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. 2019 Oct 23;4(19):18304–18311. doi: 10.1021/acsomega.9b02376

Air-Stable Aerophobic Polydimethylsiloxane Tube with Efficient Self-Removal of Air Bubbles

Jinyoung Park 1, Seeun Woo 1, Seongmin Kim 1, Moonsu Kim 1, Woonbong Hwang 1,*
PMCID: PMC6844094  PMID: 31720531

Abstract

graphic file with name ao9b02376_0006.jpg

The adherence of underwater air bubbles to surfaces is a serious cause of malfunction in applications such as microfluidics, transport, and space devices. However, realizing spontaneous and additional unpowered transport of underwater air bubbles inside tubes remains challenging. Although superhydrophilic polydimethylsiloxane (PDMS) tubes are attracting attention as air bubble repellents, superhydrophilic PDMS, which is fabricated via oxygen plasma treatment, has a disadvantage in that it is weak against aging. Here, we present a tube with the ability to self-remove air bubbles, which overcomes the drawback of rapid aging. PDMS containing Silwet L-77 with a hierarchical nano–microstructure exhibiting subaqueous aerophobicity was fabricated. We conducted adherence and saturation experiments of air bubbles using the fabricated PDMS tube with Silwet L-77 to investigate the mechanism of bubbles adhering to and separating from the fabricated tube surface. The developed PDMS with Silwet L-77 exhibits a strong self-removal effect with an air bubble removal of 97.7%. The adherence and saturation experiments suggest that the transparent superhydrophilic–underwater aerophobic PDMS is a potentially exceptional tool for spontaneously separating air bubbles attached to tube surfaces.

1. Introduction

The unique wetting behavior of many natural surfaces such as a lotus leaf and a water strider has been of great interest and an inspiration for many applications such as self-cleaning, anti-icing, and anti-fogging.1−5 Superhydrophobic surfaces are characterized by a high water contact angle (greater than 150°) and a low sliding angle (less than 10°). In contrast, superhydrophilic surfaces are generally defined by very low water contact angles (less than 10°), which means that they allow complete dispersion of water droplets in a matter of seconds. In recent years, the behavior of this special wettability surface in water has attracted wide attention considering the frequent occurrence of air bubbles in water, which may cause severe damage to various systems.

Considering that buoyancy is proportional to the volume and adhesive force is proportional to the area, adhesive force becomes relatively stronger with decreasing size. Accordingly, a problem arises where adhesive force becomes more dominant than buoyancy in microfluidics devices.6,7 Furthermore, in transportation devices, such as heat exchangers or artificial blood vessels, air bubbles trapped in pipelines often accumulate and cause blockage to fluid transportation.8−10 Air bubbles are also problematic in space devices because buoyancy is not observed in environments without gravitational force.11,12 Several workarounds have been investigated to address the problem of air bubbles attached to such surfaces. In an effort to create bubble-repellent surfaces, special wettability surfaces such as superhydrophilic materials have been used. Despite few exceptions,13 superhydrophilic surfaces generally exhibit underwater superaerophobicity and superhydrophobic surfaces exhibit underwater superaerophilicity—with superhydrophobic and superhydrophilic surfaces, air bubbles behave similar to oil droplets in water.14−16 Superaerophobic surfaces are generally defined by a high air bubble contact angle (greater than 150°) and a low sliding angle (less than 10°). In contrast, superaerophilic surfaces are generally defined by very low air bubble contact angles (less than 10°).17−21 Therefore, air bubbles frequently adhere to a superhydrophobic surface but not to superhydrophilic surfaces. Many studies have attempted to fabricate superhydrophilic polydimethylsiloxane (PDMS) (aerophobic), but the fabrication processes are very complicated and involve oxygen plasma treatment, which reduces the longevity of surface characteristics.22−24 Despite these disadvantages, oxygen plasma treatment is required because it ensures high surface energy for attaining aerophobicity in PDMS25−27 and PDMS surface modification is difficult using other methods due to chemical inertness.27−29

