Controlling Directional Liquid Motion on Micro- and Nanocrystalline

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Controlling Directional Liquid Motion on Micro- and Nanocrystalline Diamond/#-SiC Composite Gradient Films Tao Wang, Stephan Handschuh-Wang, Lei Huang, Lei Zhang, Xin Jiang, Tiantian Kong, WenJun Zhang, Chun-Sing Lee, Xuechang Zhou, and Yongbing Tang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04072 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Controlling Directional Liquid Motion on Micro- and Nanocrystalline Diamond/β-SiC Composite Gradient Films

Tao Wang1,ϯ, Stephan Handschuh-Wang2,ϯ, Lei Huang1, Lei Zhang3, Xin Jiang4, Tiantian Kong5, Wenjun Zhang6, Chun-Sing Lee6, Xuechang Zhou2,* and Yongbing Tang1,* 1

Functional Thin Films Research Center, Shenzhen Institutes of Advanced Technology, Chinese

Academy of Sciences, Shenzhen 518055, P. R. China 2

College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060, P.

R. China 3

Institute of Materials, China Academy of Engineering Physics, Mianyang 621907, P. R. China

4

Institute of Materials Engineering, University of Siegen, Paul-Bonatz-Str. 9-11, 57076 Siegen,

Germany 5

Department of Biomedical Engineering, School of Medicine, Shenzhen University, Shenzhen, 518060, P. R. China 6 Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. ϯ

Both authors contributed equally to this work.

*Corresponding author Email: [email protected]

(Yongbing Tang)

[email protected]

(Xuechang Zhou)

Abstract: In this article, we report the synthesis of micro- and nanocrystalline diamond/β-SiC composite gradient films, using a hot filament chemical vapor deposition (HFCVD) technique and its application as robust and chemically inert means to actuate water and hazardous liquids. As 1

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by scanning electron microscopy, the composition of the surface changed gradually from pure nanocrystalline

diamond

(hydrophobic)

to

nanocrystalline β-SiC

surface

(hydrophilic).

electron microscopy and Raman spectroscopy were employed to determine the presence of diamond, graphite and β-SiC phases. The as-prepared gradient films were evaluated for their ability to actuate water. Indeed, water was transported via the gradient from the hydrophobic (hydrogen-terminated diamond) to hydrophilic side (hydroxyl-terminated β-SiC) of the gradient surface. The driving distance and velocity of water is pivotally influenced by the surface roughness. The nano-gradient surface showed significant promise as the lower roughness combined with the longer gradient in transport distances of up to 3.7 mm, with a maximum droplet velocity of nearly 250 mm/s measured by a high-speed camera. As diamond and β-SiC are chemically inert, the gradient surfaces can be used to drive hazardous liquids and reactive mixtures, which was signified by the actuation hydrochloric acid and sodium hydroxide solution. We envision that the diamond/β-SiC gradient surface has high potential as actuator for water transport in microfluidic devices, DNA sensor and implants which induce guided cell growth.

Keywords: nanocrystalline diamond/β-SiC composite film, gradient surface, wettability, water transport, HFCVD

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Introduction Control of the wetting of surfaces and the directed transport of liquids aroused copious research interest due to its broad application range, such as laminar flow actuator for microfluidic devices or fuel cells, water collection devices, water repellent, in oil water separation devices, biodetection, cell adhesion, cell migration and differentiation, smart catalysis and liquid adhesion.1, 2, 3, 4, 5, 6 The actuation of liquids can be achieved by plenty of methods. Common to all these methods is the exertion of a force to the liquid. However, the conventional driving forces like mechanical pumps, centrifugal force, and hydraulic force have difficulties to meet the requirements of small size integration, low energy consumption and fast response. Novel actuation methods were developed, such as photocontrol,7 thermal energy, electrochemical,8 evaporation driving,9 surface tension,10 and asymmetric external forces, i.e. asymmetric oscillation, ratchet like surfaces in combination with symmetric oscillation, chemical gradients and structural gradients.11,

