Article pubs.acs.org/Langmuir
Collective Effects in Microbubble Growth by Solvent Exchange Shuhua Peng,† Tony L. Mega,§,‡ and Xuehua Zhang*,† †
Soft Matter & Interfaces Group, School of Engineering, RMIT University, Melbourne, VIC 3001, Australia Revalesio Corporation, 1200 East D Street, Tacoma, Washington 98421, United States
‡
S Supporting Information *
ABSTRACT: Regulating the formation and growth of microscopic bubbles at solid−liquid interfaces is essential in many physical, chemical, and catalytic processes, such as the electrolysis of water or a dry−wet transition of a superhydrophobic surface. The growth of bubbles in a group is influenced by the neighboring bubbles as well as the overall gas concentration in the system. In this work, we have investigated the growth of multiple microbubbles on highly ordered hydrophobic microcavity arrays, seeded by pre-existing gas pockets trapped inside the cavities. A pulse of gas oversaturation at an extremely low level was supplied in a process we call solvent exchange. Our results show that the distance between the seeding air pockets has significant effects on the location, number density, and size of bubbles on the array. With closely spaced microcavities, growing microbubbles self-organized into symmetric patterns. Their growth rate was enhanced at the corners and edges of the array, and interior bubbles dissolved because of the competitive growth. By contrast, no symmetric patterns were observed when the space between the microcavities was large. The findings reported in this work provide important insights into solvent exchange and collective interactions in the formation of surface nanobubbles.
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INTRODUCTION The latest research has significantly advanced our understanding of the fundamental properties of a single nanobubble.1 Notably, the stability and contact angle of a nanobubble are closely associated with pinning effects at the three-phase contact line and the oversaturation level in the system.2−7 A pinned nanobubble can reach a stable state in contact with a mildly oversaturated liquid through negative feedback of the Laplace pressure inside a bubble with a varying contact angle.2,7 Recently, White et al. have suggested the nucleation mechanism of a single nanobubble on the surface of a nanoelectrode based on the current−voltage signals during the formation of a bubble from electrochemical reactions.8−10 In many experimental studies,11,12 a nanobubble is not sitting alone on the surface but is surrounded by many others. In these cases, the nucleation, growth, and stability of bubbles are determined not only by the overall concentration in the system but also by the local concentration gradient imposed by the neighboring bubbles. The cooperative interactions are intimately related to the level of the external oversaturation level and the distance between the bubbles. The bubbles grow slowly at a low oversaturation level, allowing a long period of time for the neighboring bubbles to interact through the diffusion field before they merge with each other or detach from the surface.13 A low oversaturation level is also important for the bubble growth to be dominated by gas diffusion and not affected by convection or advection.13 Indeed, the depletion from a growing microbubble at an extremely low oversaturation level © 2016 American Chemical Society
(∼1.2) was nicely shown in the recent work by Enriquez et al.13,14 Their experiments showed the memory effect; that is, the growth of the first bubble slowed the growth of the second bubble from the same location as a result of the depletion of the dissolved gas in the surrounding liquid. However, such depletion effects were not observed at a high oversaturation level.13 Solvent exchange has been one of the most-used approaches for creating surface nanobubbles or nanodroplets.11,15−18 In a typical solvent-exchange process, the substrate is in contact with air-equilibrated ethanol (a good solvent for air) that is then displaced by air-equilibrated water (a poorer solvent for air). During the displacement, an oversaturation of air is created at the mixing front of the two solvents, leading to the formation of surface nanobubbles on highly ordered pyrolytic graphite or several other substrates.11,17,19−21 The collective interactions among growing nanobubbles during solvent exchange were implied by the spatial distribution of nanobubbles from Voronoi tessellation analysis.22 The size of the depleted surface area surrounding the bubbles was found to be well-correlated with the bubble size. A larger bubble was usually accompanied by a larger bare zone, suggesting that the depletion of dissolved gas by the growing bubble could hinder Special Issue: Nanobubbles Received: June 1, 2016 Revised: July 12, 2016 Published: July 19, 2016 11265
DOI: 10.1021/acs.langmuir.6b02066 Langmuir 2016, 32, 11265−11272