In this research, we fabricated a PDMS tube with hierarchical nano–microstructures and superhydrophilicity–underwater-aerophobicity, using a method of structure replication with the addition of Silwet L-77. Silwet L-77 is a wetting agent and a type of silicone surfactant. Similar to other surfactants, it comprises hydrophilic hydrophobic moieties (Figure S1).30 Silwet L-77 does not react with PDMS and spreads evenly in PDMS when mixed. When curing PDMS, mixed Silwet L-77 is evenly distributed in PDMS.31,32 The prepared PDMS tube successfully realizes self-removal of air bubbles attached to the surface. We conducted an underwater air bubble adhesion experiment to investigate the mechanism of bubbles adhering to and separating from the fabricated tube surface. The tube exhibited excellent self-removal performance, and the bubbles were found to be removed when their accumulation reached a saturated state. This is the surfactant diffusion phenomenon that occurs in PDMS containing surfactant such as Silwet L-77, and it has been confirmed that the same phenomenon occurs in underwater environments.33,34 The developed process used Silwet L-77 to fabricate the superhydrophilic PDMS, which exhibits underwater-aerophobicity. This surface overcame the drawback of rapid weakening aerophobicity of O2 plasma-treated surface with aging.35−37 Overall, the developed PDMS tube exhibited a high self-removal ratio for air bubble formation and excellent stability, and it can be stable in air. We believe that the information gained from this research can greatly simplify air bubble detachment methods for solving air-bubble related issues in various transportation pipelines.

2. Experimental Section

2.1. Materials

Industrial Al (99+%, Aluko Co., LTD) plates and rods were used to prepare replica molds. Al plates of 30 × 30 mm were used to prepare molds for flat replicated surfaces, and Al rods were used for tube-type replicated surfaces. The diameter and length of the Al rods for replication were 10 and 100 mm, respectively. Hydrochloric acid (HCl), sodium hydroxide (NaOH), and n-hexane (C6H14) were purchased from Samyoung Chemical Co., Ltd. Heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane (HDFS) was obtained from Alfa Aesar. PDMS (Sylgard 184, monomer and curing agent) and Silwet L-77 were purchased from Dow Corning.

2.2. Replication of PDMS/PDMS Silwet L-77 with Mold

An ultrasonically cleaned Al plate was etched in 1 M NaOH solution at room temperature for 10 s for cleaning. After washing the plate in distilled water, it was etched in 1 M HCl solution at 70 °C for 1 min to fabricate microcubic structures. Then, the plate was immersed in 1 M NaOH solution at room temperature for 1 min and dipped in deionized (DI) boiling water at 100 °C for 10 min to fabricate nanoflake structures. After the process, the fabricated aluminum plate developed nano–micro hierarchical structure and superhydrophilicity. The plate was dipped in a 1:1000 solution (v/v) of HDFS in n-hexane for 10 min to apply a superhydrophobic coating onto the surface. By dipping the Al mold in the HDFS solution, a self-assembled monolayer (SAM), which is a horizontal molecular coating with low surface energy on the target surface. After the SAM coating on the surface, the surface became superhydrophobic by very low surface energy developed on the surface without any structure change.38

PDMS and PDMS with Silwet L-77 were added to the abovementioned molds. The replication process was inspired by the work of Kim.39 Sylgard 184 and a curing agent were applied in a weight ratio of 10:1 to the prepared surface in order to generate a superhydrophobic plate. In the case of the superhydrophilic plate, 0.1, 0.3, and 0.5 wt % of Silwet L-77 was added. The PDMS and mold structure were placed in a vacuum chamber for 2 h to remove bubbles between them, such that the structure adhered well to the surface. Then, the surface was maintained horizontal at room temperature until holes that formed as air bubbles escaped were flattened. With the addition of Silwet L-77, viscosity increased, depending on the ratio of Silwet L-77; the waiting time also varied depending on the ratio. After flattening, it was cured in a 70 °C chamber for 6 h and the fabricated PDMS was carefully removed from the mold. For replicating the superhydrophobic surface, the same procedure was applied but Silwet L-77 was not added. In addition, to prepare the bare surface, the same procedure was applied, but without the Silwet L-77 and nano–micro fabricated molds. All molds of bare surfaces had flat surfaces.