12, 13, 14

Among them, surface tension driving force is especially attractive because external energy is not required. Especially for the transport of small liquid quantities, chemical gradients, structural gradients and hybrids of both are developed.15 Here, the so called drag, the force pinning the droplet to the surface, can be reduced by use of microstructured surfaces and reduced pressure.16 Gradient surfaces feature, on top of the driving force for liquid actuation, the possibility to optimize a desired property by testing a large range of surface compositions on a single sample, yielding a high throughput experiment.17, 18, 19 Furthermore, sample to sample variations, which originate from slight deviations in sample preparation or chemical purity, can be avoided. Therefore, chemical gradients and their generation garnered plenty research attention in the scientific community. Typically, the synthesis of gradient surfaces, surfaces exhibiting a gradual shift in 3


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chemical characteristics, involve either self-assembled monolayers (SAMs), microcontact printing or lithography approaches.6, 15, 20 Methods for fabrication of chemical gradients, however, have broadly relied on SAMs, for example Lai et al. fabricated a wettability gradient by employing a hybrid out of a structural gradient and a SAM gradient. Advantageous is the inclined amplitude of water contact angle between the hydrophilic ( ̴ 40°) and the hydrophobic side ( ̴ 151°), which was improved by the texture of the sample, and the significantly extended length of water self-transport.15 A further extension of the chemical gradient to the cm scale was achieved by Fioravanti et al. by an electrochemical approach, utilizing organothiol SAMs. Firstly, an alkanethiole SAM was prepared on the substrate, which was subjected to partially reductive desorption via cyclic voltammetry (CV). The desorption was controlled by the withdrawing speed of the sample out of the desorption solution. Hereafter, the substrate was dipped in a hydroxyl modified thiol and the content of this new thiol was gradually inclined with increasing exposition time to the CV.21 These SAM strategies yield long chemical gradients for water transport, but they lack chemical inertness. Harsh chemical environments, such as organic solvents, chemical reaction mixtures and even strong acids/bases, are challenging for gradients prepared by SAMs and often the chemical gradient is swiftly degraded. In this regard, chemical gradients on carbon derivatives, such as graphene and diamond, are promising candidates. Hernandez et al. proposed the use of a physical mask during electron beam-generated plasma processing to functionalize graphene with either a smooth oxygenated or fluorinated chemical gradient that can push or pull water or dimethylethylphosphonate across the gradient.22 Though this approach is suitable for harsh environments, it lacks ability for fast and long distance transport of liquids. Therefore, other materials for chemical inert and highly resistant gradients need to be devised. Diamond, a famous gemstone, offers a number of remarkable properties, such as high hardness, 4


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high thermal conductivity, semiconductivity, chemical inertness and biocompatibility.23,

24, 25

Though diamond is considered chemically inert,26 the surface chemistry of diamond is rich and flexible and offers a variety of surface functionalization.27,

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All these properties recommend

diamond for applications in the field of implants, chemical and biological sensors, and DNA and protein chips.29, 30, 31, 32 Similarly, nanocrystalline silicon carbide (SiC) is an attractive substrate for copious applications, including biosensing, power devices and single photon sources,33, 34 because SiC possesses unique electronic properties, mechanical robustness, chemical inertness, thermal stability, non-toxicity, and biocompatibility.35, 36, 37, 38, 39 Furthermore, diamond and SiC exhibit a different surface termination,40 which implies the possibility of combining different surface energies. As the composition of diamond and SiC gradually changes, the surface free energy changes accordingly, which has grave impact on and may drive protein adsorption.17,

18

Most gradient

surfaces published are microgradient surfaces, rendering an application in water transport challenging due to contact line pinning.41, 42 Therefore, a lot of studies are devoted to the synthesis of diamond films with nano-sized crystals due to their low surface roughness and high wear resistance. One possible way to achieve such a combination is the synthesis of diamond/β-SiC composite gradient films, which have been deposited successful by microwave plasma and hot filament chemical vapor depositions (MWCVD and HFCVD).43, 44 In this article, we systematically study the film growth process of the novel nanocrystalline diamond/β-SiC gradient surfaces, and investigate their applicability to water transport. Furthermore, the ability to transport water is compared between microcrystalline and nanocrystalline diamond/β-SiC composite films with continuously varied surface chemistry. Here, especially the movement distance of advancing and receding contact line of the water droplet is investigated as the reduced roughness is envisaged to enhance the water transport. Furthermore, the transport speed of 5


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water on the nanogradient is investigated and corroborated with the surface coverage of diamond. Finally, the transport of hazardous aqueous liquids, such as 1 M NaOH and 1 M HCl, was analyzed. We envision that the chemically inert and mechanically durable diamond gradient surfaces possess potential applicability in transport of hazardous aqueous liquids, biodetection and cell adhesion and proliferation studies.

