only for moderate values of Rf. For high roughness, the densely distributed caves result in the formation of air pockets, leading to a composite solid−liquid− air interface. Cassie and Baxter28 extended Wenzel’s equation, which was originally developed for the composite interface under the droplet involving solid−liquid, liquid−air, and solid− air interfaces. In order to calculate the contact angle for the composite interface, the following equation is used: cos θ = R f fSL cos θ0 − fLA
(4)
where f SL and f LA are fractional flat geometrical areas of the solid−liquid and liquid−air interfaces under the droplet, respectively. This model shows that as the roughness increases, the contact angle approaches 180°, which explains the increase in the contact angle in this work. The chemical replacement reaction produces many pores on the sample surface and greatly increases the surface roughness. When a droplet contacts the surface with pores, air in the pores can be trapped by the droplet, forming sealed air pockets.29 The sealed air greatly increases the area of the air−liquid interface and improves the hydrophobicity, which can be explained by the Cassie−Baxter model shown as above. When the substrate is turned, the droplet gradually retracts from the surface, and the air−liquid contact line in the pore changes from concave to convex, as shown in Figure 7B. The volume of the sealed air increases, leading to a decrease in the air pressure, which can be explained by Boyle’s law.30 The decrease in the air pressure results in the formation of a negative pressure, which increases the adhesion between the droplet and the surface. This phenomenon can be considered as the “gecko state”, which is inspired by the gecko that can firmly
Figure 8. (A) Schematic illustration of microdroplet transportation. (B) Digital images of the microdroplet transportation process.
represents a superhydrophobic surface with low adhesion, surface (b) is the superhydrophobic surface with high adhesion prepared in this work, and surface (c) represents the destination surface with higher adhesion. As indicated in Figure 8A, when a droplet is placed on the low-adhesion superhydrophobic surface (surface (a)) and surface (b) with higher adhesion gets close to it slowly, surface (b) attracts the water droplet and catches it. The droplet moves with surface (b) and finally to surface (c) with even higher adhesion. The microdroplet transportation process can be achieved without loss because of its superhydrophobic property. The digital images in Figure 8B also confirm the microdroplet transportation application, showing that a droplet was transited from one surface to another surface without loss. On the basis of this property, a microdroplet transportation device can be proposed, constructed as in the schematic shown in Figure 8A. The maximum lifting droplet size of the prepared substrate was also investigated by a designed transport test, and the results are shown in Figure S2. It was found that the prepared substrate can lift a droplet with the size of 11 μL, indicating good transport ability. 3.4. Stability of the Prepared Surface. Good stability would extend the application range of the prepared surface. To
Figure 7. Digital images and schematic diagrams of contact states of (A) superhydrophobicity and (B) high adhesion. E
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Figure 9. (A) Schematic diagram of the abrasion test. (B) Contact angles of the sample after different numbers of cycles in the abrasion test. (C) Schematic diagram of the flush test. (D) Contact angles of the sample after different flush test times.
remained larger than 150° at a heating temperature of 180 °C, indicating good high-temperature resistance. Figure S3B shows the CAs of the prepared substrates after immersion in solutions with different pH values.35 The CA remained larger than 150° when the pH ranged from 7 to 14, indicating good stability in alkaline solution. When the prepared substrate was immersed in acid solution (pH < 7), the Zn reacted with the H+ in the solution, and the CA decreased to nearly 65°, indicating that the binary cooperative structure was destroyed. Overall, it can be concluded that the prepared conversion coating presents extreme stability, which is very meaningful in industrial applications.
further investigate the stability, the following experiments were carried out, and the results are shown in Figure 9. To further observe the abrasion resistance of the prepared sample, a scratch test was carried out with 800# SiC sandpapers, which served as an abrasive surface.32 The prepared sample to be tested was faced with the rough side of the sandpaper, as shown in Figure 9A. A 50 g load was applied to the sample, and then the sample was moved 10 cm on the sandpaper longitudinally and transversely, which was defined as an abrasion cycle. Figure 9B shows the changes in the surface contact angle after several abrasion cycles. The water contact angle changed slightly within 15 cycles and then decreased gradually. The sample that underwent 20 abrasion cycles still maintained superhydrophobic with a CA of about 150.5 ± 0.3°, which indicates that the micro/nanostructure of the superhydrophobic surface was slightly damaged during the abrasion test. The results indicate that the prepared superhydrophobic surface possesses good abrasion resistance. A flush test was also carried out to further verify the stability of the superhydrophobic surface. A schematic diagram of the flush test is shown in Figure 9C. The prepared surface was flushed with deionized water from 50 cm above the surface at a speed of 5 mL/s.33 After a continuous test lasting for several hours, the sample was moved away to test the CAs at the place where water flushed was applied in order to evaluate the flush resistance. The results in Figure 9D show that after a continuous flush test of about 48 h, the CAs of the prepared surface remain as high as 152.5 ± 0.3°, indicating that the surface superhydrophobicity decreased slightly during the flush test. In spite of this, the surface still has excellent superhydrophobicity. The results of the flush test indicate that the prepared superhydrophobic surface has good resistance to flushing. The superhydrophobic state in this work was achieved without using a chemical coating.34 Thus, the prepared substrate might be extended to microdroplet transportation at high temperature. To confirm this, the superhydrophobic stability of the prepared substrates against high temperature was evaluated by heating the samples to a series of different temperatures for 24 h. Figure S3A shows the CAs of the prepared substrates heated to different temperatures. The CA
4. CONCLUSION In this work, a facile chemical immersion method was employed to fabricate a binary nanoscale interfacial material. The surface was prepared by chemical replacement treatment on a Zn-coated A3 carbon substrate with superhydrophobicity and high adhesion. The special wetting behaviors can be attributed to the binary cooperation of the hexagonal Zn particles and round pores. The effect of the chemical replacement time on the surface morphology and wetting behaviors of the substrate was also studied. The stability of the prepared surface was also investigated though a scratch test and a flush test, and good performance was observed. A microdroplet transportation device based on the prepared surface was proposed.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01455. Detailed high-resolution SEM images of the samples attained at different reaction times, digital images from the water droplet transition test for maximum weight, and contact angles in the high-temperature resistance and pH stability tests (PDF) F
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +86 1082317113. Fax: +86 1082317113. Author Contributions
All authors contributed to the development of the experimental design, discussion of the results, and preparation of the manuscript. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are very grateful for the support from the National Natural Science Foundation of China (Grant 51401011) and the support from the Academic Excellence Foundation of BUAA for Ph.D. Students.
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