2.3. Fabrication of Aluminum Mold and PDMS Tube

To fabricate the Al mold, an Al rod was cleaned with 0.5 M NaOH solution for 1 min. Subsequently, microstructures were formed by etching the Al rod for 15 min in 2.5 M HCl solution. Then, impurities in the Al rod were cleaned by dipping it in a desmut solution, which was composed of DI water, nitric acid, and ammonium bifluoride (50:50:3 weight ratio), for 45 s. Nanostructures on the Al rod were formed by soaking in a 95 °C DI water for 5 min after immersing the microstructured Al rod in a 0.5 M NaOH solution for 5 s. From the above process, a micro- and nano-structured Al rod was obtained. To prevent adhesion between the Al rod and PDMS replica, an antiadhesion layer was generated on the Al rod surface by soaking it in a coating solution for 10 min. The soaking solution was a mixture of n-hexane and HDFS (1000:1 volume ratio).

Silwet was mixed with PDMS prepolymer (0.1, 0.3, and 0.5 wt %), which was composed of a PDMS monomer and a curing agent with a 10:1 weight ratio. Then, the Silwet–PDMS mixture was diluted to increase fluidity by adding 20 wt % toluene. The mixture was degassed in vacuum for 30 min, and the mixture was filled between the outer cases and the Al mold. Thereafter, the air trapped between the Silwet–PDMS mixture and micro–nano structures of the mold was eliminated in vacuum for 30 min. Then, the Silwet–PDMS mixture was cured in a 70 °C oven for more than 8 h. From the above process, the micro–nano structured Silwet–PDMS was obtained. To obtain the superhydrophilic PDMS tube, the cured polymer was detached from the Al mold by removing the outer cases, and the cured polymer was dipped in an n-hexane solution for 20 min. In this process, the polymer was separated from the mold by swelling. The swollen tube was dried in air for 20 min, after which the superhydrophilic PDMS tube was finally obtained. Unless otherwise stated, all experiments were conducted at 25 °C. The replication process was inspired by the work of Kim (Figure S2).40

2.4. Bubble Adhesion Test

A PDMS tube containing Silwet L-77 was prepared as shown in Figure 1a. The fabrication method for the internal structure of the tube was adopted from Kim,40 where Silwet L-77 was added to impart superhydrophilicity. The PDMS tube containing Silwet L-77 was more rigid than the original PDMS tube but it had sufficient flexibility. Figure 1b schematically shows the experimental setup of the bubble adhesion test. The experiment was carried out in a water-filled tank with a water pump and bubble stone, and a syringe was set outside the tank to inject air bubbles. The generated air bubbles flow along the water and adhere to the tube surface. Bare, superhydrophobic, and superhydrophilic surfaces were applied to the tubes, and comparative experiments were conducted with each tube. For each experiment, 5 mL of air bubble was injected to the system for 5 s with the syringe pump; the water flow rate was 13.2 mL/s. The behavior of air bubbles in the transparent tube was observed above the water surface.

Figure 1.

Figure 1

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Photograph of fabricated tube (a) and schematic of the experimental setup for bubble adhesion test (b).

2.5. Characterization

The droplet contact angle and sliding angle were measured with 5 μL DI water droplets using the SmartDrop equipment (Femtofab, Korea) at ambient temperature. The air bubble contact angle and sliding angle were measured with 5 μL of air bubbles also using SmartDrop at ambient temperature. All contact angles and sliding angles were obtained as the average of five measurements at different positions on the surface. Scanning electron microscopy (SEM) measurements and energy-dispersive X-ray spectroscopy (EDS) were performed using a JEOL JSM-7401F with Dual EDS field-emission scanning electron microscope (JEOL, Japan). Atomic force microscopy (AFM) measurements were performed using the SPM system (Veeco Instruments Inc., USA). Oxygen plasma treatment was performed using a plasma surface treatment system COVANCE (Femto Science Inc., Korea).

