Properties of Blisters Formed on Polymer Films and Differentiating

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Properties of blisters formed on polymer film and differences from surface nanobubbles Dayong Li, yuanlin liu, Litao Qi, Juan Gu, Qingju Tang, Xuehui Wang, and Bharat Bhushan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03965 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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Revised Jan. 31 2019

Properties of blisters formed on polymer film and differences from surface nanobubbles Dayong Li,1,3* Yuanlin Liu,1 Litao Qi,1Juan Gu,2 Qingju Tang,1 Xuehui Wang, 1 Bharat Bhushan3* 1 School of Mechanical Engineering, Heilongjiang University of Science and Technology, Harbin, 150022 China; 2 School of Science, Heilongjiang University of Science and Technology, Harbin, 150022 China; 3Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLB2), The Ohio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA Abstract When studying surface nanobubbles on film coated substrates, a kind of bubble-like domain called blisters are probably forming at the solid-liquid interface together with nanobubbles. This may easily lead to misunderstanding of the characteristics and applications of surface nanobubbles and thus continue to cause problems within the nanobubble community. Therefore, how to distinguish surface nanobubbles from blisters is a problem. Herein, the morphology and properties of blisters are investigated on both smooth and nanopitted polystyrene (PS) film in degassed water. The morphology and contact angle of blisters are similar to that of surface nanobubbles. However, blisters were observed to be punctured under the tip-blister interaction and be torn broken by atomic force microscope (AFM) tip in the process of scanning. At the same time, nanopits on the surface of blisters that formed on pitted PS film can be seen clearly. These provide direct and visual evidence for distinguishing blisters from surface nanobubbles. In addition, surface nanobubbles and blisters on smooth and pitted PS film in air equilibrated water are studied. No punctured surface nanobubble was observed, and the force curves obtained on surface nanobubbles as well as the change in height of blisters and surface nanobubbles under large scanning force show that surface nanobubbles are much softer than blisters. Keywords: Blisters, surface nanobubbles, force curves, tip-blister interaction, tip-bubble interaction

*[email protected], [email protected] 1

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Introduction Surface nanobubbles are typical gaseous domains in micro/nanoscale forming at solid-liquid interface[1-4], which has attracted significant interest because of their potential application[5]. Such as liquid flow friction reduction[6-8] fouling prevention[9], heterogeneous cavitation[10] and froth flotation[11,12]. Most studies about surface nanobubbles are on atomically flat substrates such as HOPG (Highly oriented pyrolytic graphite)[13,14]

and

mica[14,15], and mineral dolomite[16], and polymer coated substrates such as PS (polystyrene) film[17-19] and PDMS (polydimethylsiloxane) film[20], hydrophobized glass[21,22] as well as self-assembled monolayer (SAM) modified Si[23,24] and SAMs on Au[25,26]. Surface nanobubbles have been investigated extensively in the last two decades. However, there is still much debate on whether the bubble-like domains on immersed substrates are gas bubbles or not. Research results show that droplets of silicone oil[27-30] on a solid-liquid interface have similar morphology to that of surface nanobubbles, and it was reported that impurities PDMS introduced by using plastic syringes may be the origin of some controversial results in the literature[27,28,31]. To differentiate nanodroplets/nanoparticles from surface nanobubbles, some methods have been proposed, such as using fluorescence microscopy to observe the movement of the three phase contact line over surface nanobubbles, polymeric droplets and hydrophobic particles[32]; evaluating the density by measuring the resonant mass and using light scattering to examine the compressibility of nanoparticles[33]; inspecting the change in morphology of nanobubbles and nanodroplets under atomic force microscope (AFM) scanning force[27,28]; exposing bubble-like domains in degassed water[34], and so on. Besides nanodroplets and nanoparticles, there are other bubble-like objects called blisters forming on polymer coated substrates[35,36], which are also a major cause for misleading and controversial results when studying surface nanobubbles. Blisters can be produced as the thin coating film fails because of osmosis; water permeates through the coated film and forms a small bladder on the substrate-coated film interface. Berkelaar et al.[35] investigated the formation mechanism of blisters in a thin PS film; they concluded that defects on the silicon−polystyrene interface will lead to the formation of blisters, and to prove their viewpoint particles were coated onto the wafer prior to spin-coating the PS film in their study. Actually, PS films are very commonly used substrate to study surface nanobubbles, but 2