Experimental Section Film Deposition. Nanocrystalline diamond/β-SiC composite gradient films were synthesized by the hot-filament chemical vapor deposition (HFCVD) process, which featured a specific filaments/substrate configuration similar to our latest work.18 P-type (100) Si-wafers were utilized as substrates. Prior to film deposition, the substrates were cleaned in a piranha solution (H2SO4:H2O2 3:1) for 30 min, and subsequently ultrasonically seeded for 30 min employing a 5 nm nanodiamonds dispersion (0.05 wt.% in water). Two tantalum wires with diameter of 0.6 mm serve as hot filaments. The distance between the filaments and the sample surface is 10 mm (see also Figure S1). The gradient film was prepared by displacing the Si-wafers from the middle (between the two filaments) to a position, where the wafers are only partly under one hot filament (see also our previous publication17). The flow rates of H2, CH4 and tetramethylsilane (TMS, 1% TMS diluted in H2) were maintained at 500, 5 and 30 sccm. The temperature of the filaments, determined by an optical pyrometer, was 2400 ± 100 °C. The depositions were carried out at a constant gas pressure of 10 mbar. The parameters for deposition of micro- and nanocrystalline composite films were the same except for the substrate temperature. For micro-gradient films, the substrate temperature was kept between 883°C (± 20°C) and 762°C (± 20°C) along the length of substrate. For nano-gradient films, the substrate temperature was kept between 783°C (± 20°C) and 632°C (± 6


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20°C) along the length of substrate. Surface Treatment. After the deposition, the surface of sample was oxidized as follows. The film was first heated at 250°C for 30 min in an oxidizing mixture of concentrated H2SO4, sulfuric acid, and KNO3, potassium nitrate, in a beaker followed by washing ultrasonically three times in Milli-Q water and dried with flowing N2. Afterwards, the samples were reduced in HFCVD with H2 (flow rate of 80 sccm) at a gas pressure of 20 mbar for 20 min. Measurement of the water droplet transport distance on gradient surfaces. The gradient surface was placed parallel to the camera lens of the contact angle microscope. Water droplet with a volume of 2 µL were generated and placed near to the gradient surface. The gradient surface was carefully moved by micrometer screws in 0.1 mm steps until a droplet movement was observed. From the droplet movement, videos with a time resolution of 15 images per second and an image resolution of 640 x 512 pixel² were prepared. Afterwards, these videos were transferred into single optical images at a time interval of 66.7 ms. The distance of the advancing and receding contact line were ascertained by applying a calibration. Measurement of water droplet motion speed on the nanogradient surface. Similar to the movement distance of a droplet, the water droplet movement speed on the nanogradient surface was ascertained. The transport speed, however, is too high to measure it with a normal camera at frame rates between 15 and 30 Hz. Therefore, the sliding of the water droplet on the diamond gradient films was recorded with a high-speed camera (Miro Lab 110, Vision Research Phantom, USA) at a frame rate of 300 Hz as a video. Subsequently, the video was converted into sequential photographs with a time interval of 3.33 ms. The movement speed was determined by measuring the moving distance of the advancing or the receding contact line between the droplet and sample surface. 7


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Characterization. The surface morphology of the nanocrystalline gradient composite films was examined using a Zeiss Ultra55 Field Emission Scanning Electron Microscopy (FE-SEM, Carl Zeiss NTS GmbH, Germany). All measurements were performed with an operation voltage of 5 kV with the Inlens secondary electron detector. To determine the phase structure of the films, transmission electron microscopy (TEM) and Raman scattering were utilized. TEM images and diffraction pattern were obtained by using TEM (FEI Tecnai F20) to identify the correct number of the crystalline components present in the composite films. Raman scattering (LAabRam HR Visibie Raman Microscope) was carried out to identify diamond and non-diamond phases by utilizing a laser wavelength of 532 nm. The surface wettability on the gradient surface was determined with a contact angle microscope (SDC-200, Sindin). The static contact angle was measured with a droplet sized of 2 µL at 25°C. The contact angles stated in this article are averaged values measured at least 5 different spots on the surface while the error given denotes for the standard deviation.