3. Results and Discussion

When PDMS is replicated on a micro–nano structured aluminum surface, even nanostructures are replicated well (Figure 2a).39 However, PDMS with 0.5 wt % Silwet L-77 had higher viscosity than general PDMS because of which efficient penetration between the structures could not be achieved. As a result, a roughness (Ra) reduction of about 29% was observed under AFM (Figure 2b). Ra for the Al mold and 0.1, 0.3, 0.5 wt % PDMS were 564, 534, 432, and 404 nm, respectively (Figure S3). The surface roughness reduction by Silwet with increasing weight percentage agrees well with the previous studies reporting on the increase of Young’s modulus for Silwet L-77-added PDMS.41 This increase of Young’s modulus makes it harder to replicate micro- and nano-structures from the mold. Despite this reduction of surface roughness, Figure 2a shows that the micro–nano hierarchical structure of PDMS containing Silwet L-77 was well maintained and the surface was successfully replicated. Although viscosity increases with the addition of Silwet L-77, it is advantageous to add a large amount of Silwet L-77 to induce superhydrophilicity in PDMS. Nevertheless, the amount of Silwet L-77 in this study was limited to 0.5 wt % in order to minimize the effects on roughness and surface structure.

Figure 2.

Figure 2

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(a) SEM image of micro- and nano-structures of replicated PDMS (i,iii) and replicated PDMS with 0.5 wt % Silwet L-77 (ii,iv). (b) Surface roughness measurement of replicated PDMS (i) and replicated PDMS with 0.5 wt % Silwet L-77 (ii).

The atomic composition of PDMS was investigated through EDS to confirm the presence of Silwet L-77 at the surface and bulk. As shown in Table S1, the oxygen/silicon rates of the 0, 0.3, and 0.5 wt % samples were 0.486, 0.598, and 0.689, respectively. The oxygen/silicon rate increased with increasing Silwet L-77 wt % of PDMS. These results show the same trends as those of the previous studies.42 Additionally, the 0.653 oxygen/silicon rate of the 0.5 wt % bulk sample indicates that Silwet L-77 was well spread from the surface to bulk.

We fabricated the superhydrophobic surfaces by replicating micro–nano structures on the PDMS surface and prepared a bare PDMS surface. The bare PDMS surface has a flat surface without micro–nano structures and no addition of Silwet L-77. For the hydrophilic/superhydrophilic surface, 0.1, 0.3, and 0.5 wt % of Silwet L-77 was added to the micro–nano fabricated PDMS surface to grant superhydrophilic properties. The contact angles were measured with a water droplet of 5 μL and the measured values were 158.7° (superhydrophobic), 109.0° (bare), 28.5° (PDMS with 0.1% Silwet L-77), 17.4° (PDMS with 0.3% Silwet L-77), and 6.5° (PDMS with 0.5% Silwet L-77), respectively (Figure 3a). In particular, the contact angle for PDMS containing Silwet L-77 was measured after 1 min of saturation to consider the phenomenon of surfactant diffusion, in which water droplets on the PDMS containing the surfactant spread over a constant time interval.33 Similar to previous research, the replicated PDMS exhibited superhydrophobicity with a water contact angle of 152° and the bare PDMS showed a contact angle of 110°. It is noteworthy that Silwet L-77 was added to the replicated PDMS to induce superhydrophilicity at a sliding angle of 8.4° (Figure 3b). In this manner, a hydrophilic surface that can be stable in air could be fabricated. We also measured contact angle hysteresis and sliding angles. The measured contact angle hysteresis were 27.6° and 7.5° and sliding angles were 21.7° and 40.3° for superhydrophobic and bare surface, respectively (Figure S4a). Although the superhydrophobic surface shows lower sliding angle, the hysteresis was relatively high. This result shows that the surface has high roughness with random micro–nano structures, which were generated by etching. The structure size on the surface possibly show large differences between the etching and micro machining processes. In this article, the nano–micro structured PDMS with 0.5 wt % Silwet L-77 is defined as a superhydrophilic surface considering its wettability. We measured the underwater air bubble contact angle and sliding angle for superhydrophilic, superhydrophobic, and untreated PDMS surfaces with only 0.5 wt % Silwet L-77 added. The contact angles for 5 μL of air bubbles were 28.5°, 114.3°, and 152.9° on the superhydrophobic, bare PDMS with Silwet, and the superhydrophilic surfaces, respectively. This showed that the underwater air bubble contact angle on each surface matches the tendency of the water contact angle and wettability. The measured contact angle hysteresis were 30.2° and 13.3° and sliding angles were 8.4° and 43.2° for the superhydrophilic and bare surface, respectively (Figure S4b). The high contact angle hysteresis can also be attributed to the irregularity of surface structure. Bubbles were observed to be pinned to some irregular point before sliding off the surface. According to the Cassie–Baxter eq 1, the droplet contact angle in air (2) and underwater air bubble contact angle (3) on solid surfaces are shown in the following equations.43

