Study on Nanobubble-on-Pancake Objects Forming at Polystyrene

Jul 8, 2016 - Study on Nanobubble-on-Pancake Objects Forming at Polystyrene/Water Interface ... School of Mechanical Engineering, Heilongjiang Univers...
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Study on Nanobubble-on-Pancake Objects Forming at Polystyrene/ Water Interface Dayong Li,*,† Yunlu Pan,*,‡ Xuezeng Zhao,‡ and Bharat Bhushan*,‡,§ †

School of Mechanical Engineering, Heilongjiang University of Science and Technology, Harbin 150022, China School of Mechanical and Electrical Engineering, Harbin Institute of Technology, Harbin 150001, China § Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLB2), The Ohio State University, 201 West 19th Avenue, Columbus, Ohio 43210-1142, United States ‡

S Supporting Information *

ABSTRACT: Surface nanobubbles, which are the main gaseous state forming at the solid/liquid interface, have received extensive attention due to their peculiar features and potential applications. Nano/micro pancakes and interfacial gas enrichment (IGE) are observed at the water−solid interface, which suggest nanobubbles may coexist with IGE. An intuitive case for the coexistence of nanobubbles and IGE is the nanobubble-on-pancake-like objects. However, it still is not clear whether nanobubbles sit on top of an IGE or the IGE surrounds a nanobubble, which increasingly is seen to be important for understanding the stability and small contact angle of nanobubbles. In this study, the nanobubble-on-pancake-like objects were investigated on a polystyrene (PS) surface. Considering the nanobubble-like objects forming on PS film might be blisters formed because of osmosis, whether such objects are gaseous state or blisters therefore was investigated first. Then, the structure of the nanobubble-on-pancake-like object was analyzed, on the basis of which the stability of nanobubbles under tip perturbation was discussed. The pancake-like domains of the bubble-on-pancake composite disappeared, but the bubble part remained. This indicates that nanobubbles do not sit on top of the pancakes, but are pinned on the solid surface. This is in good agreement with the contact line pinning theory, and is helpful to understanding the abnormal long lifetime (stability) of nanobubbles.

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INTRODUCTION Experimental and theoretical studies of accumulated gases at water−solid interface have grown significantly since the early 2000s. This is especially true for surface nanobubbles, which are the main gas state. They have attracted intense research interest because of their peculiar features and potential applications in many fields.1−5 Up to now, the characteristics,6−8 formation methods,9−13 influence factors11,14−19 and some potential applications20−27 of surface nanobubbles have been investigated. However, a comprehensive understanding of the anomalous large contact angle (in this paper, contact angle means water side contact angle) and longevity (stability) of nanobubbles is still an open question. The proposed theoretical explanations for the mechanism of nanobubble superstability include dynamic equilibrium,28,29 contamination,30−32 contact line pinning,33−36 and the comprehensive effect of the cases mentioned above.37 Although these theories are supported by some experimental phenomena, none of them could be indisputably accepted. Some explanations of the abnormally large contact angle (the contact angle of nanobubbles measured through water side is much larger than that of macrobubbles) have been proposed such as contamination,30 electrostatic force,38 gas type,39 line tension,10,17,40−42 and interfacial gas enrichment (IGE, a kind of gas state means a gas molecules layer accumulated at solid− liquid interface, shown in Figure 1).33,39,43 Among them, IGE © 2016 American Chemical Society

Figure 1. Sketch of the morphology of nanobubble, pancake/IGE, and blister formed on a solid surface.

has been attracting the ever-growing attention of the researchers.37,44−52 Limbeek and Seddon39 and Weijs et al.33 hypothesized that IGE coexisted with nanobubbles and contributed to the stability and anomalous contact angle of nanobubbles. If an IGE coexists with nanobubbles, the surface free energies of solid−liquid, liquid−gas, and solid−gas should be changed by the role of IGE in the thermodynamics of wetting. Based on Young’s equation, the reason for the large contact angle of nanobubbles can be explained. Furthermore, Special Issue: Nanobubbles Received: May 19, 2016 Revised: July 7, 2016 Published: July 8, 2016 11256

