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Is Superhydrophobicity Equal to Underwater Superaerophilicity: Regulating The Gas Behavior on Superaerophilic Surface via Hydrophilic Defects Moyuan Cao, Zhe Li, Hongyu Ma, Hui Geng, Cunming Yu, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05410 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Materials & Interfaces

Is Superhydrophobicity Equal to Underwater Superaerophilicity: Regulating The Gas Behavior on Superaerophilic Surface via Hydrophilic Defects Moyuan Cao,†* Zhe Li,† Hongyu Ma,‡ Hui Geng,† Cunming Yu,‡* and Lei Jiang‡ †

School of Chemical Engineering and Technology, State Key Laboratory of Chemical

Engineering, Tianjin University, Tianjin 300072, China. ‡

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

Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, China.

ACS Paragon Plus Environment

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ABSTRACT

Superhydrophobic surfaces have long been considered as superaerophilic surfaces while being placed in the aqueous environment. However, versatile gas/solid interacting phenomena were reported by utilizing different superhydrophobic substrates, indicating these two wetting states cannot be simply equated. Herein, we demonstrate how the hydrophilic defects on the superhydrophobic track can manipulate the underwater gas delivery, without deteriorating the water repellency of the surface in air. The versatile gas transporting processes can be achieved on the defected superhydrophobic surfaces; on the contrary, in air, water droplet is able to roll on those surfaces indistinguishably. Results show that the different media pressures applying on the two wetting states determine the diversified fluid delivering phenomena, i.e., the pressureinduced hydrophilic defects act as a gas barrier to regulate the bubble motion behavior under water. Through the rational incorporation of hydrophilic defect, a series of gas transporting behavior are achieved purposively, e.g. gas film delivery, bubble transporting, and anisotropic bubble gating, which proves the feasibility of this underwater air controlling strategy. KEYWORDS: superhydrophobic, superaerophilic, gas transport, controllable, hydrophilic defect


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1. INTRODUCTION Nature generously provides inspirations for us to develop advanced science and technology.1-5 During the last two decades, lotus-inspired superhydrophobic surfaces with extreme water repellency have drawn considerable attentions and exhibited great potential in the field of corrosion prevention, anti-icing/fogging, liquid separation, etc.6-12 As a complementary strategy for organism survival, the superhydrophobic property of diving bell spiders (Argyroneta aquatica), which is found in their legs and backs, effectively guarantees the underwater oxygen supply.13, 14 In aqueous medium, hydrophobic substrate manifests a significant adhesion of gas bubbles, which is also known as aerophilic surface.15, 16 Further incorporation of micro-/nanostructure to the aerophilic surface can facilitate a rapid process of bubble absorption and spreading.17-19 This phenomenon can be well understood in terms of the Cassie-Baxter state of superhydrophobicity, i.e., the “air cushion” upon the micro-structure acts as a channel for underwater gas transport.20-23 Taking advantage of this gas-involved special wettability, a series of researches have been proposed to optimize the current applications such as directional airflow delivery, efficient water remediation, gas consumption reactions, etc.24-33 Due to the insufficient researches of underwater wetting phenomena, the superhydrophobic (SHB) surface has been regarded as the equivalent to underwater superaerophilic (SAL) surface in the long term. However, although exhibiting similar water-repellent characteristics including contact angle and rolling-off angle, the SHB surfaces may display remarkably different gas transporting processes underwater, such as rapid gas spreading, bubble attachment, slow drainage of bubble.34-36 These experimental results reveal that it is inappropriate to equate the superhydrophobicity with underwater superaerophilicity. The deeper understanding of the difference between the SHB surfaces and underwater SAL surfaces should focus on the


