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Controlling the Accumulation of Water at Oil-Solid Interfaces with Gradient Coating Yan Li, Qiaomu Yang, Ran Andy Mei, Meirong Cai, Jerry Y.Y. Heng, and Zhongqiang Yang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05062 • Publication Date (Web): 18 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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The Journal of Physical Chemistry

Controlling the Accumulation of Water at Oil-Solid Interfaces with Gradient Coating Yan Li,† Qiaomu Yang,† Ran Andy Mei, †, § Meirong Cai,‡ Jerry Y.Y. Heng,§ Zhongqiang Yang*, †

†

Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of

Education, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics Chinese

Academy of Sciences, Lanzhou 730000, China §

Department of Chemical Engineering, Imperial College London, South Kensington Campus,

London SW7 2AZ, UK *Corresponding author. Corresponding Tel: +86-010-62796082

Email address: [email protected] (Y. Li), [email protected] (Q. Yang), [email protected]

(R.

A.

Mei),

[email protected]

(M.

Cai),

[email protected] (J. Y. Y. Heng), [email protected] (Z. Yang)

ACS Paragon Plus Environment

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ABSTRACT

We demonstrate a strategy to control the accumulation of water at the oil-solid interface by using a gradient coating. Gradient chemistry on glass surface is created by vapor diffusion of organosilanes, leading to a range of contact angles from 110° to 20°. Hexadecane is placed on the gradient substrate as an oil layer, forming a ‘water/hexadecane/gradient solid substrate’ sandwich structure. During the incubation time, water molecules spontaneously migrate through the micrometer thick oil layer and result in micrometer-sized water droplets forming at the oilsolid interface. It turns out that water droplets at more hydrophobic regions tend to be closer to a regular spherical shape, which is attributed to the higher contact angle that the water droplets have with the hydrophobic substrate. However, along the gradient from hydrophobic to hydrophilic, the water droplets gradually form more irregular shapes, as hydrophilic surfaces pin the edges of droplets to form distorted morphology. It indicates more hydrophilic surfaces containing more Si-OH groups lead to higher electrostatic interaction with water and higher growth rate of interfacial water droplets. This work provides further insights to the mechanism of spontaneous water accumulation at oil-solid interfaces and assists in the rational design for controlling such interfacial phenomenon.


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1. INTRODUCTION An interface is the area which separates two phases from each other1. Scientific study of interfaces is essential to understanding and advancing our knowledge concerning interfacial phenomena2,3, such as lubrication,4 capillarity,5,6 adsorption,7 and nanobubbles,8 while also enhancing our ability to further exploit these interfaces for practical applications from friction,9,10 oil-water separation,11,12 assembly,13-17 to chemical reactions.18-20 Interfacial phenomena are diverse and widely exist, and unique physical and chemical properties arise when a new interface forms. In terms of water/oil/solid sandwich structure, although scientists have gained broad knowledge on the interface created by any two out of the above three phases,21,22 the coexisting state of water/oil/solid still lacks a full understanding. It is easy to accept that these three phases are immiscible and well separated or involve very little diffusion, until Yang and Abbott reported an "abnormal" interfacial phenomenon that water can interact with the solid substrate across a micrometer thick oil layer in between, resulting in water spontaneously accumulated at oil-solid interfaces.23 From the perspective of thermodynamics, this interfacial phenomenon cannot be explained by simple theoretical calculation, but it is generally observed in a range of different oils and substrates. The revealing of this interfacial phenomenon would be intriguing for solving the problems such as efficiency loss of lubricants over time, oil-cleaning of transportation pipelines in oil fields24-26 and antifouling of surfaces etc.27,28 In order to further understand such formation of interfacial water droplets and potentially manipulate them, herein, we demonstrated a strategy to control the accumulation of water at the oil-solid interface by using gradient solid substrates.