Results and Discussions Growth of the diamond/SiC gradient film

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Figure 1. (a) Top view of the filament/substrate configuration (see front view in Supporting Information). (b) Fractional diamond coverage versus distance of the diamond/SiC gradient films, blue circles denote for nanodiamond and black squares microdiamond gradient surface. (c) SEM micrographs of nanogradient (scale bar 2 µm) and microgradient (scale bar 5 µm) diamond/SiC film deposited on Si along the gradient axis. The distance in (c) relates to the x-axis in (b).

Figure 1 shows the local SEM surface morphology of micro- and nanocrystalline diamond/β-SiC composite gradient films at several positions. At a position far away from the filaments, the film surface features exclusively pure β-SiC crystallites (distance 0 mm). The distance in Figure 1 represent the length of the gradient away from the pure β-SiC areas. Along the length of the sample, 9


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the content of diamond increased in the film (see Figure 1c). In Figure 1c, the bright areas correspond to diamond crystalline and the dark areas correspond to SiC. With decreasing distance to the filaments (increasing temperature during fabrication), the diamond fractional coverage is increasing. The formation of SiC at long distances is ascribed to less activated gases for diamond growth, which results in a lower content of diamond in the film. In contrast, the gases are sufficiently activated (H radical, methyl radical and temperature) to form diamond nearer to the filament. For the micro-gradient, the grain size of diamond is about 500 – 600 nm. At a low surface fractional coverage of diamond, single round-shaped diamond islands can be observed (i.e. at a distance 0.45 mm). At higher fractional coverage (at distance 0.72 mm), diamond and SiC were homogenously mixed at the surface. At pure diamond areas, microcrystalline diamond forms a coherent film. Likewise, in the case of nano-gradient, the SEM micrographs reveal the size of the nanodiamond crystallites to be in the order of tens of nanometers. The difference in surface roughness of micro and nanodiamond/SiC surfaces was already discussed previously.17 Briefly, the rms (root-mean-square) roughness of a microdiamond surface with a fractional coverage of around 6.4 % (10 x 10 µm2) was 67 nm, while the rms of nanodiamond surface signified with 40 nm (10 x 10 µm2) and 16 nm (1 x 1 µm2) a lower surface roughness. SiC areas showed a smooth surface with a rms of 8.8 nm.17 Revealed by the surface and corresponding cross-sectional SEM images in Figure S2, with increasing deposition time, the content of diamond increases, whereas that of β-SiC decreases. In contrast to the growth process of micro-gradient,44 a stage is reached where a larger diamond surface is available for the incoming species than β-SiC surface (see Figure S2). At this stage, β-SiC starts nucleating and growing on the diamond surface as well as on the already formed β-SiC surfaces. In this case, β-SiC grows faster than diamond, which occupies the growth space of 10


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diamond columns. The content of β-SiC increases, and will continue to increase, if TMS flux is kept constant during the deposition. Therefore, the growth of nano-composite film is attributed to a space competition between diamond and β-SiC nanocrystals. The existence of nanocrystalline diamond and β-SiC was confirmed by Raman spectroscopy and TEM. The Raman spectra at different positions of the nano-gradient film are given in Figure 2. From the areas consisting of diamond crystals, the Raman spectra are similar. The peaks at 1333 cm-1, 1350 cm-1, 1560 cm-1 and 1150 cm-1 are assigned to diamond, D band,45 G band and segments of trans-polyacetylene, respectively. For all measured diamond and composite films, the diamond peak appears to be broadened. The peak broadening is attributed to a reduction in the phonon’s lifetime that can be correlated with the concentration of point defects in diamond crystallites46 and decreasing crystallite size.47 The strong D and G bands of amorphous carbon and the existence of trans-polyacetylene indicate the existence of nanometer sized diamond crystals.48

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Figure 2. Raman spectra of nano-gradient composite film at different positions (several mm apart). The percentage given in this figure denotes for the surface fractional coverage of diamond on the nanodiamond gradient film.