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Figure 2. Nanobubbles, nanopancakes, and nanobubble-on-pancake-like objects were imaged on PS film by TM-AFM. The scan time of each image is 10 min. Therefore, t = 10 min signifies the finished time of image (a). Height images (a) and (b) show nanobubble-like, nanopancake-like, and nanobubble-on-pancake-like objects formed on PS film. Image (b) is the magnified section of the rectangle region in height image (a). Image (c) shows the section analysis of the nanobubble and nanopancake in image (b), and the inset is the 3D image of the corresponding nanobubble and nanopancake. Image (d) shows the height image of the rectangular region (1.2 μm × 1.2 μm) in image (a) scanned at an amplitude set-point of 65%. Image (e) is the height image at a scan area of 3 μm × 3 μm with an amplitude set-point of 95%. Image (f) to image (i) shows the height images obtained at amplitude set-point of 95% after rescanning the location of image (d) with an amplitude set-point of 65%. Z range of image (d) is 25 nm; z range of other images is 40 nm.

not clear whether the nanobubbles sit on top of an IGE or whether an IGE surrounds the nanobubble. This will influence the pinning force of the contact line, which is important to the stability of nanobubbles according to the contact line pinning model.34,35 Suppose a nanobubble sits on top of an IGE, the contact line pinning force will be determined by the binding energy of gaseous molecules inner the bubble to the gaseous molecules of IGE. If nanobubble is located on the solid surface and surrounded by an IGE, the binding energy of gaseous molecules to the underlying solid should be considered when calculating the contact line pinning force. Therefore, to investigate the exact structure of the coexisting nanobubble and IGE is significant for studying and understanding the stability of nanobubbles. In this paper, nano/micropancake and nanobubble-onpancake-like objects on PS/water interface were investigated by using tapping mode AFM (TM-AFM). Given that the nanobubble-like objects forming at PS films might be blisters that are formed as the thin PS film fails because of osmosis (the sketch of morphology of blister, nanobubble and nano/ micropancake/IGE can be seen in Figure 1), the hydrophobic

the coexistence of IGE and nanobubbles supports the dynamic equilibrium theory,28 i.e., IGE can provide a gas source for the gas influx at the three-phase contact line to balance the diffused gas molecules from the gas−liquid interface of nanobubbles. Some experimental studies43 showed that IGE coexists between nanobubbles at the highly ordered pyrolytic graphite (HOPG) surface after solvent-exchange. This provides strong evidence for the existence of an IGE. Except for nanobubbles and IGE, nano/micropancakes37,46,52 including monolayer nano/micropancakes, bilayer nano/micropancakes, and nanobubble-on-pancake objects were also observed and studied on a variety of surfaces using an AFM. Zhao et al.52 investigated the stiffness of micropancakes forming at the ethanol/water interface by using the peakforce quantitative nanomechanics (PF-QNM) mode of an AFM in their latest study. The question remains whether the pancake-like objects are a kind of IGE, or whether there is a difference between the pancake-like objects and IGE. If they are IGE, the nanobubble-on-pancake objects should be a special case of the coexistence of nanobubble and IGE. However, the exact coexisting structure of nanobubbles and IGE still remains puzzling, that is, it is still 11257