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Herein, we report our experimental design, as illustrated in Scheme 1, where the gradient substrate

is

created

via

vapor

deposition

of

t-F8H2

(1H,1H,2H,2H-perfluoro-

decyltrichlorosilane). A micron meter thick oil layer, (e.g. hexadecane in this study, acting as the oil layer), is dripped on a TEM nickel grid, forming the oil-solid interface on the gradient substrate through capillary force. The whole system is submerged in pure water and monitored over time. Tailoring the surface chemical gradients, it clearly reveals the critical driving force, i.e., Si-OH groups, contributes to the migration of water molecules through the immiscible oil layer to absorb at the solid substrate. The main advantage of using such design is to allow indepth investigation of combinatorial interfacial phenomena on one substrate in parallel fashion.

Scheme 1. Schematic illustrations of (A) the chemical structure of t-F8H2, (B) controlled-vapor transport for preparation of surface gradients, and (C) cross section of experimental setup.

2. EXPERIMENTAL SECTION Materials. The glass microscope slides (Fisher’s Finest Premium grade) were obtained from Fisher Scientific (Pittsburgh, PA). t-F8H2 (1H,1H,2H,2H-perfluoro-decyltrichlorosilane) (CAS: 78560-44-8) and hexadecane were obtained from Alfa Aesar (Shanghai, China). Fluorescein


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sodium was purchased from Beijing Chemical Reagents Company (Beijing, China). Paraffin oil (CAS: 8012-95-1) was obtained from Sigma-Aldrich (Shanghai, China). Hydrogen peroxide (30% w/v) was obtained from Tianjin Damao Chemical Reagent Co. Ltd. Sulfuric acid was obtained from Beijing North Fine Chemicals Co. Ltd. Ethanol was obtained from Tianjin Guangfu Technology Development Co. Ltd. Nickel grids (20 µm thickness, 333 µm pitch, and 55 µm bar width) and glass cover slips (Cat. #72204-01) were obtained from Electron Microscopy Sciences (Fort Washington, PA). Syringes were obtained from Hamilton (Switzerland). Deionization of a distilled water source was performed using a Milli-Q system (Millipore, Bedford, MA) to provide water with a resistivity of 18.2 MΩcm. Preparation of Surface Gradients. Glass microscope slides were cleaned with “piranha” solution for 1 hour at 80 °C, then rinsed with MilliQ water and dried with nitrogen gas.29 The gradients of t-F8H2 on glass slips were created following reported methodology.30 40 µL of the diffusing solution (t-F8H2 : paraffin oil = 1:5 (w/w)) was placed onto a small Teflon strip (3 mm wide). The piranha-cleaned glass substrate was positioned horizontally 2 mm away from the diffusing solution, and the whole system was enclosed in a Petri dish and kept at ambient conditions. The humidity of the experimental environment was about 10%. After 5 minutes, the substrate was removed from the container, washed copiously with ethanol to remove any physically adsorbed t-F8H2 molecules, and dried with N2. Contact Angle (CA) Measurements. Static contact angle experiments were carried out using an optical contact angle measuring device (OCA 20, Dataphysics Instruments GmbH). We measured the contact angle against the gradient substrate of water droplets in air and in hexadecane. For the CA in air, a water droplet (3 µL) was deposited onto the glass gradient substrate and then measured. For the CA in hexadecane, a drop of water (3 µL) was added by


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syringe in the contact angle machine on the gradient substrate surface and then the substrate was directly immersed in hexadecane. Then the contact angles of water were measured accordingly. Each data point reported in the paper represents an average over three measurements on a fresh portion of the sample. X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS experiments were carried out using a PHI Quantera SXM Scanning X-ray Microprobe (Ulvac-PHI. INC. Japan). Binding energy is calibrated with C1s = 284.8 eV. Each data point reported in the paper represents an average over three measurements on a fresh portion of the sample. Atomic Force Microscope (AFM) Measurements. AFM experiments were carried out using a Dimension Icon atomic force microscope (Bruker Technology Co., Ltd, Germany). The contact mode was ScanAsyst. Formation of Interfacial Water Droplets. The Ni grid was placed onto the predetermined positions along the surface (2.5 cm × 7.5 cm) of the gradient substrate. A drop of hexadecane was deposited into the grid, and the excess oil was removed by using a 10 µL syringe in order to obtain a uniformly filled grid. The hexadecane acts as an oil layer placed on the gradient substrate. The whole system was placed into 10 mL glass wells containing 8 mL of distilled and deionized water. The experiments were carried out at room temperature (25 °C). Optical Characterization of Interfacial Water Droplets. A Nikon ECLIPSE Ti microscope with a digital camera (Nikon DS-U3) was used to image the samples. Differential Interference Contrast (DIC) images were obtained by adding a DIC prism. LSCM (Laser Scanning Confocal Microscopy) Characterization of Interfacial Water Droplets. Zeiss LSM800 with Airyscan model inverted laser scanning confocal microscope was