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application scenarios of the functional interfaces. Compared to the droplet-on-SHB surface-in air system, the bubble-on-SAL surface-under water system suffers a much higher media pressure i.e. static water pressure (Pswp). Under a certain external pressure, the physical chemistry of the SHB surfaces could be changed by the random water infusion, especially when the SHB surface is highly defective.35 It is necessary that making random water infusion be controllable. The rational regulation of the water impregnation on SHB surfaces should offer great opportunity to reinforce the functions of SAL surface, promoting the development of fluid delivery systems. Here we report that the hydrophilic defects on superhydrophobic surface can effectively control the gas/bubble delivering behavior in aqueous environment. The hydrophilic (HL) defects were precisely fabricated by the laser ablation; accordingly, the bubble delivery processes including bubble transporting, gas film transport, anisotropic gas delivery, etc., have been fulfilled on the SAL surface via the design and integration of HL defects. More intriguingly, although the various gas delivering phenomena on the as-prepared defected SAL surfaces are observed beneath the water surface, the droplet rolling behavior in air exhibits no difference. The current finding should update our understanding of superhydrophobic state and serve as an inspiration for designing fluid manipulating interfaces. 2. EXPERIMENTAL SECTION 2.1. Preparation of silicone-based superhydrophobic surfaces: 1 g of polydimethylsilixone (PDMS) prepolymer (Dow Corning, Sylgard 184), containing 10% curing agent, was dissolved in 10 mL n-hexane. The glass slide was dipped into the as-prepared solution and coated with a thin film of PDMS. After solvent was mostly evaporated, the hydrophobic fumed silica (R-972,


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Evonic) was deposited onto the surface. The final superhydrophobic surface was obtained after cured at 80 °C for 2 hours. 2.2. Preparation of superhydrophobic testing samples: Superhydrophobic steel mesh was fabricated by two-step coating. In the first step, the mixed solution of 1 M CuCl2 and 1 M hydrochloric acid was used to construct microstructure on the mesh surface. In the second step, the mesh was modified by the 50 mM stearic acid/ethanol solution, and then the surface becomes superhydrophobic. For the fabrication of superhydrophobic copper film, the typical alkaline corrosion was applied. The clean copper film was treated by the mixture solution containing 2.5 M of sodium hydroxide and 0.13 M of ammonium persulfate. In subsequent, the superhydrophobic copper film was prepared by soaking in a ~ 5 mM dodecanethiol/ethanol solution for 24 hours. 2.3. Instrument and Characterization: All the contact angles of the substrates were measured by a SCA-20 contact angle geometer (Dataphysics, German), and the high-speed videos were captured by the high-speed video camera (i-speed 3, Olympus, Japan). The morphology of the substrates was observed by the environmental scanning electron microscope (ESEM) (Phenom G2-Pro, Phenom-World, Netherlands). The laser ablation of the superhydrophobic substrates is achieved by a nanosecond pulse CO2 laser with a fixed wavelength of 1064 nm (Chutian Laser Group, China). The gas flow was continuously supplied by a syringe needle with a speed of 300 mL/h. 3. RESULTS AND DISCUSSION Silicone-based materials have shown great priority to the fabrication of SHB surface with high quality, owing to this intrinsic hydrophobicity and high inertness.37 In order to generate the


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effective hydrophilic defects by laser ablation, the polydimethylsilioxide (PDMS) pre-polymer was uniformly coated on the pristine glass slide with hydrophilic surface. In subsequent, the dry powder of hydrophobic fumed silica (R-972, with an average diameter of 16 nm) was deposited on the surface to construct micro-/nano-scale surface roughness (Figure 1a and 1a’). With the coating layer cured at 80 °C, the as-prepared SHB surface exhibits excellent SHB properties, i.e., the contact angle is 153.4° ± 0.9° and the rolling-off angle of a 10 µL droplet is smaller than 2° (Figure 1d). The direct laser writing on the SHB surface can remove the hydrophobic layer and expose the hydrophilic glass surface with a contact angle of 34.2°± 1.0° (Figure 1e). The diameter of HL defect fabricated by single-time laser ablation is approximately 300 µm, and the thickness of the coating is ~50 µm (Figure 1b and 1c).