To obtain more detailed structural information of the nanocrystalline diamond/β-SiC composite film, TEM images along with the selected area electron diffraction (SAED) of nanocrystalline composite film at the chemical gradient are shown in Figure 3. The crystal size of both diamond and β-SiC is mostly smaller than 20 nm. The diffraction pattern (Figure 3b) shows rings indexed to the (111) diamond, (111) β-SiC and (220) β-SiC phases. Reflexes from both diamond and β-SiC appear in the same diffraction pattern. This clearly shows the co-existence of diamond and β-SiC in the composite film. In addition, the uniformity of the rings indicates a homogeneous distribution of the crystallites. High-resolution TEM (HRTEM) images of diamond and β-SiC nanocrystals are shown in Figure 3c and 3d, respectively. Figure 3c and 3d show characteristic lattice fringes of diamond and β-SiC with interplanar spacing of their planes of 0.206 nm and 0.252 nm, respectively.

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Figure 3. (a) Bright-field TEM image and (b) SAED pattern of nanocrystalline diamond/β-SiC composite. HRTEM images of diamond crystal and of β-SiC crystal from the nanocrystalline composite film are shown in (c) and (d).

Water transport Surface wettability and maximum transport distance of water The diamond/β-SiC composite gradient films were utilized to study water transport. For liquid motion, the surface free energy and wettability are of pivotal importance. The surface wettability, surface free energy and water contact angles of a microgradient diamond/β-SiC surface were 13


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already discussed previously.18 Briefly, the water contact angle was found to change from ~20° on the β-SiC side to ~90° on the hydrogen treated microdiamond side. This change in water contact angle (wettability and surface energy) was attributed to the different surface termination. H-treated SiC featured silanol, C-OH and C-H termination.18 In contrast, H-treated microdiamond was found to be H-terminated. The H-termination on diamond is the reason for its hydrophobicity, whereas hydrophilicity of the SiC surface originates from its OH-termination. The total surface free energy was slightly lower on H-terminated diamond than OH-terminated SiC. Especially, the electron-donicity (basic,

) part was much lower on H-terminated diamond surface than that on

OH-terminated SiC surface. The water contact angle was linearly increasing with increased surface fractional coverage of microdiamond due to its H-termination.18 Because of the gradually changed contact angle on the diamond/SiC composite films, it is possible to realize droplet motion at small length scales. In contrast to the H-treated gradient surface, it is also possible to fabricate a hydrophilic gradient by oxidation.18 Here, the surface free energy was found to be slightly higher than on the H-treated surface while both, the diamond and SiC side featured surface termination (i.e. -OH) and thus, contact angles of 20° or lower were determined.18 The oxidized gradient, however, cannot be utilized for water actuation due to its small change in its wettability. As the surface roughness highly influences the water contact angle, both micro- and nano-sized diamond/SiC composite films were investigated. The water contact angle on H-treated nanodiamond was found to be around 97° (see Figure S3), which is slightly higher than the microgradient surface. This difference may be ascribed to the combined effect of the surface roughness and the existence of amorphous carbon and graphene at diamond grain boundaries, which has a grave efflux on the surface free energy of nanocrystalline diamond surfaces. It is well-established that amorphous carbon and graphene are considered to exist at the grain 14


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boundaries,49 which is a prominent factor for nanocrystalline diamond surfaces, increasing the apparent water contact angle while decreasing the surface free energy.50 For both diamond surfaces, the asperities are small enough to afford the droplet to be in the Cassie-Baxter state,51 resulting in an inclined water contact angle (WCA). However, the nanocrystalline diamond surface features small, yet abundant air pockets (lower roughness). Thus, the fractional surface coverage with air is higher than on microcrystalline diamond, resulting in higher WCA and lower surface free energy. The values of the contact angles are consistent with results published earlier.18, 52 In contrast to the diamond surface, the water strongly wets the SiC surface due to its smoothness17 and hydrophilicity. The water contact angle on SiC was observed to be around 20°, as measured by contact angle microscopy (Figure S3), and ≤ 10° (see video S1) for nanodiamond/SiC gradients. The wettability of H-treated diamond, SiC and the diamond/SiC composite gradient (both micro and nano) is assumed to be constant due to its inherent chemical inertness. For micro- and nano-sized gradient films, the surface roughness (micro- and nano grain size) and pH of the droplet on the motion distance was evaluated. Moreover, other parameters, such as the gradient steepness, which determines the gradient length, affect the displacement ability and displacement velocity towards liquids.53 The fractional diamond coverage on the gradient films is shown in Figure 1a. The micro-gradient surface gradient appears to be steeper (157 ± 14 %/mm) than the nano-gradient surface (110 ± 10 %/mm). This difference can also be observed in the length of the gradient films, which were calculated to be around 0.55 mm and 0.8 mm for the micro and nano-gradient surface, respectively. Figure 4 b and 4c shows clearly that the water drop moves spontaneously from diamond side to the SiC side on the nano- and micro-gradient films, respectively. In the sketch in Figure 4a, the movement of a droplet on the gradient film is illustrated. The oval shaped droplet signifies the 15