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used. A micrometer thick film of hexadecane supported on the gradient substrate was submerged into water and incubated for 3 to 4 days. Subsequently, fluorescein sodium (ex/em 496 nm/518 nm) was added to get a final concentration of 4 µM. The sample was incubated overnight followed by exchanging aqueous solution with 300 mL of water to remove free fluorescein sodium. In order to match the working distance of LSCM, the sample was inverted over a coverslip and supported by a spacer (adhesive tape) to ensure the hexadecane layer was roughly 60 µm above the coverslip and maintained in aqueous solution during sample handling and imaging. LSCM was employed and interfacial water droplets were scanned at the Z axis direction. A 488 nm laser was used to excite fluorescein sodium and an emission filter was used to collect the fluorescent light at 509 nm.

3. RESULTS AND DISCUSSION Characterization of the Surface Gradients. The aim of our first set of experiments is to obtain the gradient glass substrate by vapor deposition of t-F8H2.30-32 The preparation is sketched in Scheme 1. Initially, t-F8H2 and paraffin oil at a weight ratio of 1:5 were mixed to form the source of t-F8H2, which was placed onto a Teflon strip next to the bare glass substrate and cleaned with “piranha” solution.29 With the vapor diffusion of t-F8H2, it generated a gradient of t-F8H2 vapor whose concentration decreased along the length of the glass substrate. As t-F8H2 reacted with the silicon hydroxyl groups on the glass substrate, a gradient t-F8H2 modified glass substrate was generated. It turned out that near the source, the substrate possessed a higher density of t-F8H2 and higher hydrophobicity which decreased over the distance. To learn more about the interfacial properties of the modified substrate, we conducted contact angle (CA) measurements upon different positions on the gradient substrate (Figure 1). It is clearly


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seen that the degrees of CA of 3 µL water droplets decreased gradually along the gradient substrate.

Figure 1. Contact angle photographs of 3 µL water droplets covering different positions along the gradient substrate, (A) 8 mm, (B) 12 mm, (C) 16 mm, (D) 20 mm, (E) 24 mm, (F) 28 mm, respectively. To better understand the chemical composition on the gradient surface, we carried out X-ray Photoelectron Spectroscopy (XPS) and found that over a distance of 20 mm, the element percentage of F1s and C1s originating from t-F8H2 decreased gradually from 49.05% to 1.25% and 30.04% to 18.24%, respectively (Figure 2 and Figure S1).33 Meanwhile, the value of Si2p and O1s originating from unreacted silicon hydroxyl groups on bare glass substrate increased from 7.28% to 20.42% and 13.63% to 58.42%, respectively. These changes revealed that along the gradient substrate the density of t-F8H2 molecules decreased, while on the other hand, the number of unreacted silicon hydroxyl groups increased.


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Figure 2. The percentage of atomic concentration versus the position along the gradient substrate measured by XPS. AFM imaging was used to further characterize the gradient t-F8H2 modification. For example, AFM results (Figure S2) demonstrated that when the deposition distance was 28 mm away from the vapor source, little t-F8H2 reacted onto the surface and the substrate was very smooth,34 which agrees well with the contact angle measurement of a slight increase to 26°. In addition, XPS demonstrated that F1s and C1s originating from t-F8H2 were low, 1.25% and 19.23% respectively. Along the substrate closer to the vapor source of t-F8H2, AFM imaging showed that the surface became rougher and protuberant circles appeared with increasing sizes, which are the direct evidence of the deposition of t-F8H2. The increasing composition of F1s and C1s in XPS analysis and higher contact angle of the gradient surface all proved that the density of tF8H2 molecules increase along the substrate.

