Preprint Title Morphological and force spectroscopy characterizations for indentification of surface nanobubbles from nanodroplets and blisters

12 Surface nanobubbles (NBs) play an important role in various practical applications, such as min13 eral flotation and separation, drag reduction, and nanostructured surface fabrication. Until now, it 14 still remains as a challenge to identify surface NBs from other spherical-cap-liked nano-objects, 15 like blisters and nanodroplets (NDs). Here we focus on the distinctions of NBs from NDs and blis16 ters using an atomic force microscopy. It is implemented through morphological characterization, 17 high load scanning, and force spectroscopy measurement. In the morphological characterization 18 experiment, contact angles of the three types of nano-objects were compared. In the high load 19 scanning experiment, the response of the nano-objects to high scanning loads was studied. The 20 mobility, deformability, and volume change of the nano-objects during the high load scanning were 21 investigated. At last, the force spectroscopy measurement was implemented. Due to the existence 22 of the three-phase contact lines on both tip-NB and tip-ND interactions, force-distance curves ex23


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Surface nanobubbles (NBs) at solid liquid interfaces have attracted significant attentions in the last 32 two decades because of their great potential in numerous applications, such as mineral flotation 33 and separation [1], drag reduction [2][3][4], nanostructured surface fabrication [5-8] and the context 34 of catalysis and electrolysis [9,10]. The properties of NBs have been investigated with numerous 35 techniques, including atomic force microscopy (AFM) [11-13], x-ray reflectivity [14,15], infrared 36 spectroscopy [16,17], and optical microscopy [18][19][20]. However, the NB community has long been 37 suffering from the confusion caused by some other spherical cap shaped nano-objects, like nan-38 odroplets (NDs) [21,22] and blisters [23]. NDs may nucleate in NB experiments because of the im-39 purities in liquid. This is because NDs basically are similar to NBs regarding the nucleation mech-40 anism. The blisters are thin polymer film wrapped water pockets at solid-liquid interfaces on a thin 41 film coated silicon substrate [24,25]. They are produced as water permeates through the thin film 42 (i.e. a polystyrene film), wet and hence detach the thin films, leading to the formation of a water 43 reservoir in between the supporting silicon substrate and the thin film [23,26]. So far it is still dif-44 ficult to distinguish NBs from these spherical cap shaped nano-objects, especially from NDs. NDs 45 and NBs are all soft in nature, which makes it more challenging to distinguish one from the other 46 with current imaging techniques. 47 Several research works have been performed to identify surface NBs [22,[27][28][29][30]. The gaseous NBs 48 tion curve on nanodroplet shows sharp kinks. This is in conflict with what was reported in An's 79 work. 80 In this work, the spontaneous formation of NBs on a hydrophobic surface was adopted to avoid 81 addition of any other solvent and then minimize the chance of contamination. By employing nano-82 manipulation and force spectroscopy measurement of AFM tips on NBs, NDs, and blisters, a sys-83 tematic investigation was conducted to distinguish NBs from NDs and blisters. We find that NBs 84 are distinguishable from three aspects: (a) volume changes before and after coalescence, (b) re-85 sponse to higher loads, and (c) force spectroscopy measurement. The polystyrene (PS) surfaces used for NB and blister nucleation were prepared by spin coating 89 thin films of PS on silicon (100) substrates at a speed of 1000 rpm. Before spin coating, the sub-90 strates were sequentially cleaned in sonication bathes of piranha, acetone, and then water, each 91 for 30 min. PS particles (molecular weight 350 000, Sigma-Aldrich) were dissolved in toluene 92 (Mallinckrodt Chemical) to make the PS solutions. Two PS films, PS sample 1 and PS sample 2, 93 were prepared. The PS concentrations for sample 1 and sample 2 are 1.0% and 0.5%, respectively.

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The lower PS concentrations leads to a thinner PS film and may cause surface defects [ changing solvent or producing temperature differences, local gas supersaturation can be achieved, 105 resulting in the nucleation of NBs. In this study, the spontaneous nucleation of NBs was adopted.