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typical nanobubble and nanopancake (in Figure 2b); the diameter and height of the nanobubble are about 250 and 25 nm, respectively; the lateral size and height of nanopancake are about 300 and 7 nm, respectively. The bubble-like objects forming at the PS/water interface might be blisters produced as the thin PS film fails because of osmosis, a claim made by Berkelaar et al. in their recent study55 (see Figure A2a in the Appendix), and the “blisters” show obvious growth with immersion time. Here, we specifically check whether the bubble-like and pancake-like objects are gaseous. A successive in situ scanning first was carried out, which can also be taken as a temporal evolution experiment. Figure 2d was a 1.2 μm × 1.2 μm area scanned at an amplitude set-point of 65% of the rectangular region shown in Figure 2a, and then Figure 2e was scanned at a scan area of 3 μm × 3 μm with an amplitude setpoint of 95%. Each image from Figure 2f to Figure 2i was obtained (set-point, 95%) after rescanning the location of Figure 2d with an amplitude set-point of 65%. Given that the scanning time of a part of the image is about 10 min, it will take about 110 min from the beginning of the first scan to the completion of Figure 2i. However, contrary to the results in ref 55, there is no obvious growth for both the bubble-like objects and the pancake-like objects by comparing the images from Figure 2a to Figure 2i. In addition, the pancake part of the bubble-on-pancake object P1 in Figure 2a (in the circle) and Figure 2b changed into a bubble and linked together with the bubble part in Figure 2e under high scanning force. After that, it changed into a pancake state in Figure 2f, then changed into scattered disorderly in Figure 2g, and disappeared in Figure 2h,i. Similarly, the bubble-on-pancake object P2 in Figure 2a,b changed into scattered disorderly in Figure 2g, and disappeared in Figure 2h,i. We also can see that bubbles b1 and b2 in Figure 2b merged together and formed bubble b3. Interestingly, as can be seen in Figure 2h, bubble b3 was shown to be surrounded by a pancake object, which should be derived from the disappeared bubble-on-pancake-like objects (P1 and P2) mentioned above. Although the coalescence of two neighboring bubble-like domains b1 and b2 is like the coalescence of blisters, the complicated behavior of P1 and P2 under high scanning force indicates that if b1, b2, P1, and P2 are blisters, the movement path of P1 and P2 will make the PS film swollen and form a large blister (sketch for the swollen process of PS film can be shown in Figure A2 in the Appendix). However, there are no images presented of the large blister in Figure 2h,i. In particular, the thin swelling PS film is likely to be fractured and appear to be peeling off under large tip-film interaction force, but there are no images presented of the PS fragments, which result from damage to blisters. In addition, the “blisters” on PS film claimed by Ahmad et al.56 show a toughness property under large scanning force, i.e., there is no change in morphology of the “blisters” obtained even with contact mode (CM) AFM. However, this is remarkably different from the observation in our experiments. 3.2. Interaction between a Hydrophilic Sphere and a PS Wafer Immersed in Purified Water. Since the long distance force between two hydrophobic surfaces was first hypothesized by Parker et al.57 to be caused by the presence of nanobubbles, a variety of chemisorbed hydrophobic surfaces, such as PS surface,58 octadecyltrichlorosilane (OTS) surface,59,60 propyltrichlorosilane (PTS) surface,61 silanized surface,62,63 and esterified surface,64 etc., have been employed to investigate the hydrophobic attraction. So far, the viewpoint that the bridging of nanobubbles is responsible for the long-

force curves between the PS substrate (with nanobubbles and nano/micropancakes) and a glass microsphere (with nanobubbles and without nanobubbles) were measured. Based on these measurements, whether the bubble-like and pancake-like objects are gaseous objects or blisters is discussed first. As the most probable configuration of the coexistence of nanobubble and IGE, the change in morphology of bubble-on-pancake-like object under large scanning force was studied. Then, the structure of coexisting nanobubble and IGE and stability of nanobubbles are discussed.