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almost all of these work[17,18,37-43] paid no attention to some of the bubble-like domains they observed might be blisters until the work of Berkelaar et al. was published in 2016. Noticeably, it is inevitable that the introduction of impurities in the processes of PS film preparation experiments, although the operators might have paid more attention to keep all the experimental system clean. So, it is very possible that there exist blisters when studying surface nanobubbles on PS film, and it might be partial to regard all the bubble-like domains on immersed PS film as surface nanobubbles or blisters. Up to now, there is still no study about differentiating blisters from surface nanobubbles. In our recent study,[44] we measured the force curves on a nanobubble/nanopancake covered PS film with a hydrophilic sphere and the long-range force was used to verify whether the bubble-like objects are gas nature or not. However, this method cannot distinguish whether a separate bubble-like object is blister or a surface nanobubble, and it cannot differentiate blisters from other bubble-like objects such as droplets either. Noticeably, blisters are a completely different matter from surface nanobubbles, particles and droplets (a sketch of the morphology of a blister, a surface nanobubble, and a nanodroplet formed on a film coated substrate is shown in Fig. 1), and it should show different characteristics under AFM tip blister interaction and large scanning force. However, a systematic comparative study of the force curves on blisters is still lacking, and there is still no effective method to differentiate blisters from surface nanobubbles. Considering the fact that AFM as an invasive technique is extensively used in studying surface nanobubbles, it is significant and essential to investigate and differentiate blisters from nanobubbles for further study of surface nanobubbles and their applications.

Figure 1. Sketch of the morphology of blister, surface nanobubble and nanodroplet formed on film coated substrate. Geometric parameters of blister are defined. 3


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In this paper, blisters are investigated on flat and pitted PS films in degassed water. The change in morphology under different AFM scanning force, size dependence of contact angle and tip-blister interaction were first studied. Especially, whether the film of blisters can be punctured by an AFM tip and under large scanning force was investigated, which provides a direct evidence for blisters. Then the blisters and surface nanobubbles formed on smooth and pitted PS film in air-equilibrated water were investigated. Based on the results of scanning under different set-points and tip-bubble interaction, the stiffness between blisters and surface nanobubbles was compared. Experiments section Materials Preparation In our experiments, two kinds of PS films including smooth PS film and pitted PS film were prepared by spin-coating PS (molecular weight 350,000, Sigma-Aldrich) solution on 1 cm  1cm silicon (100). The smooth PS film was prepared through spin-coating PS solution (0.2 %, weight), the nanopitted PS film was prepared by coating emulsion of PS solution (0.2%, weight) and water with a ratio of 500:1 (volume). A span speed used for preparing the two kinds of PS films is 2000 rpm. To obtain clean silicon (100) substrates, 2-3 silicon wafers (too many wafers may lead to residual impurity) were put in glass container and boiled in a piranha solution for 30 min, then acetone for 10 min, and then ultrasonic rinsed in ethanol and ultrapure water several times (each for 2 min). Finally, the clean PS coated substrates were dried with nitrogen and put in an oven at (52°C) for 5 hours. The thickness of smooth and pitted PS film was measured by AFM nanoshaving to be about 31 ± 2 nm and 30 ± 2 nm, respectively. Ultrapure water (purified with Milli-Q system, Millipore, Corp.) with a conductivity of 18.2MΩ.cm was used in our experiments. To prepare air equilibrated water, about 50 mL ultrapure water was equilibrated in a clean glass beaker for about 12 hours. When preparing degassing water, 100 mL ultrapure water stored in a glass beaker was degassed in a vacuum chamber (DZF-6050LC, equipped with a vacuum pump, 2 XZ-2, China) at 20 mbar for about 5 hours before use. To avoid the oils and minimize the introduction of particles in experiments, glass syringe and glass beakers were carefully cleaned (ultrasonic rinsed in acetone, ethanol and ultrapure water several times (each for 2 min), similar procedure was adopted to clean the AFM liquid cell and AFM tip (the AFM tip was hold up with tweezers and ultrasonic rinsed in acetone and ethanol 10 seconds and then ultrapure water several times (each for 5 seconds). 4