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Figure 1. The morphology and water contact angle of the gas delivering surfaces. (a) and (a’) The Scanning electronic microscope (SEM) images of the defect-free SHB surface. Classic micro-/nano-scale cooperative structure is observed. (b) and (b’) The SEM images of the linear HL defect on SAL surface. After the laser ablation, the SHB coating is removed and the glass surface is released accordingly. (c) The SEM image of the cross-section of the SHB surface reveals that the coating thickness is approximately 50 µm. The contact angle of (d) SHB surface and (e) glass surface (similar to the HL defect). The underwater gas delivering experiments were conducted on the SAL channels with an inclined angle of 10°, and the dots of HL defect were written on the channels with densities of ~ 20 dots/cm2 (Slight Defects), ~ 50 dots/cm2 (Medium Defects), and ~ 120 dots/cm2 (Serious Defects). The gas delivering processes are obviously changed along with the degree of surface defects. For a horizontal defect-free SHB surface, the gas primarily spreads on the surface and then retracts to original site due to the bubble formation (Figure 2a). Even with a tiny inclined angle (~ 1°) of the platform, gas can directionally transport driven by the buoyancy. During the gas delivery on defect-free surface, the continuous air plastron on the SHB surface acts as the channel for gas transport, and no visible bubble is found until the gas reaches the upper rim of channel. The bubble at the upper rim of channel keeps on accumulating until releasing to the water by overlarge buoyancy. In comparison, the defected SHB surfaces exhibit a different behavior when placing in aqueous environment. After wetted by water, the HL defect should serve as a motion barrier for gas transport. With a constant velocity of gas injection (300 mL/hour), a tiny protrusion can be seen on the slightly defected SHB surface (Figure 2b). The running bubble becomes big as the surface defects is increasing (Figure 2c), and finally the gas


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no longer delivers on the channel with serious surface defects due to the overlarge motion resistance (Figure 2d).

Figure 2. The gas delivering behavior on the SHB channels with various HL defects. (a) The defect-free SHB surface with the ideal air-cushion can facilitate a continuous and rapid gas transport even under a small tilt angle. The gas firstly spreads as a film and then starts to move driven by the buoyancy. During the gas delivery test, (b) a tiny bubble and (c) a big bubble was noticed on the SHB surfaces with slight and medium HL defects respectively. (d) The gas failed


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to deliver on the SHB surface with serious defects, indicating the overlarge resistance from the gas barriers. Scale bar is 2 mm. The critical spreading resistance and the maximum gas flux cooperatively contribute to the regulation of bubble behavior (Figure 3). The maximum width between two neighboring HL defects on the SAL surface determines the critical motion resistance of gas, i.e., the Laplace pressure (P).36,

37

When the gas starts to penetrate through the two HL dots, the gas/liquid

interface appears to be meniscus, resulting in the Laplace pressure against the penetrating direction (which equals to the quotient of water tension to the radius of meniscus).38 Meanwhile, the effective width (W) of gas penetrating channel plays an important role in the volume of running bubble. Under a constant gas injecting speed, bubble volume is increasing as the shrinking of the channel of flux. In brief, the critical defects gap determines whether the gas can successfully delivery on the SAL surface, and the width of effective channel influences the shape and volume of the running bubble. In detail, defect-free surface with maximum channel width and minimum resistance facilitates a gas-film delivery (Figure 3a). The front surface of gas film starts to deform when contacting with the hydrophilic defects, and subsequently passes through the barriers (Figure 3b). Slight increase of the hydrophilic defects may not influence the motion resistance but decrease the effective width of gas channel (Wc ≈ 0.67Wb), leading to the further accumulation of bubble (Figure 3c). The SAL surface with serious defects possesses a smaller gap-width of two HL defects and an enlarged resistance (Pd ≈ 4Pa ≈ 4/3Pb), on which the bubbles cannot move easily (Figure 3d). The increase of surface defects synchronously narrows the barrier gap and effective gas channel, which makes gas delivery difficult.


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Surprisingly, no matter what degree of the surface defects the SHB surface has, a 10 µL water droplet in air can easily roll off the surfaces in less than 0.5 sec (Figure S1). The significant difference between two systems can be explained by the background pressure. When rolling on the SHB surface in air, the droplet only presses the surface and the defects with a small water pressure (several times of the pressure from the droplet gravity). This Pswp cannot propel the liquid infusion of the defects, which suppresses the wetting risk of HL defects in air. The situation of underwater system is completely changed due to the enhanced Pswp, i.e., the surface defect becomes valid after being immersed in the aqueous solution. It is worthy noted that the additional pressure of water in air phase, such as droplet collision and water jetting, can also wet the hydrophilic defects and change the surface wettability, which do not discussed in this work.