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Figure 1. Array of microcavities for microbubble formation by solvent exchange: (a) Top-view SEM image of a microcavity array. Length of the scale bar: 40 μm. (b) Sketch of the microcavity layout. The cavity diameter L was 10 μm, and the center-to-center distance of two adjacent cavities D was 13 μm on a closely spaced array and 40 μm on a loosely spaced array. The cavities are labeled from 01 to 100. (c) Optical profile of the microcavity array. The cavity depth was 50 μm. (d) Contact angles of ethanol aqueous solutions at different ethanol fractions on the cavity surface with a hydrophobic coating. (98%), hydrogen peroxide (35%), and ethanol (100%) were from Merck Pty Ltd. All chemicals were used without further purification. Fabrication of Hydrophobic Microcavity Arrays. A standard photolithography process was employed to prepare circular patterns on the silicon wafer substrates. The prepatterned substrate with circular domains/arrays surrounded by a protecting photoresist was subsequently applied for deep reactive-ion etching (DRIE, Bosch etcher Oxford PlasmaPro 100 Estrelas). The depth of microcavities was controlled by the etching duration of DRIE fabrication. After the desired cavity depth was achieved, the substrate was sequentially cleaned with acetone, ethanol, water, and then piranha solution (30% H2O2, 70% H2SO4). The hydrophobilization treatment was carried out by a chemical vapor deposition of PFDTS. The samples were placed under vacuum together with the silane at room temperature for 2 h. The design of the cavity surface is shown in Figure 1(a). The cavity surface consisted of 10 × 10 cylindrical microcavities in an array. The diameter of the cavity (L) was 10 μm, and the center-to-center distance between two cavities (D) was 13 μm for the closely spaced array and 40 μm for the loosely spaced array, respectively. A typical top-view scanning electron microscopy (SEM) image of the cavity surface with D = 13 μm in Figure 1(a) shows that the microcavities were uniformly distributed in the array. SEM analysis was performed using an FEI Nova NanoSEM equipped with an Oxford X-MaxN 20 energy-dispersive X-ray (EDX) detector operating at 15 kV and a 5 mm working distance. Samples were coated with about 5 nm Au before imaging. The depth of these cylindrical cavities was confirmed by 3D optical surface profilometry (Bruker, ContourGT-K1), as shown in Figure 1(c). In this work, the cavity substrates with 50 μm depth were applied for all solvent-exchange experiments. Hydrophobilization of the cavity surface was subsequently carried out by a chemical vapor deposition coating of PFDTS. The contact angles of aqueous ethanol solution as a function of ethanol fraction on the hydrophobic cavity surface are shown in Figure 1(d). The water contact angle of the hydrophobic cavity surface was about 115°, which decreased to about 89° when a 20% fraction of ethanol was added to water. It decreased further to about 40° when pure ethanol was deposited on the surface, suggesting that ethanol can readily penetrate the cavities. Solvent-Exchange Procedure. The substrate was exposed to water, which was then displaced by ethanol. All of the solvent
the formation of other nanobubbles in the adjacent area. The collective effects during solvent exchange were also suggested by the self-organization of surface nanodroplets. During the growth of multiple surface nanodroplets confined at the circular rim of a microcap on the substrate, the droplets organized themselves into symmetrical arrangements, dividing the circumference of the microcap almost equally. The competitive growth between neighboring droplets was proposed to explain the evolution of those symmetrical patterns.23 The goal of this work is to better understand the collective interactions in the growth of surface bubbles in response to the oversaturation created by solvent exchange. To follow the bubble growth in time from optical microscope images, we have quantified the size of bubbles that are larger than surface nanobubbles (typically less than 100 nm in height). The initial distance between growing microbubbles was well controlled by the spacing of the hydrophobic microcavities on the substrate. The substrate was exposed to water first and then to ethanol in an order opposite to the usual procedure in the literature.2,18 The air pockets trapped in the hydrophobic microcavities under water acted as bubble precursors, expanding under an extremely low oversaturation level when water was displaced by ethanol. We found remarkable symmetrical patterns of growing microbubbles on a closely spaced array due to the collective interactions among the bubbles. The growth rate of the bubbles reveals the local oversaturation level depending on their proximity to larger earlier-grown bubbles, providing a quantitative measure of the collective interactions among slowly growing microbubbles on the surface. The findings in this work provide insights into the concentration profile during solvent exchange and the formation of nanobubbles by this process.
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EXPERIMENTAL SECTION
Chemicals. 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTS) was purchased from Sigma-Aldrich. Chemicals including sulfuric acid 11266
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Langmuir exchanges were performed in the same fluid cell at room temperature, the height of the fluid channel was kept constant (2.66 mm), and the flow rate of solvent exchange was precisely controlled by a syringe pump. All of the water, ethanol, and ethanol aqueous solutions were air-equilibrated under ambient conditions for at least 24 h before use. The growth of bubbles on the ordered array of microcavities was monitored by optical microscopy (Huvitz HRM-300, Scitech Pty. Ltd., Australia). In all experiments, the bubble growth was measured before water was completely displaced by ethanol. The concentration of ethanol was not high enough to cause the detachment of the microbubbles from the cavities.