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difference in surface wettability between the diamond and the SiC side of the gradient. To the right, the diamond side is located, which is hydrophobic and possesses a high contact angle (low surface energy). To the left, the SiC surface is located, which is hydrophilic and possesses a low contact angles (high surface energy). Driving force ( Fγ ) is dependent on the change of surface energy and contact angle according to the following equation:54 dFγ = [(γ SV − γ SL ) a − (γ SV − γ SL ) r ]dx

(1)

dFγ = γ LV (cos θ a − cos θ r ) dx

(2)

where, γ LV, γ SV and γ SL are the surface free energies of the liquid-vapor, solid-vapor and solid-liquid interfaces, and dx is the thickness is the thickness of the section of the water drop. If the contact angle at point a is smaller than that at point r, the water drop will move in the direction of higherγSV. Thus, the droplet moves from the hydrophobic side (diamond, lowerγSV) toward the hydrophilic (SiC, higherγSV) side. The contact line between the droplet and the surface arranged toward the SiC is called the advancing contact line, where the water contact angle is called advancing contact angle θa. On the other hand, the receding contact line is located on the opposite side of the droplet, where the receding contact angle θr is located. The driving force becomes higher when the difference between contact angle θa and θr is larger. The change in contact angle over time/during the movement of the droplet and the displacement of the advancing and receding contact lines can be determined by facilely measuring the droplet movement over time. One important parameter of gradient films for applications in water actuation is the displacement distance of the water. In Table 1, the droplet displacement of water (and harsh water based chemicals, such as hydrochloric acid) on nano-gradient and water droplet on micro-gradient diamond/SiC film is shown. The advancing contact line of the water droplet (2 µL) was displaced by 3.73 mm on the nano-gradient surface, while on the micro-gradient surface a shorter 16


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displacement distance was observed (3.14 mm). The receding contact line, however, was moved for both gradient surfaces by a similar extent (around 1 mm). The difference of the advancing contact line results partly from the length difference of the gradient (0.25 mm difference). We ascribe the other part to the surface roughness of the micro-gradient surface, which may induce contact line pinning, thus, reducing the displacement distance. As stated previously, one feature of these diamond gradient films is the chemical inertness, which renders them useful for the transport of hazardous liquids, such as water based acids. The transport of hydrochloric acid (pH 0) and sodium hydroxide solution (pH 14) on the nano-gradient surface was investigated. Figure 4d and 4e shows the movement of HCl and NaOH drops on the nano-gradient surfaces. The displacement of the advancing contact line of hydrochloric acid and sodium hydroxide solution was 1.13 mm and 1.34 mm, respectively. Acidic condition were found to reduce the displacement distance of a water droplet, which was assigned to reversible protonation of the SiC (SiOH and an incline in water contact angle (WCA), resulting in lower driving force. Using HCl solution, the contact angle on SiC at a pH of 0 is with around 34° (see Figure S3) significantly higher than the contact angle observed for MilliQ water on the nanodiamond/SiC surface, yielding in a shorter transport distance of the droplet. The transport of 1M HCl is impeded by a rather high contact angle on SiC surface, which originates from protonation similar to the work of Mierczynska et al.,55 who prepared a pH responsive chemical gradient with amine and carboxylic acid surface termination. Indeed, carboxylic acid and amine are pH responsive and changed the wettability with varying pH. In contrast, at basic conditions, the driving force is increased (deprotonation of SiOH and reduced WCA). In addition, the sodium hydroxide solution by sticking to the needle during deposition of the droplet, as illustrated in Figure 4e. During this sticking period, the droplet moved already slightly. Thus, the collected speed and its accompanying energy is lower compared to the 17


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MilliQ droplet and a subsequent spreading is reduced, yielding a rather high contact angle. The adherence of the sodium hydroxide solution to the metal needle is attributed to surface oxidation of the metal, rendering the metal rather hydrophilic. Considering the bigger driving force at basic conditions (bigger ∆θ, contact angle difference), this nanodiamond/SiC gradient film has great potential for actuation of chemical mixtures at neutral and basic conditions. Moreover, the transport distance of aqueous liquids can be extended by use of multi gradients,56 such as a combination of chemical gradient and structural gradient, and by lengthening of the chemical gradient, which reduces the steepness of the chemical gradient.