2. METHODS 2.1. Materials Preparation. Water used in our experiments was obtained from a Milli-QA10 system (conductivity, 18.2 MΩ.cm). PS substrate was prepared by spin-coating PS (molecular weight 350,000, Sigma-Aldrich) solution with a concentration of about 0.1% (weight) on 1 μm × 1 μm silicon (100) at a speed of 2000 rpm. Before spincoating, the silicon (100) wafers were boiled in a piranha solution of a 3:1 (volume ratio) mixture of 98% sulfuric acid and 30% hydrogen peroxide solution for 30 min, then acetone for 30 min, and then rinsed with ultrapure water followed by ethanol at least five times (each for 2 min) and dried with nitrogen. After spin-coating, the PS sample was put in a culture dish and left in an oven at 52 °C for 5 h to remove the remaining solvent. The root-mean-square (RMS) value obtained in air at a scan area of 5 μm × 5 μm was 0.91 nm, and the contact angle of a 5 μL droplet of purified water on the newly prepared PS sample was measured as 95° ± 2°. The PS film thickness was measured by AFM nanoshaving53 and found to be about 23 ± 2 nm. To ensure gas saturation, about 50 mL of water was equilibrated in a stainless steel container for about 10 h at atmospheric pressure before imaging in water. About 0.8 mL water was injected into the liquid cell with a clean glass syringe before imaging in water. 2.2. Measurement Methods. TM-AFM (NTEGRA platform, NT-MDT Company, Zelenograd, Moscow) was used to image the PS substrates both in air and purified water. A rectangular cantilever (CSG30, NT-MDT Company) with a nominal spring constant k = 0.13−2 N m−1 (a measured spring constant of k = 0.51 ± 0.02 N m−1 determined by the resonance frequency method54), resonance frequency in air of ω0 = 65 kHz, resonance frequency in water of ω0 = 23 kHz, and nominal tip radius of Rt ≤ 10 nm (actual Rt = 13 ± 2 nm measured by SEM imaging) was used. While imaging in water, an open fluid cell with about 1 mL water was used. The height images were scanned at set-point ratios of 95%, 75%, and 65% with a typical value of free amplitude of 4.0 ± 0.2 nm and a scanning frequency of 1 Hz. All experiments were carried out at a room temperature of 25 ± 1 °C and humidity of 45−55% RH. A borosilicate microsphere (GL018B/45-33, MO-Sci Corporation) with a diameter about 32 μm obtained with an optical microscope was used to measure the interactions between the PS substrate covered with bubble-like objects and the microsphere. The borosilicate microsphere was attached to a rectangular cantilever (ORC8, Bruker, nominal spring constant k = 0.38 N m−1) using epoxy (Araldite, Bostik, Coubert). To minimize the hydrodynamic contribution to the measured force, the PS substrate was driven with a low constant velocity of V = 0.6 μm/s to approach to the sphere.

3. RESULTS AND DISCUSSIONS 3.1. Imaging of Nanobubbles, Nano/Micropancakes, and Nanobubble-on-Pancake-Like Objects. After immersing the PS substrate into water, we scanned the PS surface at an amplitude set-point of 95% and scan frequency of 1 Hz. Figure 2a shows the topography of surface nanobubbles, nano/ micropancakes and nanobubble-on-pancake-like objects obtained from TM-AFM on the prepared PS film (RMS = 0.91 nm) at a scan area of 3 μm × 3 μm. The diameter and height of these nanobubbles are generally in the order of ∼500 nm and ∼35 nm, respectively. Figure 2c shows the section analysis of a 11258

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Figure 3. Typical retraction curves between a hydrophilic sphere and PS surface immersed in purified water. (a) Force curves between a hydrophilic sphere (without nanobubbles) and PS surface (with nanobubbles); (b) long ranged force curves between a contaminated hydrophilic sphere (with nanobubbles) and PS surface (with nanobubbles). The insets in image (a) and image (b) are schematic diagrams of the relationship between a hydrophobic surface covered with nanobubbles and a microsphere (without nanobubbles)/(with nanobubbles), respectively.