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The water was first kept in air for more than 10 hours for air diffusion. To avoid the possible con-  AFM images also reveal that the three objects exhibit different aspect ratios. Figure 2 depicts the 156 contact angle of NBs, NDs, and blisters from the gas, oil, and solid side, respectively. It clearly 157 shows that increases with for both NBs and NDs. On the contrary, is independent of for 158 7 blisters. In addition, NBs and NDs have larger than that of blisters, and for NBs is slightly 159 lower than ND. 160 The response of NBs to higher load scanning is a typical approach to study their physical prop-161 erties [31,38]. Here a higher scanning load (setpoint 60%) was applied for imaging. After that, a 162 98% setpoint lower load scanning was applied to the same areas to observe the change of sample 163 surfaces. The results are shown in Figure 1 (g-i). It shows that the NBs and NDs were moved and 164 coalesced during the higher load scanning. As a result, larger NBs and NDs were nucleated ( Figure   165 1g and h). However, no apparent change was observed for blisters. They all remained at the same  before coalescence in Figure 1 (d) and (e) are 4.55×10 6 nm 3 and 1.05×10 7 nm 3 for NBs and NDs, 184 respectively. After coalescence ( Figure 1 g and h), the total volumes are =1.1×10 7 nm 3 and 185 =1.11×10 7 nm 3 for NBs and NDs, respectively.

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190 where 0 is the ambient pressure, is surface tension of water, and is the radius of NBs.  suming that the NBs are equilibrated with each other and the gas concentration in the edge of 192 bubbles is constant, the inner pressure rapidly reduces with increasing . This has two con-sequences. First, based on the ideal gas law, the gas molecules originally trapped in NBs before coalescence will occupy more space when is reduced. This leads to increased bubble volumes.
NBs. This further increases NB volume. Regarding the NDs, as expected, the volumes are almost 197 the same before and after the coalescence. The results indicate that the content in the spherical ob-198 jects on PS surface 1 are indeed gas.

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After the 60% setpoint high load scanning, the successive AFM scans with setpoints from 98% to Notably, the AFM tip used here is silicon, which is hydrophilic. Previous studies demonstrate that 239 the hydrophobicity of AFM tips can significantly influence tip-bubble interactions and hence the 240 bubble imaging [46,47]. For hydrophobic tips the bubble interface may jump towards the tips. This  The tip-ND interaction is very similar to the tip-NB interaction ( figure 4 (b)). However, tip-blister 247 interaction exhibits distinct behaviors from that of tip-NB/ND interactions, as shown in figure 4 (c).

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The force curves measured on the blisters is close to that measured on the bare PS substrate. The can see that blisters more behave as solid objects in tip-sample interaction measurement.

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With the above force spectroscopy measurement, it is easy to distinguish NBs from blisters. How-252 ever, force-distance curves on the soft objects of NBs and NDs are very close to each other. In or-253 der to further investigate the difference between NBs and NDs, we extract several key indicators 254 from force distance curves.

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As shown in Figure 4   The parameter ℎ extracted from the force distance curves is the length of pulled capillary 279 bridge, namely, the deformation of the NB/ND when the tip detaches from them. In most cases, 280 the capillary bridge can be pulled out for a limited distance before the AFM tip eventually detaches. 281

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As shown in Figure 7, the ℎ of the NB is smaller than that of the ND. This result indicates that 282 the ND has a much larger capillary bridge than that of the NB. The medium value of ℎ on the 283 ND is about 36 nm, which is about three times of the value measured on the NB. This is because 284 that the droplet of PDMS is non-Newtonian liquid. That can stretch longer, since the stretching 285 modifies the stress balance anisotropically.   In this study, the distinction among surface NBs, NDs and blisters was systematically investigated 302 with an AFM through three approaches: morphological characterization, high load scanning, and  The NBs and NDs have similar contact angles. They are all movable and deformable, and exhibit 308 similar response at force-distance curves. However, they can be well distinguished by volume and 309 16 force spectroscopy measurement. The volume of NBs significantly increased after coalescence, 310 while it remains the same for NDs. This is because of the reduced inner pressure and gas diffusion 311 from liquid to the coalesced NBs. In the force spectroscopy measurement, three parameters, the 312 prefactor of linear tip-NB/ND interaction region, the adhesion force ℎ , and the deforma-313 tion ℎ are extracted. The results show that NBs have larger , ℎ , which agrees well with 314 the fact that the water/air surface tension is higher than water/oil. On the contrary, ℎ on NBs is 315 much smaller than that on NDs, due to the lower viscosity of water. We believe that this work pro-