Figure 3. The direct observation on the front-surface of spreading bubble and the proposed mechanism of gas delivery on the SAL surfaces with various HL defects. (a) Gas rapidly spreads on defect-free SAL channel with a negligible resistance. (b) The incorporation of HL defect acts


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as gas barriers, generating the Laplace pressure against the gas spreading. (c) With the increase of defects, the effective channel of gas flux was continuously decreased, resulting in the enlargement of running bubble. (d) The serious surface defects finally hamper the gas delivery on the SAL surface due to the overlarge resistance. The track width (w) and defect diameter (d) are about 5 mm and 0.4 mm respectively. r, P, and W represent the radius of gas front-surface, the motion resistance from the Laplace pressure, and the effective channel of gas flux, respectively. To verify the hypothesis of hydrostatic pressure in aqueous environment, two linear HL defects with diameters of 0.3 mm (Similar to the laser written defect) and 1 mm (Control group) were drawn on the SHB surface (Figure 4a). The patterned SHB slide was vertically and gradually inserted to the water, and then the classic air “cushion” was observed on the defect-free SHB surface. When water submerges the 0.3 mm defect, the air film keeps unchanged and continuous. Once the 1 mm defect is submerged, water will instantaneously wet the defect and deteriorate the air film. The wetting of 0.3 mm defect starts to appear only when the upper water height exceeds 25 mm, indicating the critical depth of gas-barrier formation. The mechanism of HL barrier formation under water can be briefly illustrated by the model shown in Figure 4b and 4c. The Laplace pressure (PLaplace) originating from the curved surface of water counteracts the upper Pswp and maintains the air cushion of SHB surface. As the Pswp is larger than PLaplace, the defect will be wetted to form a barrier to gas delivery. The critical depth of barrier formation (h), determined by the width of the defect (w) and the thickness of SHB coating (d), can be predicted by following equation (1):


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8 ∙ ∙ 2

∙∙ 1 2 ∙ 2 ∙ ∙

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where γ, ρ, and g represent the surface tension of water, water density, and gravitational acceleration, respectively. ɛ is the characteristic parameter of the defect, i.e., 1 for linear defect and 2 for dotted defect. The critical depth is plotted in Figure 4d as a function of defect diameter and coating thickness (Figure S2). According to the calculation, the critical depth of a 0.3 mm line-defect (Coating thickness is ~50 µm) is about 25 mm, which is matched with the experimental observation. The operation depth throughout the gas delivering experiment is larger than 50 mm, ensuring that the laser-created HL defect is able to act as the gas barriers.


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Figure 4. The effect of background pressure on the wetting condition of SAL surfaces. (a) The SAL surface is decorated with linear HL defects with diameters of 0.3 and 1 mm. The wetting process of the defects is strongly influenced by the Pswp, i.e., the 0.3 mm defect retains the air cushion until the immersion depth exceeds 25 mm. (b) the non-wetted and (c) wetted state of the defects are dominated by the interaction between the PLaplace and Pswp, and the anti-wetting ability of defect is determined by the coating thickness (d) and the defect width (w). (d) The calculated critical depth of defect wetting. The solid and dashed lines represent the linear and dotted defects respectively. For the testing sample in our experiment, the critical depth is about 25 mm. After knowing the effect of surface defect on the gas delivery, we can achieve a series of manipulation relating to the underwater gas behavior on SHB surface. Anisotropic interface has been regarded as a fascinating and efficient platform for controlling the fluid delivery.40-43 Employing an asymmetrical gas barrier, viz. angular shape defect, the channel can facilitate a smart gas gating process to manipulate the bubble delivery pathway (Figure 5 and Figure S3). As the gas is continuously supplied, gas firstly accumulates at the surface defect and turns to a visible bubble. While moving against the tip of the defect (Transporting direction), the bubble can break through the barrier and keep on running (Figure 5a). In the opposite direction (Releasing direction), the enlarged resistance from the long-side of barrier blocks the bubble penetration, resulting in the bubble detachment from the SHB surface (Figure 5b). By switching the direction of angular water barrier, the transport and release of bubble can be tuned. The major reason of the distinct gas delivering processes is that the effective lengths of three-phase contact lines (TCL) between two directions are clearly different. A tip shaped barrier with a shorter TCL is easier to stab into the bubble (Figure 5c), whereas the long TCL tends to hamper the gas delivery completely (Figure 5d).