3. 1
3. 2
3. 3

where cos θE is the equilibrium contact angle, cos θ* is the apparent contact angle, and f is ratio of the surface in contact with the droplet or bubble. The droplet contact angle was measured on nano–micro structured superhydrophobic PDMS, and the air bubble was measured on the nano–micro structured aerophobic PDMS with Silwet L-77.

Figure 3.

Figure 3

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(a) Droplet contact angle of superhydrophobic, bare, and hydrophilic/superhydrophilic surfaces. (b) Contact angle and sliding angle of underwater air bubbles on bare, superhydrophobic, and superhydrophilic (0.5 wt % Silwet L-77) surfaces.

The value of air bubble contact area on the aerophobic surface fPDMS′ (0.186) calculated by the above equations is 78% higher than the value of water droplet contact area on the superhydrophobic surface fPDMS (0.104). As a result, the superhydrophilic surface showed a superaerophobic property with an underwater contact angle of 150° or higher, but the superhydrophilic surface was found to have a larger contact area than the superhydrophobic surface in air. However, because it still has a small contact area of less than 0.2, superaerophobicity of the surface can be deduced. This is a phenomenon in which water is trapped between superhydrophilic surface structures and air bubbles cannot make contact with the surface in the aqueous environment, such as the behavior of water droplets on superhydrophobic surfaces in the atmosphere. Similarly, the bubble sliding angle of the superhydrophilic surface was very low at 8.4°, which is in contrast to superhydrophobic surfaces with very high bubble drop sliding angles. This is because the bubble pinned to the surface structure of the superhydrophobic surface in a flattened shape, but the pinning phenomenon does not appear in the air bubble on the superhydrophilic surface.