35,36

It also noted that the more deposition of t-

F8H2, the rougher of the substrate, the increase of the roughness may further increase the contact angle than smooth surface with the same chemistry composition.37,38 The presence of the surface roughness may also contribute to the water nucleation and accumulation, considering the trend of interfacial water droplets formation, the contribution from the surface roughness comparing to


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the surface chemistry is not dominating. The design of a substrate with same surface chemistry with various roughnesses is worth in-depth investigating. Optical Characterization of Interfacial Water Droplets. With the gradient surface prepared, we used the substrate to manipulate the formation of interfacial water droplets. In a typical experiment, a Ni grid (20 µm thickness, 333 µm pitch, and 55 µm bar width) was placed onto the gradient surface at the stated position along the substrate. A drop of hexadecane was filled into the Ni grid with a 10 µL syringe and subsequently the excessive oil was extracted away to form a micrometer thick oil layer. The sample was placed into a glass weighing bottle containing 8 mL MilliQ water. The bottle was covered with a lid to prevent water evaporation. The sample was monitored with a microscope over a period of 9 days. The oil layer, hexadecane, is somewhat volatile, but the hexadecane is immersed under a large bulk of water, with the oil layer being integrated during the experiment. The upper water phase is large enough to resist the slight evaporation. The pH of the water was also monitored, which was kept stable at around 6.37 to 6.70 for 9 days (Figure S3). First, examination of each column from the top to bottom in Figure 3 reveals that interfacial water droplets during incubation grew gradually. The images in each row from left to right corresponded to the interfacial water droplets formed at a specific position along the substrate. For example, the diameter of water droplets growing at the 8 mm position along the substrate increased to 20.2 µm on the 9th day. Other samples incubated at different positions on the gradient substrate exhibited similar trends. This agrees well with previous study that interfacial water droplets coalesce and grows to bigger ones. It is also noticed that once the sample was submerged under water and immediately examined by microscope within 10 minutes, a few micronmeter sized water droplets appeared. The source of water accumulated was likely from the


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trace amount of water in the oil, the water adsorbed on the solid substrate in air and/or the water diffused from the bulk solution above the oil layer. Due to the consistent diffusion of water through the oil layer in and out, it is almost impossible to distinguish where the origin of water comes from within the first 10 minutes, which is worth further investigation. Next, over a specific incubation time, the interfacial water droplets at different positions along the substrate grew at different rates with different morphologies. In general, the more hydrophobic regions which were close to the t-F8H2 source during preparation appeared to form fewer and smaller interfacial water droplets. In contrast, the more hydrophilic regions towards the right end of the substrate promoted water accumulation, resulting in more and bigger interfacial water droplets. This result suggests that in a gradient substrate, the more hydrophilic surface favors interfacial droplet formation over a hydrophobic one. This is likely because the hydrophilic surfaces, as reported previously, possess more charges arising from the residual silicon hydroxyl groups. It is clear that hydroxyl groups (Si-OH in this study) play an important role in promoting water molecules accumulated at the oil-solid interface. The long-range attraction between hydroxyl groups and water molecules is the key driving force.

39-43

From a

chemical viewpoint, we describe that the increase of Si-OH groups would enhance the driving force of the migration of water molecules through the immiscible oil layer to condense at the solid substrate. From a physical viewpoint, the increase of Si-OH groups on the substrate will lead to the decrease of total interfacial free energy of the system. The interfacial water droplets also tended to form a more regular spherical shape over hydrophobic regions, especially at positions shorter than 24 mm along the substrate, corresponding to the images in columns A-E in Figure 3. This is attributed to the hydrophobicity of the substrate, which tends to dewet water into round droplets. The threshold of contact angle of the underlying substrate in this study to


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promote a spherical morphology of interfacial water droplets was ca. 35°. In contrast, the hydrophilic surface with contact angle less than that caused the formation of an “island” like water film. The irregular shape likely arises from the pinning effect of hydrophilic substrates.44,45 These results suggest that the rate of formation of water droplets and their shape can be controlled by tuning the underlying substrate.