Figure 4. (a) Sketch of a droplet moving on a gradient surface with advancing contact angle θa at the advancing contact line, highest point P and receding contact angle θr at the receding contact line. The solid line denotes for the position of the droplet before movement and the dashed line during/after movement. Movement of a water droplet on diamond/SiC microgradient (b) and 18


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nanogradient (c) surfaces measured with contact angle machine (15 frames per second). Here, right side is diamond. Transport of HCl (d) and NaOH (e) on the nanogradient surface, measured with high speed camera (300 frames per second). Here, left side is diamond.

Table 1 Movement distance of water droplets (2 µL) on diamond/SiC gradient surfaces.

Substrate, liquid

Movement of advancing

Movement of highest point

Movement of receding

contact line (mm)

of water droplet (mm)

contact line (mm)

Nanogradient, H2O

3.73

2.32

0.93

Microgradient, H2O Nanogradient, HCl

3.14 1.13

2.19 0.64

1.04 0.05

Nanogradient, NaOH

1.47

0.72

0.05

Velocity of water transport on nanogradient film To evaluate the movement of a water droplet on the nanogradient surface and to evaluate the movement of the hazardous liquids, the displacement speed of the liquids was measured, as illustrated in Figure 5a. For this measurement, a normal camera with only 15 frames per second was not enough. Therefore, a high-speed camera, capable of capturing up to 1000 images per second, was used. A frame rate of 300 pictures per second showed to be sufficient for quantitative measurement of the movement velocity of droplets. The movement of a water droplet as a function of time on the nanogradient surface was captured with the aforementioned high-speed camera and is depicted in Figure 5a. Here, we determined 3 displacements (advancing contact line, receding contact line and highest point), which are depicted in Figure 4a. The most important two are the advancing and the receding contact line, as a droplet in such an experiment are known to spread und change in length and shape. The displacement distance of a droplet versus time is depicted in the sequential pictures in Figure 5a. At first glance, the moved distance of the advancing contact line (Figure 5b) appears to possess a sigmoidal shape. The corresponding velocity profile of the advancing contact line of the droplet with time shows a steep increase in velocity of the droplet at 19


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the beginning (0-0.013 s). Afterwards, the advancing contact line slows down (0.015 s) and accelerates again (0.02 s). This behavior of slowing down and accelerating is observed several times. Concurrently, a similar wavelike behavior of the velocities of the highest point and the receding contact line can be observed. This behavior is also observable in the non-ideal sigmoidal curves in Figure 5b. The images from Figure 5a suggest that this behavior is due to the fast movement of the advancing contact line, which is as fast as the water droplet movement on a shape gradient with an opening angle of 8° by Zhang et al.53 However, compared to the shape gradient, the transport distance of the diamond/SiC gradient film is greater. In the aforementioned images, the water droplet appears to be deformed. The deformation originates from the fast velocity of the advancing contact line in comparison to slow reaction of the water droplet towards this pulling force and to sticking to the hydrophobic surface, which may be related to contact line pinning due to surface roughness. Furthermore, the droplet jumps into contact with the gradient surface, inducing an out of equilibrium force on the droplet. These effects potentially afford variations of the velocity of droplet advancing contact line, especially, as a similar velocity profile was not found for the slow movement of 1M NaOH solution. In contrast to this idea, Qiang et al. pointed out that the variation in the velocity profile mainly originates from the variation of surface energy gradient.57

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Figure 5. (a) Displacement of a water droplet (2 µL) on a nanogradient diamond/SiC gradient surface, measured with a high-speed camera. The total displacement was measured to be 3.77 mm, comparing Table 1 - measurement with contact angle microscope. (b) Excerpt of the distance covered by the droplet at specific times. Movement after 0.1 s appears to be rather slow. (c) Determined velocity of the advancing contact line, receding contact line and highest point of the droplet during displacement.