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Measurement methods Tapping mode™ and contact mode (CM) atomic force microscope (AFM, NTEGRA Prima, NT-MDT Company, Zelenograd, Moscow) was used in our experiments. Rectangular cantilever (CSG30, NT-MDT Company) with a nominal spring constant k = 0.13 ~ 2 Nm-1 (the measured spring constant k = 0.65 ± 0.02 Nm-1) and a nominal tip radius of Rt ≤ 10 nm was used. Experiments in water were carried out in an open fluid cell, and a typical resonance frequency of the cantilever in water was 56 kHz. A typical scanning frequency of 1 Hz with a calculated value of free amplitude A0 of 4.3 ± 0.2 nm and different set-point ratios sp 

A1 A0

were adopted (a detailed depiction can be seen in the text), where A1 is the reduced amplitude. With these data, the applied force (Fa) on blisters or surface nanobubbles is given by

Fa  k * A0 * (1  sp ) . Room temperature and humidity were 25 ± 1 °C and 45-55 % RH, respectively. Results and Discussions Imaging of blisters: their morphology and tip-blister interaction Figure 2a(i) shows the topography of bubble-like domains on smooth PS film, which is obtained at a scan area of 5 μm × 5 μm, a scan frequency of 1Hz and an amplitude set-point of 95 % after immersing the PS substrate in degassing water. Due to the fact that water degassing can effectively constrain the nucleation of surface nanobubbles at the solid-liquid interface, the bubble-like domains observed here are very probably blisters. The ‘blisters’ in this experiment are generally cap-shaped (as can be seen from the 3D image of Fig 2a(iv)) and with a lateral size less than 3μm. This is similar to the morphology of surface nanobubbles[1,3,4,17,45] in previous literature. In Fig. 2a(ii) and Fig. 2a(iii), we scanned a typical blister labeled as “blister 1” in Fig. 2a(i)) with a reduced set-point value of 85% and 70% respectively, this means an increase in scanning force, and then enlarge the scan region to the original size (Fig. 2a(iv)) at set-point of 95%. The height of the blister shows a slight decrease, first with the increase of scanning force (set-point 85%), then decreases to about 50 nm from about 70 nm at set-point of 70%, and then recovers to the initial height with a reduction of the scanning force (set-point 95%). The morphology change of blister 1 can also be seen from the section analysis in Fig. 2c, which is in line with the change in morphology of surface nanobubbles under different scanning force[17,46,47]. But differently, blisters show more robust as compared with surface nanobubbles in the literature[17,46,47], i.e., surface nanobubbles are much softer than blisters. 5


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Based on captured images, the height h and literal size L can be measured from the dimensional cross-sections along the line on blisters, and then the contact angle θ (inner side) can be calculated (h, L and θ are defined in Fig. 1). Figure 3 plots the height and contact angle of blisters as a function of their lateral size. The height of blisters is shown as linearly dependent on their lateral size in Fig. 3a; the contact angle of blisters exhibits a nonlinear increase with the increase of their lateral size in Fig. 3b. This is consistent with the results of surface nanobubbles reported in literature [13, 45].