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Figure 3. Differential Interference Contrast (DIC) images of hexadecane hosted in gold grid supported on positions along the gradient substrate (A) 8 mm, (B) 12 mm, (C) 16 mm, (D) 20 mm, (E) 24 mm and (F) 28 mm in contact with water at room temperature. Scale bar is 50 µm. Two questions may arise: first, the uniformity of the oil layer, the current setup we employed is to use Ni grid to hold oil layer by capillary force, such that a relatively uniform oil layer can be achieved. The thickness of the oil layer is roughly tens micrometers, whereas the precise the thickness of the oil layer is challenging to control with current techniques. Our experiment may cause undulation of the oil layer, considering the thickness of oil layer is in the same scale, which did not show a dominating influence on the interfacial water droplets formation from sample to sample. The future study to investigate the relationship between the interfacial water droplets formation and a wide range of oil layer thickness is ongoing; second, the pathway of water molecules migrating to the underlying substrate, we proposed that water molecules migrate from the top bulk water phase to the oil-solid interface for the following reason: if the grid or the side of the oil-solid interface play a role, for example, water migrates along the walls of the grid or through the side of the oil-solid interface, which would cause a gradient of the water droplets size along incubation, which was not observed in our experiments. LSCM Characterization of Interfacial Water Droplets. In order to examine the morphology of the interfacial water droplets in three dimensions, we used water soluble dye fluorescein sodium which is expected to diffuse into interfacial water droplets. The hexadecane layer supported on gradient substrate was incubated in water for approximately 3-4 days, and then fluorescein sodium was added to the aqueous solution with a final concentration of 4 µM. After overnight incubation, the aqueous solution above the hexadecane layer was exchanged with 300 mL of water to remove free fluorescein sodium. LSCM with an Airyscan model was


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used and interfacial water droplets were scanned at the Z axis direction.41 Figure 4A-F showed a clear trend, very similar to Figure 3, that along the gradient substrate, the wettability varied from hydrophobic to hydrophilic with a contact angle decreasing from 152° to 55°. In contrast, the contact angles of water droplets (3 µL) on gradient substrate measured in air ranged from 110° to 20°. The contact angle data is summarized in Figure 4G. In order to distinguish them with interfacial water droplets, we labelled the water droplets in the air and in hexadecane as artificial water droplets, since they were deliberately made by putting one drop of water on the substrate. It was found that on average the spontaneously formed interfacial water droplets in oil had a higher CA than the 3 µL artificial water droplets in air measured at the same position along the gradient substrate. This is because the solid-oil surface tension is higher than solid-gas surface tension.46,47 From the results, in the range of distance from 8 mm to 20 mm on the gradient surface, the contact angles of artificial water droplets in hexadecane were almost the same as those of the interfacial water droplets measured by confocal imaging. However, from 20 mm to 36 mm on the gradient surface, the contact angles of artificial water droplets were smaller than those of the interfacial water droplets. We also noticed that from 28 mm to 36 mm on the gradient surface, the contact angles of artificial water droplets were more similar to those of water drops (3 µL) measured in air. This could be attributed to the more pronounced pinning effect from a more hydrophilic surface, where the interfacial water droplets are simply pinned during growth. The difference of artificial water droplets is those droplets already exist. They do not grow as interfacial water droplets do, and therefore, they are pinned in the first case upon putting in the oil. Another important piece of information extracted from this experiment is that the morphology of the interfacial water droplets can be controlled by choosing a desirable position on gradient


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substrate. It can be envisioned that this setup allows for a series of experiments to be done simultaneously and in a parallel manner on one piece of gradient substrate.