range hydrophobic force has achieved wide consensus. Therefore, to verify further that the nanobubble-like and nanopancake-like objects are gaseous, force measurements were carried out between the nanobubble/nanopancake-covered PS surface and a borosilicate microsphere. Figure 3a shows two typical retraction curves measured on the nanobubble/nanopancake covered PS film with a clean hydrophilic sphere. Both the retraction curves are long-ranged (>20 nm), and there are several “pull off” steps (such as position A, B, C, D, and E, etc.) in each retraction curve. This is characteristic of nanobubbles−hydrophilic sphere interaction, and is consistent with the results of previous studies.2,59,62−64 The distance of the long-ranged attraction force is about 40 nm, which is near the height of nanobubbles captured on PS films in our experiments. The inset in Figure 3a shows a schematic diagram of the relationship between the retracted hydrophobic surface covered with nanobubbles and a clean hydrophilic sphere (without nanobubbles). Figure 3b shows two typical retraction curves obtained on the same substrate after capturing the images in Figure 4 (a double check of the force measurements on the nanobubble/ nanopancake covered PS film). Both retraction curves contain several “pull off” steps (such as position A, B, C, D, and E, etc.), similar to the retraction curves in Figure 3a. However, the distance of the long ranged attraction is about 240 nm, which is much longer than that in Figure 3a. This should be caused by the gas bridges that are formed by the nanobubbles on both the PS film and the sphere/water interface (To ensure the same experimental environment, the water used was the same while carrying out a series of experiments in Figure 2, Figure 3, and Figure 4. Meanwhile, the AFM tip and microsphere need to be changed several times. This is quite liable to introduce contamination to the water. Therefore, nanobubbles might form at the contaminated microsphere surface while measuring the force curves.). A schematic diagram of capillary bridge of nanobubbles between the PS surface and a contaminated hydrophilic sphere can be seen from the inset in Figure 2b. The long distance attraction forces measured between the microsphere and PS surface indicates that the nanobubble-like and nanopancake-like objects formed at the PS film are gaseous. Furthermore, a degassed experiment was carried out on the PS surface (shown in Figure A1 and Table A1 in the Appendix).

Figure 4. Morphology of nanobubble-on-nanopancake under tip perturbation. (a) The height image of nanobubbles, nanopancakes, and nanobubble-on-nanopancake objects (set-point, 95%). (b) The magnified section of the rectangle region in height image (a). (c) The enclosed region by dotted line in image (b), which was scanned at a set-point of 75%. (d) The height image obtained at the scan region of image (b) at a set-point of 95% after image (c) was taken. z range of image (c) is 25 nm, and z range of other images is 35 nm.

The total number and volume of the preformed surface bubbles are decreased noticeably when we twice injected the degassed water into the liquid cell to replace the air-equilibrated water. The force curves were also measured on the nanobubble covered PS film before and after twice replacing air-equilibrated water with partially degassed water (as shown in Figure A1d,e, respectively). Both the two typical retraction curves are longranged with several “pull off” steps (such as positions A, B, and C). The separation distances of the two retraction curves are near to the height of nanobubbles. Together with the results of force measurements carried out on nanopancake covered PS 11259

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Figure 5. Section analysis of the bubble-on-pancake object under AFM tip disturbance in Figure 4 is shown in image (a). Image (b) is a sketch of the possible structure of the bubble-on-pancake object.

film above, this can effectively show the gaseous nature of the nanobubble-like and nanopancake-like objects on PS films in our experiments. 3.3. Change in Morphology of Bubble-on-Pancake Object under Tip Perturbation. The bubble-on-pancake-like objects in Figure 2 show an interesting behavior under the tip perturbation. To further investigate the structure of bubble-onpancake object, we rescanned the PS film (with a set-point of 95%), the morphology of nanobubbles, nanopancakes, and nanobubble-on-nanopancake objects can be seen in Figure 4a. Figure 4b is the magnified section of the rectangle region in Figure 4a. After that, we intentionally scanned the bubble-onpancake objects (P3 and P4) at a scan area of 1 μm × 1 μm (the rectangle region enclosed by the dotted line in Figure 4b) with a magnitude set-point of 75%. The corresponding height image can be shown in Figure 4c. Then, an enlarged scan area of 2 μm × 2 μm was selected and scanned at a magnitude setpoint of 95% (shown in Figure 4d). Both pancake parts of the two bubble-on-pancake objects (P3 and P4) disappeared under the tip-pancake interaction. The gas molecules in which it can be judged to move and form a gas ridge, can be seen in the elliptical region enclosed by the dotted line in Figure 4d. This is similar to the phenomenon in Figure 2 and the result of ref 48, i.e., the tip−nanobubble/pancake interaction can change the morphology of nanobubbles and pancakes. It is noteworthy that there is no swollen blister or fragment of broken PS film in the possible moving range of the disappeared nanopancakes in Figure 4d. This again clearly shows that the bubble-like and pancake-like objects are not blisters. Furthermore, it is noticeable that all the nanobubbles of the bubble-on-pancake objects remained, which can be seen intuitively from the height images in Figure 4 and the section analysis of P3 (as shown in Figure 5a). This indicates that the nanobubbles might not sit on top of pancakes in the bubble-onpancake assembly, but pin on the solid surface (as shown in Figure 5b). Although it is possible that the AFM tip destroys the pancake but not bubbles, or even bubbles on pancake, if the nanobubble sits on top of the pancake, when the pancake part is destroyed or moved by the AFM tip, it is very likely that the nanobubble part is also moved just like taking a boat trip. However, the nanobubble parts are very stable and did not move with the pancake parts (the pancake parts moved and formed a gas ridge), as shown in Figure 4b,d. This indicates that the nanobubble parts do not sit on top of the pancakes, but pin on the surface.