Experimental results of the bubble adhering test for the bare, superhydrophobic, and superhydrophilic surfaces in the fabricated system are shown in Figure 4. Each tube had minute air bubbles in its initial state (Figure 4a(i)), which is very small compared to the amount of air bubbles injected thereafter. The initial minute bubbles are the small amount of bubbles that adhered to the surface without being removed by the water when it first flowed in the tube. Immediately after injecting 5 mL of air bubbles into each tube, the three wettability surfaces exhibited different behaviors of air bubble adhesion (Figure 4a(ii)). After waiting for the air bubbles to stabilize for a while, the experiment was terminated when there was no further bubble movement (Figure 4a(iii)). The ratio of air bubble area in the entire tube area of Figure 4a(ii,iii) was measured and compared to determine the self-removal ratio. In the superhydrophobic tube, the surface showed an aerophilic property in the underwater environment with a large area of air pockets developed inside the tube surface. On the tube surface, the attachment of air pockets is different from that of air bubbles. Unlike air bubbles, air pockets are large flat air layers merged along the tube length direction. Different sizes of air pockets are shown in Figure 4a(A), and one large air pocket in Figure 4a(B). These air pockets adhered to the surface very strongly, such that they did not disperse with the water flow inside the tube, and they were difficult to remove to within 10% even after reaching the saturated state (Figure 4b). In the superhydrophilic tube, the surface showed an aerophobic property in the underwater environment, with air bubbles adhering to the surface in the shape of a bubble. Immediately after air bubble injection, the area of bubbles adhered to the surface was 67.8%, which is higher than that of the superhydrophobic tube surface (64.5%). There are two reasons for the similarity in the bubble adhesion area of the two surfaces. First, the observed area of the bubble shape is larger than the actual contact area between air bubbles and the surface. In the case of air pockets, the area of the air layer attached to the large surface is accurately observed. This is because air pockets are attached to the surface with low contact angle, and the edges of air pockets have low curvature. However, in the case of the air bubbles, with their high contact angle, their edges have very high curvature. Therefore, the area observed from the vertical view is exaggerated. Moreover, air bubbles have even higher rate of edges in the observed area than air pockets. Second, this phenomenon is explained by surfactant diffusion. The superhydrophilic tube contains Silwet L-77, a kind of surfactant, which causes surfactant diffusion on the tube surface when wetted with water.33,34,44−46 The surfactant is diffused from the PDMS surface to water. When the surfactant diffuses in water, the surface tension of water decreases. The droplet contact angle and the bubble contact angle were measured considering this phenomenon. Because of this phenomenon, the wettability of the superhydrophilic tube was observed to change over time as surface tension decreased.34 In contrast, air bubbles were observed to be fastened in water (Figure 4c). Surfactant diffusion did not occur at the air–PDMS boundary. The surfactant diffused only at the water–PDMS boundary. Therefore, when the surface is occupied by air bubbles, the surfactant diffuses only in water around the air bubbles. Consequently, the surface tension of water around the bubble drops to easily penetrate into the bubble–PDMS boundary because of which the bubbles appear to be fastened by water. Moreover, air bubble fastening induces the air bubble self-removal effect. As shown in Figure 4a(iii,C), most bubbles were removed at the saturation state and the bubble area ratio was reduced by 97.7% (Figure 4b). This is in contrast to the removal of less than 30% of the air bubbles during saturation in the bare tube, where bubble shape and air pockets were observed (Figure 4a(A),b).

Figure 4.

Figure 4

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(a) Air bubble behavior for the (i) initial, (ii) adhesion, and (iii) saturation states on (A) bare, (B) hydrophobic, and (C) hydrophilic tube surfaces. (b) Bubble area ratio and self-removal ratio by tube wettability type. (c) Schematic diagram of the type of air bubble adhesion.

We compared PDMS with oxygen plasma treatment, which is one of the methods of inducing superhydrophilicity on PDMS surfaces and superhydrophilic PDMS containing Silwet L-77 in terms of their stability in ambient air. The surface showed excellent surface droplet contact angle and underwater air bubble contact angle after the oxygen plasma treatment. The oxygen plasma-treated PDMS surface shows better aerophobic effect at the beginning of the test. However, as is well known, oxygen plasma treatment has serious aging problem, which is hydrophobic recovery in ambient air.36 The surface that comes into contact with air loses its OH group, thus losing its hydrophilicity rapidly. We measured the air bubble contact angle of the surface experiencing aging (Figure 5). Within the first 10 h, the surface suddenly lost its hydrophilicity and the air bubble contact angle slowly decreased. These results show that underwater air bubbles are propagated from the Cassie state to the Wenzel state at about 100°, which is also in agreement with previous studies.36 An underwater aging test was also conducted under conditions matching those of the expected application. The test was conducted for 2 weeks at a flow of 13.2 mL/min, same as the bubble adhesion test. After 2 weeks of underwater aging test, the oxygen plasma-treated PDMS surface maintained the bubble contact angle of 160°. However, the bubble contact angle of 0.5 wt % Silwet L-77-added surface decreased from 153° to 134°. As shown in Figure S5, no surface structure change was observed. Therefore, the bubble contact angle drop in the underwater aging test can be attributed to surfactant diffusion. After 2 weeks of underwater aging, a significant amount of surfactant diffused to the water, which weakened the aerophobicity of the Silwet L-77-added PDMS surface. This result is consistent with that of the previous studies.34,45 After the experiments, the results of EDS showed that the carbon, oxygen, and silicon ratios were 33.39, 24.65, and 41.96, respectively, and the oxygen/silicone ratio was 0.587. It can be seen that this is significantly reduced compared to the oxygen/silicon ratio (≥0.650) of the 0.5 wt % Silwet L-77-added PDMS (Table S1). It is also lower than the oxygen/silicon ratio (0.598) of the 0.3 wt % Silwet L-77-added PDMS, indicating that leaching of surfactant occurred in the 0.5 wt % Silwet L-77-added PDMS.