Figure 2. Morphology of blisters formed on PS film. Image (a) shows blisters that were captured on PS film in degassing water; image (b) shows the 3D image of image a (iv) and image, the square around the blister 1 should be the indentation caused by AFM tip after large force scanning (image a(iii)). A similar result can refer to a relevant study[48] (c) is the section analysis of blister 1 from a(i) to a(iv), the zero height was selected at the lowest point of the section curve. The blister shows a similar property to that of surface nanobubbles under different scanning force.

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Figure 3. Height and contact angle of blisters as function of blister size. The height of blisters shows a linear dependence on its lateral size in image (a), and the contact angle of blisters show a nonlinear increase with its lateral size in image (b).

To further study the property of blisters, we increase the scanning force to inspect whether the film of these water pockets will rupture. However, most of the blisters can endure large force TM-AFM scanning, and even remain intact after being subjected to invasive contact mode (CM) AFM scanning. Surprisingly, a blister was observed as it ruptured under large scanning force (set-point 50%). As can be seen in Fig. 4a(iii), the blister 2 in a(i) is broken completely. This can be explained that blister 2 was punctured first by the vibrative AFM tip, and then the AFM tip was trapped in the surface of blister and tore the blister broken, leave a hole and piece of broken PS film. This phenomenon provides direct and visual evidence for the bubble-like domains forming at PS film in degassed water to be blisters (not surface nanobubbles, drops and particles). Similar phenomenon can also be seen in Figure S1 in supporting information, where the blister was punctured first by AFM tip after repeated tip-blister interaction, then was torn broken even at the scan with set-point of 95%. On the contrary, blisters in Figure S 2 show intact with scan at set-point of 90% and 80% and even under the contact mode (CM) scanning. This indicates that blister can be punctured by tip-bubble interaction or large force scanning (TM), and it is easy for a punctured blister to be torn broken. Figure 4b shows the dimensional cross-section along the line in Fig. 4a(iii), based on which the PS film thickness can be evaluated about 30 nm. This is in accord with the result measured by AFM nanoshaving (31 ± 2 nm).

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Figure 4. Film of blister ruptured under large scanning force. Image (a) shows the process of blister 2 ruptured under large scanning force; image (b) is the dimensional cross-section along the line in image a (iii). The blister 2 bursts after subjecting to large scanning force.

Tip-blister interaction was also carried out in our experiments. Figure 5 shows that the blister 3 was punctured by AFM tip after tip-bubble interaction, corresponding force curves are provided in Fig. 6a. The force of AFM tip exerted on blister surface shows a rapid increase from point A to B in the extension curve, interestingly, there is a transient spring-back from point B to C. This indicates that the PS film was punctured at point B with the force about 20 nN. Then the force gradually increases accompanied by several small fluctuations such as point D (the fluctuation in extension force curve should be caused by the tip shape (backward pyramid shape), and eventually rises to the maximum value (about 30 nN), the tip at this moment should have touched the silicon substrate (point E). Obviously, the force of tip-blister interaction is much larger than that of tip-bubble interaction (a maximum value of tip-bubble interaction evaluated in our previous work is about 4.2 nN[17]. 8


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In addition, the blister height can also be estimated from the force curve, the AFM tip comes into contact with the blister surface at point A, and get to the hard substrate at point E, which means that the height of blister 3 should be the distance from A to E, i.e., about 80 nm. This is consistent with the evaluated height of blister 3 in Fig. 6b. The inset in Fig. 6b shows an intact 3D image of blister 3, and the insect in Fig. 6a is the 3D image of punctured blister 3 after tip-blister interaction. The punctured blister provides another visual proof for differentiating blisters and surface nanobubbles.

Figure 5. Punctured blister under the tip-blister interaction. The blister was punctured by AFM tip after tip-bubble interaction.

Figure 6. Force curves of tip-blister interaction. Image (a) shows the force curves obtained on blister 3 in Fig.5, and image (b) is the dimensional cross-section along the line on blister 3 in Fig. 5 (i), the insets in image (a) and (b) are 3D image of blister 3 after and before subjecting tip-blister interaction, respectively. The thickness of PS film and height of blister can be evaluated according to the force curve and section analysis.