Figure 4. Confocal fluorescent images of interfacial water droplets grown on the different positions along the gradient substrate (A) 8 mm, (B) 12 mm, (C) 16 mm, (D) 20 mm, (E) 24 mm and (F) 28 mm, respectively. (G) The contact angle curve of water droplets (3 µL) on gradient substrate measured in air (black squares) and in hexadecane (blue triangles) by contact angle machine. Interfacial water droplets spontaneously formed at the oil-solid interface measured in hexadecane (red circles) with a fluorescent confocal microscope. The dotted lines are used to mark the transformational point clearly.

4. CONCLUSIONS


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We have successfully controlled the accumulation of water droplets between oil and solid interfaces via a gradient solid substrate, in particular the growth rate, size and morphology of interfacial water droplets. The more hydrophilic sites on the gradient substrate accelerated interfacial water accumulation than hydrophobic ones. The former possessed a lower density of hydrophobic molecules and on the other hand more silicon hydroxyl groups providing a stronger interaction with water molecules, consequently leading to a faster formation of interfacial water droplets, and vice versa. Another important conclusion drawn from this study is that interfacial water droplets at more hydrophobic regions tended to be regular spherical shape, attributed to the hydrophobicity of the underlying substrate dewetting the water droplets to be rounded. In contrast, along the gradient from hydrophobic to hydrophilic, the interfacial water droplets gradually turned to irregular shapes, as hydrophilic surfaces wetted water better than hydrophobic surfaces, while pinning the edge of interfacial water droplets to distorted irregular shapes during interfacial water accumulation. This work not only further investigates the mechanism of interfacial water droplets formation, but also provides better understanding of how to employ gradient surfaces to control water molecules accumulating at the oil-solid interface. It also opens up another broad question regarding the water/oil/solid structure, especially in the use for water-oil separation, since the long incubation time may induce water accumulation at the oil/solid interface which is usually neglected in past studies.

ASSOCIATED CONTENT Supporting Information XPS analysis of the different positions on the gradient substrate; AFM images of the different positions on the gradient substrate; the pH of the upper water during 9 days.


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NOTES The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21474059, 21421064), and the National Basic Research Program of China (2013CB932803). We would like to acknowledge the assistance of Imaging Core Facility, Technology Center for Protein Sciences, Tsinghua University, for assistance of using Zeiss LSM800 with Airyscan model instrument.

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TOC Graphic


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Scheme 1. Schematic illustrations of (A) the chemical structure of t-F8H2, (B) controlled-vapor transport for preparation of surface gradients, and (C) cross section of experimental setup. 334x133mm (120 x 120 DPI)


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Figure 1. Contact angle photographs of 3 µL water droplets covering different positions along the gradient substrate, (A) 8 mm, (B) 12 mm, (C) 16 mm, (D) 20 mm, (E) 24 mm, (F) 28 mm, respectively. 344x54mm (120 x 120 DPI)



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Figure 2. The percentage of atomic concentration versus the position along the gradient substrate measured by XPS. 289x203mm (300 x 300 DPI)


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Figure 3. Differential Interference Contrast (DIC) images of hexadecane hosted in gold grid supported on positions along the gradient substrate (A) 8 mm, (B) 12 mm, (C) 16 mm, (D) 20 mm, (E) 24 mm and (F) 28 mm in contact with water at room temperature. Scale bar is 50 µm. 315x287mm (120 x 120 DPI)



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Figure 4. Confocal fluorescent images of interfacial water droplets grown on the different positions along the gradient substrate (A) 8 mm, (B) 12 mm, (C) 16 mm, (D) 20 mm, (E) 24 mm and (F) 28 mm, respectively. (G) The contact angle curve of water droplets (3 µL) on gradient substrate measured in air (black squares) and in hexadecane (blue triangles) by contact angle machine. Interfacial water droplets spontaneously formed at the oil-solid interface measured in hexadecane (red circles) with a fluorescent confocal microscope. The dotted lines are used to mark the transformational point clearly. 336x242mm (120 x 120 DPI)


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