Reference 47 shows that nanobubbles can move from one side to another of the pancake, which means the nanobubbles might sit on the pancake. It is noticeable that the pancake part of the bubble-on-pancake assembly expanded and merged with other pancakes under the tip-bubble/pancake interaction in the study of ref 47. In this case, the nanobubble part, even pinned on the surface, might move with the expansion movement of pancake part. Therefore, the opinion that nanobubbles in the bubble-on-pancake assembly pin on the solid surface seemingly is apt to be accepted. Our results provide direct experimental evidence for the contact line pinning of nanobubbles. The pinned three phase contact line resulting from nanoscale surface roughness has been hypothesized and studied by Weijs and Lohse34 to contribute to the stability of nanobubbles.65 Recent simulated studies35,36 also show that the pinning of the contact line can contribute to a decreased internal pressure Pi as contact angle θ increases with the diffusion of gas molecules inside the nanobubbles (based on the Laplace’s equation, the inner 2γlg

2γlg

pressure Pi can be given as Pi = R = r sin(90° − θ ), where γlg is the liquid−gas surface tension, and R and r are the curvature radius and base radius, respectively). This provides a negative feedback for the dissolution process of nanobubbles, and thus prolongs the bubble’s lifetime. However, a question remains whether the matter contained in the pancake part and in the bubble part of the bubble-onpancake object is of the same chemical nature. If they are the same, the pancake part and the nanobubble part of the bubbleon-pancake composite should be a whole, and the pancake part should be easy to merge into the bubble part, rather than separating with tip perturbation. In fact, there are still no studies that have proven that the matter contained in pancake objects and nanobubbles are the same. In our measurements, the separation of the two parts under the disturbance of the AFM tip seemingly indicates that there is a difference between the two gaseous states. Considering the gas states observed at the solid/liquid interface in other works including IGE33,39,43 and water depletion layer,66,67 it seems safe to assume that the nanopancakes we observed are still a kind of “water depletion layer” or “IGE” in which the contained gas molecules density is higher than that in the water depletion layer, but lower than that in nanobubbles. This is supported by a recent study52 where the stiffness of micropancakes measured by PF-QNM mode AFM is much higher than that of nanobubbles; thus, the pancake objects could be called “visible IGE”. Therefore, the 11260

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Figure 6. Contact angle as a function of nanobubbles’ size and the relationship between cos θ and 1/r. (a) The dependence of contact angle on base radius r, (error bars correspond to one standard deviation). (b) The relationship between cos θ and 1/rl; the slope (−τ/γlg) values of the fit lines are 0.54 and 1.39 for the bubbles-on-pancakes and independent nanobubbles, respectively, based on which the line tension τ can be calculated.

pancakes (the height of pancakes is about 8 nm), and a reduced γlg will be responsible for the larger contact angle of nanobubbles.