Moreover, the velocity profile of advancing contact line of MilliQ water droplet and the fractional coverage of diamond on the gradient film was plotted versus displacement of the droplet on the gradient surface, as shown in Figure 6a. The strong acceleration of the droplet motion at the beginning of droplet motion coincides with the steep gradient between 0.0 and 1.0 mm displacement. Afterwards, a slight incline of the velocity was observed, which is attributed to excess 21


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acceleration at the end of the gradient surface. Subsequently, the droplet is decelerated by several forces, amongst them are surface tension and hysteresis at the tailing end of the droplet, as well as friction and viscosity.58 The comparative plot in Figure 6a signifies that the variation in the velocity profile does not originate from variations in surface energy in the gradient as these variations have to originate in the diamond/SiC gradient film from fractional coverage gradients of diamond. In Figure 6b, the velocities of 1 M HCl, MilliQ water and 1 M NaOH dependent on the time are given. The fastest transport was observed for MilliQ water, while the maximum speed for 1 M hydrochloric acid was halved. This can be attributed to the lower driving force due to wettability change, resulting from the low pH value. Unsurprisingly, 1 M NaOH showed an even lower transport velocity on this gradient (due to attachment to the needle). However, at around 1.1 seconds, the liquid detached from the needle and further motion, albeit with low velocity was observed, and the final distance is in the end with 1.47 mm longer than for 1M HCl due to the diminished driving force at acidic conditions. We envision that this gradient film can be used for flow actuation in microfluidic devices, especially at basic conditions.

Figure 6. (a) Fractional diamond coverage and velocity of a water droplet versus displacement on the nanodiamond/SiC surface. (b) Comparison of the velocity of 1 M HCl, MilliQ water and 1 M NaOH on the nanogradient surface. The red circle denotes for the detachment of the NaOH solution 22


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from the needle.

Conclusions We demonstrated the fabrication of micro-/ and nanocrystalline diamond/β-SiC composite gradient films by careful manipulation of the sample position during the HFCVD process using H2, CH4 and TMS. Due to the wettability variation between the diamond side (hydrophobic) and SiC side (hydrophilic), a wettability gradient was obtained, which yielded spontaneous droplet motion in the created chemical gradient. The gradient length on the nanodiamond/SiC gradient surface was with 0.9 mm slightly longer than the microgradient with 0.6 mm, while the nano-gradient surface was smoother. Both the micro-/ and nanogradient film were employed in transport of water, 1 M NaOH and 1 M HCl. The micro-gradient surface showed a shorter transport distance of water (3.14 mm), compared to the nano-gradient surface (3.73 mm), where a maximum droplet velocity of nearly 250 mm/s was measured with a high-speed camera. Furthermore, the gradient was able to transport both 1 M HCl and 1M NaOH without chemical alteration of the film. In contrast, most other methods, such as self-assembled monolayers, lack this chemical inertness. Furthermore, the gradient films are mechanically durable and reusable. In future work, the droplet motion of diamond/SiC gradient films will be prolonged by extending the chemical gradient and geometric gradient. Considering that the surface chemistry of the gradient film is changeable, this gradient has not only great potential in water and hazardous liquid actuation but also in attaching biomolecules for investigation of biomolecular interactions, sensors (i.e. for DNA) and protein- and cell-adhesion studies.

Acknowledgments

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Generous financial support by the National Nature Science Foundation of China (51702350), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013C090), Shenzhen Peacock Plan (KQTD20161129150510559, KQJSCX2017033116124476), China Postdoctoral Science Foundation Grant (2017M612760), Shenzhen Science and Technology Planning Project (JCYJ20160122143155757,

JSGG20160301173854530,

JSGG20160301155933051,

JSGG20160229202951528,

JCYJ20170307171232348,

JCYJ20170307172850024,

JSGG20170413153302942), Guangdong Engineering Technology Research Center Foundation (No. 20151487), Shenzhen Engineering Laboratory Foundation (No. 20151837), and Scientific Equipment Project of Chinese Academy of Sciences (GJHS20170314161200165) are gratefully acknowledged. Supporting Information Additional figures and movies are available free of charge via the Internet at http://pubs.acs.org/.

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Controlling Directional Liquid Motion on Micro- and Nanocrystalline

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