Figure 5.

Figure 5

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Graph of underwater air bubble contact angle against aging time under ambient air conditions for replicated PDMS with 5 wt % Silwet L-77 and replicated PDMS with O2 plasma treatment.

We conducted further experiments to find the maximum operating conditions required for bubble removal. Bubble removal occurs well at faster flow rates, and the bubbles experience greater force from the fluid in the tube. For this reason, we measured the minimum flow rate for bubble self-removal of each tube. Experiments with the bare, superhydrophobic, and superhydrophilic samples showed that the lowest flow rates for bubble self-removal were 23.9, 31.5, and 9.5 mL/s, respectively (Figure S6). Each sample is identical to the sample used in Figure 4, and it can be seen that 0.5 wt % Silwet L-77-added PDMS exhibits bubble self-removal even at the lowest flow rate.

4. Conclusions

We successfully fabricated a PDMS tube with hierarchical nano–micro structures, and by adding a silicon surfactant (5 wt % Silwet L-77), air-stable aerophobic characteristics could be achieved. SEM images, AFM images, and measured contact-angle, and sliding-angle showed that this PDMS has the requisite surface roughness and wettability for efficient self-removal of air bubbles. In the subsequent air bubble adhesion experiment, a similar ratio of bubbles adhered to the superhydrophobic, bare, and superhydrophilic surfaces at the initial state. However, the superhydrophilic tube (aerophobic surface) showed an efficient self-removal of 97.7% through surfactant diffusion in the saturated state, in contrast to the bare and superhydrophobic surfaces, which showed low bubble removal efficiencies of 28.5 and 8.3%, respectively. In addition, aging experiments confirmed that the proposed surface exhibits air-stable aerophobicity unlike the aerophobic surface generated through oxygen plasma treatment. Therefore, considering the behavior of bubbles in the tubes over time, the air-stable PDMS tube with aerophobicity has high potential as a useful tool for spontaneously detaching air bubbles from tubes.

Acknowledgments

This research was financially supported by the National Research Foundation (NRF) of Korea (no. 2016K1A3A1A20005945), which is funded by the Ministry of Science, ICT and Future Planning, Korea.

Glossary

Abbreviations

FE-SEM

field-emission scanning electron microscopy

AFM

atomic force microscope

HDFS

heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane

PDMS

polydimethylsiloxane

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02376.

  • Chemical structure of Silwet L-77; replication process of the superhydrophilic PDMS tube; surface roughness measurement of replicated Silwet L-77-added PDMS; atomic composition of Silwet L-77-added PDMS; dynamic contact angles of the droplet in air and bubble in water on surfaces; SEM images of aging tested surfaces; and graph of minimum flow rate for bubble self-removal (PDF)

  • Air bubble behavior for the bare, hydrophobic, and hydrophilic tube (ZIP)

Author Contributions

J.P. contributed the most to the experimental work and drafted the manuscript. S.W., S.K., and M.K. designed the study. W.H. is the guarantor. All authors reviewed the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao9b02376_si_001.pdf (458.8KB, pdf)
ao9b02376_si_002.zip (9.9MB, zip)

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