Blisters and surface nanobubbles on nanopitted and smooth PS films In this section, we examined blisters and surface nanobubbles forming on nanopitted and smooth PS films. Figure 7a shows the height image of pitted PS film captured from contact 9


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mode AFM in air. The diameter and depth of these nanopits are generally on the order of ~200 nm and ~10 nm, respectively. Figure 7b shows the cross-section on a typical nanopit labeled as nanopit 1 in Fig. 7a, from which the diameter of nanopit 1 is about 150 nm and the depth is about 6 nm. This indicates that the pits do not destroy the PS film with a pitted film thickness (30 ± 2 nm). Thus, it is very possible for the formation of nanopitted blisters on the PS film. Fig. 7c(i) shows the morphology of blisters observed with TM-AFM after immersing the pitted PS film in degassed water, nanopits can be seen dotted among varisized bubble-like domains. By scanning the largest bubble-like object (the square region enclosed with blue line in Fig. 7c(i)), Fig. 7c(ii) was obtained and nanopits can be seen clearly located on the surface of the bubble-like object. This again demonstrates the bubble-like domains forming at the pitted PS film in degassed water are not surface nanobubbles but blisters.

Figure 7. Blisters forming at pitted PS film in degassing water. Image (a) shows nanopitted PS film captured in air (CM-AFM) and image (b) is the section analysis of nanopit 1 in image (a), the insect shows the 3D image of nanopit 1; image c(i) shows blisters forming at the pitted PS film, c (ii) was obtained by scanning the square region in c(i), the nanopits on the blister surface can be seen clearly.

When immersing the pitted PS film in the air-equilibrated water, both surface nanobubbles and blisters are captured forming at the solid-liquid interface. Figure 8(i) shows a large number of nanobubbles and two blisters on the pitted PS film, nanopits can be seen distinctly dotted among the surface nanobubbles. Noticeably, there are several nanobubbles 10


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forming on the top of blisters. Fig. 8(ii) and Fig.8 (iii) are the height image and phase image of the enclosed area (yellow line between two blisters) in Fig. 8(i), respectively; a number of surface nanobubbles dotted with a few nanopits can be seen clearly. When changing set-point from 95% to 80%, most of small surface nanobubbles show being squashed (Fig. 8(iv)) and then recover partially at the set-point of 95% (Fig. 8(v)). This is in line with the result of surface nanobubbles in the work of An et al.[27], who studied the mechanical responses of nanobubbles and nanodroplets; the drops in their study show a complete recovery after suffering large load scanning. Figure 8(vi) shows the enclosed area (blue line) in Fig. 8(i) captured at set-point of 80%, where one nanopit on the blister surface can be seen clearly. After the scanning of Fig. 8(vi), Fig. 8(vii) was captured at the original site with set-point of 95%. By comparing Fig. 8(vii) with Fig. 8(i), the phenomenon of bubble coalescence can be observed both on the blister surface and beside the blister (bubble 1 formed after bubble coalescence). Tip-nanobubble interaction was also carried out on nanobubbles, the force curves obtained on nanobubble 1 in Fig .8(viii) can be seen in Fig. 9a. The extension curve shows a ‘drop in’ at about 16 nm and touch the hard substrate at about 5 nm, which means a bubble height of about 11 nm. This is coincident with that deduced from the section analysis in Fig. 9b. Noticeably, the force exerted on nanobubble 1 is about 1 nN, which is about a thirtieth of that on blister 3 in Fig. 6a. This again indicates that surface nanobubbles are much softer than blisters, thus we also can distinguish the two different bubble-like domains from their stiffness.

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Figure 8. Surface nanobubbles and blisters forming at pitted PS film in air equilibrated water. (i) shows a mass of surface nanobubbles and two blisters forming on pitted PS film in air-equilibrated water; (ii) and (iii) are the height image and phase image of the enclosed area (yellow line between two blisters) respectively, (iv) and (v) are successive scanning at set point of 80% and 95% respectively after (ii) was captured; (vi) shows the enclosed area (blue line) in Fig. 8(i) captured at set-point of 80%, (vii) was obtained at the original site of (i) at set point of 95%; (viii) shows the yellow enclosed area in (vii).