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From another point of view, we can conveniently evaluate the quality of SHB surfaces via the underwater gas/solid interacting test. Although the droplet-based measurement on SHB surface are informative and common to characterize SHB properties, the information gained from the air phase may be not enough to identify the detail properties of SHB surfaces. We fabricated two kinds of SHB surfaces, i.e., the stearic acid coated steel mesh and the alkylthiol modified copper film (Figure S4). Both SHB surfaces show a large contact angle (> 150°) and a small rolling-off angle (< 10°). However, the underwater test clearly distinguishes the quality of SHB surfaces from each other. While the test sample was placed in the water with a 5 cm depth, the SHB steel mesh achieves a bubble-movable SAL surface; meanwhile, the bubble is merely stuck on the SHB copper film without further movement (Figure S5). Results quantitatively reveal that the surface defects of the SHB copper film is more serious than that of the SHB steel mesh, demonstrating a possible method for characterizing the surface superhydrophobicity.


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Figure 5. The anisotropic gas delivery on the SAL channel with asymmetrical HL defect. (a) The bubble can pass through the angular shaped defect from the tip side (Transporting direction). (b) In the opposite direction, the gas delivery is completely blocked by the long side of the defect, resulting in the bubble release (Releasing direction). The corresponding mechanism of anisotropic motion resistance is proposed in (c) and (d). The length of TCL has been considered the decisive factor for the manipulation of bubble behavior. CONCLUSION In the manuscript, we have investigated the difference between the SHB surfaces and the underwater SAL surfaces by introducing the defect effectiveness. In air environment, the hydrophobic capillary force existing in the defects is capable of maintaining the air cushion, i.e., the Cassie state, to endow the substrates with similar SHB properties. After immersing the SHB surfaces in aqueous environment, the hydrostatic pressure will overcome the hydrophobic capillary force, and the wetted defect on the SAL channel can act as a barrier to regulate the gas delivery. Based on the understanding of the underlying mechanism, we can realize versatile gas delivering phenomena on the SAL surface via the rational incorporation of HL defects, e.g., the gas-film transport, bubble transporting, smart gas gating process, etc. The current findings can provide valuable information on the research and applications of superhydrophobicity underwater and promote the development of gas-related interface chemistry as well.

ASSOCIATED CONTENT Supporting Information Available: The droplet-on-SHB surface-in air test of the SHB surfaces with different densities of surface defects. The top view of anisotropic gas gating process. The


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SEM images of the testing sample. (a) & (b) The SHB steel mesh, and (c) & (d) the SHB copper film. The formula derivation of the critical depth of water infusion. This material is available free of charge via the Internet at http://pubs.acs.org. Notes: The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author Email: [email protected] Email: [email protected] ORCID: Moyuan Cao: 0000-0002-1528-3096; Cunming Yu: 0000-0002-6824-9035; Lei Jiang: 0000-0003-4579-728X. Author Contributions The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript. M. Y. Cao, C. M. Yu and L. Jiang conceived and designed the experiments. M. Y. Cao, Z. Li, H. Y. Ma, H. Geng, and C. M. Yu performed the experiments. M. Y. Cao and C. M. Yu wrote the manuscript. ACKNOWLEDGMENT This work was supported by the State Key Laboratory of Chemical Engineering (SKL-ChE16B04), China Postdoctoral Science Foundation (BX201700020, 2017M620013), and the National Natural Science Foundation (21431009), and the 111 Project (B14009).


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