bubble-on-pancake objects are a special form of the coexistence of nanobubbles and IGE. In this case, if all nanobubbles are surrounded by IGE, the gas molecules could be provided continuously from the IGE to form a gas influx at the three phase contact line and to sustain a long lifetime for surface nanobubbles.28 Furthermore, we notice that both the large contact angle and the coexistence of nanobubbles and IGE are helpful to the stability of surface nanobubbles. Therefore, it would be interesting to examine the effect of the IGE on the contact angle of nanobubbles. We calculated the contact angle of nanobubble-on-pancake objects in Figure 4a. To ensure an accurate measurement, nanobubbles surrounded partially by nanopancakes were chosen (such as bubble b4 in Figure 4a). As a comparison, the contact angle of nanobubbles independent of any pancake in Figure 4a was also calculated. Figure 6a shows the contact angle as a function of bubble radius r. The contact angle of nanobubbles surrounded partially by IGE is usually larger than that of independent nanobubbles for bubbles with a same radius. The average contact angle of the nanobubbles surrounded by IGE is about 171.1° ± 0.1°, which is larger than that of the independent nanobubbles of 169.6° ± 0.1°. The difference for the contact angle between two kinds of nanobubbles should be ascribed to the presence of the gaseous pancake. According to the modified Young’s equation, the contact angle of nanobubbles can be expressed as cos θ = cos θY −

γls − γgs τ τ = − γlgr γlg γlgr

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CONCLUSIONS In summary, in this study we investigated the nanobubble-onpancake-like objects formed on PS film by using TM-AFM. Force measurements between a hydrophilic microsphere and a PS film immersed in pure water were carried out. The change in morphology of nanobubble-like and nanobubble-on-pancakelike objects with immersion time and increased scanning force were determined. We then discussed whether the nanobubblelike and nanobubble-on-pancake-like objects are gaseous state or are blisters. We found that (1) the typical distances of the long-ranged attraction force are about 35 nm (in the range bubble height) and about 240 nm as the microsphere is clean or contaminated, respectively; (2) the bubble-like objects and the pancake-like objects did not grow with about 110 min evolution; (3) there were no renegade PS fragments or swollen phenomenon observed after the movement of the pancake part of the nanobubble-on-pancake composite; (4) degassed experiment (shown in Appendix A) showed that the total number and volume of nanobubbles formed on PS films noticeably decreased after replacing air-equilibrated water degassed water. The separation distances of the retraction curves measured on PS film in air-equilibrated water and partial degassed water are comparable to the height of nanobubbles formed on the PS film in corresponding liquids. All of these experimental results indicate that the nanobubble-like and nanobubble-on-nanopancake-like objects observed in our experiments are gaseous. On this basis, the structure of nanobubble-on-pancake-like objects, which can be taken as a special case for the coexistence of nanobubble and IGE, was analyzed. The pancake-like domains disappeared under tippancake interaction, but the nanobubble domains remained. This indicates that the nanobubbles might not sit on top of the pancakes, but are pinned at the solid surface, which is consistent with the contact line pinning theory and provides direct experimental evidence for the contact line pinning of nanobubbles. Furthermore, according to our results, the nano/micropancakes or the pancake part of nanobubble-on-pancake objects is thought to be still a kind of “water depletion layer” or “IGE”. The density of gas molecules in the pancake should

(1)

Based on eq 1, the line tension of nanobubbles can be estimated from the relationship between cos θ and 1/r (see in Figure 6b).17 With a liquid−gas surface tension γlg = 0.072 N/ m, the magnitude of calculated line tension τ for the bubbles surrounded by pancakes (−0.04 nN) is smaller than that of independent nanobubbles (−0.1 nN; this value is very close in magnitude to the results of −0.3 nN,10 and of ∼ − 0.2 nN68). The value of the liquid−solid surface tension γlg should be reduced because of the effect of nanopancake. However, both the smaller line tension and of liquid−solid surface tension γlg will contribute to a smaller contact angle. Therefore, a possible reason for the larger contact angle of nanobubbles surrounded by pancakes than that of independent nanobubbles might be that the liquid−gas surface tension γlg is also changed by the 11261

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Figure A1. Change of surface bubbles on PS substrate in degassed experiments. Image (a) shows the preformed surface bubbles in air-equilibrated water. Image (b) and image (c) show the surface bubbles after the water was twice replaced with the degassed water. Image (d) and image (e) show typical retraction curves measured on the nanobubble covered PS film before and after twice replacing the air-equilibrated water with partial degassed water, respectively.

be higher than that in IGE, but lower than that in nanobubbles. The contact angle of nanobubbles surrounded partially by nanopancakes were also calculated and found to be larger than that of independent nanobubbles, and the theoretical explanation was provided based on the modified Young equation.