Figure 9. Force curves of tip-nanobubble interaction. Image (a) shows the force curves obtained on nanobubble 1 in Fig.8, and image (b) is the dimensional cross-section along the line of nanobubble 1.

We also investigated the formation of blisters and surface nanobubbles formed on smooth PS film in air-equilibrated water. Considering surface nanobubbles are softer than blisters, we used a relatively large scanning force to separate surface nanobubbles from blisters. As can be seen in Figure10 a(i), both blisters and surface nanobubbles were captured, surface nanobubbles are squashed obviously as compared with the blisters (set-point 80%), but in Fig. 10 a(ii), the height of surface nanobubble show recovered at set-point of 95%, and 12


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at this time we cannot distinguish the blisters from surface nanobubbles nearly. However, after replacing the air-equilibrated water by degassed water, as can be seen in Fig. 10 b(ii), almost all the surface nanobubbles are disappeared as compared with that in Fig. 10 b(i). This provides direct evidence for the gas nature of nanobubbles. Together with the experimental results above, we can conclude that the bubble-like objects formed on smooth PS films in this study are surface nanobubbles and blisters other than drops and particles, and these two kinds of bubble-like objects can be differentiated in degassed water or by using relatively large scanning load in air-equilibrated water.

Figure 10. Blisters and surface nanobubbles forming on smooth PS film. Image a(i) shows surface nanobubbles being squashed at set-point of 80% as compared with the blisters, but all surface nanobubbles show recovered at set-point of 95% in image a(ii); image b(ii) shows that the surface nanobubbles disappeared but blisters remain there after using degassed water to replace the air-equilibrated water (image b(i)).

Conclusions In this paper, the morphology and properties of blisters are investigated on smooth and pitted PS film in degassed water, at the same time, surface nanobubbles and blisters on smooth and pitted PS film were studied in air equilibrated water. Blisters exhibit the 13


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following characteristics in our experiments (1) cap-shaped; (2) can form on both smooth and pitted PS film in both degassed water and air-equilibrated water; (3) much stiffer than surface nanobubbles; (4) the contact angle and height of blisters are as a function of their size; (5) can be punctured by tip-blister interaction and be torn broken by AFM tip under large scanning force. Based on the results in this study, some methods can be used to distinguish blisters from surface nanobubbles, drops and particles: including scan with large scanning load, tip bubble/blister interaction and using pitted film as substrate; to distinguish surface nanobubbles from blisters and drops, degassing water can be used; when to separate blister from drops, large force scanning and tip-blister/drop interaction should be more effective. Our work presents a direct and practical method to distinguish blisters from surface nanobubbles, drops and particles by using AFM, which should be of great interest to the nanobubble community as well as to other researchers who study soft matter such as graphene nanobubbles[49][50]. Supporting Information Images of blister was punctured first by AFM tip after repeated tip-blister interaction, then was torn broken at the scan with set-point of 95%; Images of blisters show intact with scan at set-point of 90% and 80% and even under the contact mode (CM) scanning. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author *E-mail: [email protected];

[email protected]

Notes The authors declare no competing financial interest Acknowledgement The authors thank the support from the Key laboratory of Micro-Systems and Micro-Structures Manufacturing in HIT and the help of Feiran Li for preparing PS film. We gratefully acknowledge the financial support from the Heilongjiang Province Natural Science Foundation of China (E2016059, E2017060), the National Natural Science Foundation of China (51775175). Dayong Li gratefully acknowledge the financial support from China Scholarship Council and Heilongjiang Provincial University Basic Scientific Research Service Project (Hkdqg201803).

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References

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Properties of Blisters Formed on Polymer Films and Differentiating

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