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Table A1. Change of Surface Bubbles in Degassed Experiments Nta Vt × 104 (nm3 μm−2)

image a

image b

image c

832 ± 5 181.3 ± 0.1

512 ± 5 57.7 ± 0.1

306 ± 5 28.2 ± 0.1

Nt is the total number of surface bubbles in the area of 30 μm × 30 μm, Vt is the total volume of surface bubbles in the area of 1 μm−2. a

APPENDIX A

Degassed Experiments

Figure A1d,e shows typical retraction curves measured on the nanobubble-covered PS film in air-equilibrated water and after twice injecting partially degassed water in the liquid cell to replace the air-equilibrated water. Both the retraction curves are long-ranged with several “pull off” steps (such as position A, B, and C). The separation distances of the two retraction curves are about 190 and 160 nm, respectively, which are near the height of nanobubbles captured on PS films in air-equilibrated water (∼175 nm, in Figure A1d) and partially degassed water (∼135 nm, in Figure A1e). There is no large long-ranged separation distance found like that in Figure 3b, which should be ascribed to clean water. Thus, there are few gas bridges formed between the PS film and the sphere (the water replacement can lead to clean water surrounding in our degassing experiment, which restricts the formation of gas bridges between PS film and the sphere).

To prove the bubble-like domains on PS substrates are gaseous, degassed experiments were carried out. Figure A1 shows the changes of the surface bubbles in degassed experiments. Figure A1a shows the surface bubbles on PS substrate in airequilibrated water. The diameter and height of these nanobubbles are generally in the order of ∼2.5 μm and ∼175 nm, respectively. After replacing the air-equilibrated water with degassed water (prepared by degassing the purified water for 30 min at 20 mbar), Figure A1b was captured immediately. The total number and the size of nanobubbles in Figure A1b shows an obviously decrease. Figure A1c shows the bubbles after twice replacing the air-equilibrated water with degassed water; the diameter and height of these nanobubbles are generally in the order of ∼1.5 μm and ∼135 nm, respectively. The size and total number of nanobubbles shows a further decrease. After that, the total number and size of nanobubbles observed ceased to decrease with injecting degassed water. Also, the total volume (shown in Table A1) of the preformed surface bubbles was calculated and noticeably decreased after we twice injected the degassed water into the liquid cell to replace the air-equilibrated water. This indicates that the quantity of gas provides a noticeable decrease

Blisters

The thin PS film on a silicon wafer might form blisters when immersed in water. A possible mechanism for the formation of blisters claimed by Berkelaar et al.55 is osmosis of water. Water permeates through the PS film at defects on the PS-silicon interface and forms small water pockets, which can lead to a peeling off of the PS film due to the osmotic pressure, and 11262

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blisters may form. If blisters formed at the silicon−PS interface in our experiments (see Figure A2a), the PS film of the formed

Figure A2. Sketch of the formation of swollen blisters on a PS surface. If the “blisters” moved under the tip−“blister” interaction, the PS film will be peeled off from the Si substrate (see in image (a)), and form a larger swollen blister (see in image (b)).

blister would continue swelling under the high tip−blister interaction. This leads to a larger block of detached PS film, and thus forms a larger swollen blister (see Figure A2b). However, there is no obvious change for both the bubble-like and pancake-like objects under high scanning force.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01910. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of Heilongjiang Province Natural Science Foundation of China (No.E2016059) and the National Natural Science Foundation of China (No. 51475118 and No.51505108).

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REFERENCES

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