microbubble growth was further confirmed by exposing the substrate to ethanol, water, and ethanol again. In this case, no microbubbles were observed because ethanol in the first step removed the air pockets. This work follows the microbubble growth from the air pockets trapped in the arrays of microcavities during the solvent exchange of water by ethanol. What is the oversaturation, ΔC, created by the solvent exchange? From the gas solubility in pure ethanol and pure water, we can know the upper boundary of the oversaturation level. The solubilities of N2 and O2 in the solutions with different ethanol fractions are plotted in Figure 3(a). The straight lines represent the amount of dissolved gas when pure ethanol or pure water are mixed in different ratios. The oversaturation of gas at a given ethanol fraction (ΔC) is obtained from the difference between the actual solubility and the point on the straight line. The plot in Figure 3(b) shows that the oversaturation level ζ is the highest at an ethanol fraction of around 0.3, where the maximum ζ is 0.5 for oxygen and 0.2 for nitrogen. However, during the solvent exchange the solution may not even reach this maximum oversaturation level. Rather than mixing pure water and pure ethanol directly at the optimal fraction, the ratio of ethanol in the solution increases gradually at the mixing front, while the water-rich solution is rinsed off. Because dissolved air is mainly supplied by ethanol, the ratio between the dissolved gas and ethanol remains almost same as the ratio in saturated ethanol. The ethanol concentration in solution varies with time and with location. We do not know the exact concentration of the ethanol concentration and hence the oversaturation level with time, although it is clear that the gas oversaturation from the solvent exchange is extremely low. We could estimate the actual oversaturation level from the growth rate of the bubble, assuming that the bubble growth was in a constant contact radius mode (CR mode) sketched in Figure 4(a). Because the diameter of the microcavities was known, the contact angle and the volume of the bubble were calculated on the basis of the bubble radius from the top view. Figure 4(d) shows the time course snapshots of a growing bubble that was far away from other cavities (L ≪ D), so the growth of the bubble was not influenced by other bubbles. Figure 4(e) shows the plots of the bubble radius as a function of time. The contact angle of the bubble through the gas phase is plotted in Figure 4(f), showing that the contact angle increased rapidly initially and then reached 150°. This maximum contact angle was close to the static contact angle of the bubble in the surrounding area measured macroscopi-
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RESULTS Oversaturation Level by Solvent Exchange and the Growth Rate of a Single Bubble. We did not observe the growth of bubbles from the microcavities when the substrate was directly immersed in pure ethanol or pure water or when in contact with ethanol that was then displaced by water. Bubbles formed only on the array, following the displacement of water by ethanol (Figure 2). These results suggest that it is crucial
Figure 2. Schematic illustration of the microcavity array in contact with (a) ethanol and (b) water. (c) Sketch of the standard solventexchange process where the good solvent, ethanol, is displaced by the poor solvent, water. (d) Sketch of solvent-exchange procedure used in this work. The substrate was in contact with the poor solvent (water) first and then the good solvent (ethanol).
that a dry substrate is in contact with water first and then with ethanol. The reason is that ethanol wets the substrate better than water does, as sketched in Figure 2(a,b). Therefore, ethanol can penetrate the cavities and remove the air pockets (i.e., the Wenzel state), while water cannot penetrate the cavities and displace the air pockets (i.e., the Cassie state). The trapped air pockets in the microcavities grew as the water was displaced by ethanol. The essential role of the air pockets for
Figure 3. (a) Scatter lines are gas solubilities as a function of ethanol fraction, and the solubility data is taken from ref 24. The solid straight lines represent ethanol dilution curves during solvent exchange, and the oversaturation ΔC can be estimated from the solubility difference between two lines. (b) Oversaturation ΔC and oversaturation level ζ as a function of ethanol fraction. The reproduced data are for individual nitrogen or oxygen, and the solubility of air in the solutions lies between the values for pure nitrogen and pure oxygen. 11267
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Figure 4. (a) Schematic illustration of a growing bubble with a constant base area and a pinned boundary. (b) The contact angle θ calculated from the bubble radius and the dotted lines indicating the largest bubble contact angle based on the wettability of the cavity surface in constant contact area growth mode. (c) The geometry factor f(θ) as a function of θ and the range of the contact angle of microbubbles in this work is labeled in the shaded area. (d) Time course snapshots of a single growing bubble on the microcavity by solvent exchange. Scale bar: 20 μm. (e) Bubble radius R, (f) bubble contact angle θ, (g) bubble volume V, and (h) calculated oversaturation ΔC according to eq 5 as a function of growing time.
2ρ dV = c∞ − cs = ΔC LDf (θ )π dt
cally in ethanol solution (Figure 1(d)). The bubble volume is calculated from the bubble radius (R) and the diameter of the microcavity (L).
V=
4πR3 − 3
⎛ π ⎜R − ⎝
2
R −
L2 4
⎞⎛ ⎟⎜2R + ⎠⎝
2
R −
L2 4
The plot of ΔC as a function of time in Figure 4(h) shows that the concentration of the dissolved air was very low in the solution, corresponding to an ethanol fraction of 0.01 from the solubility plot in Figure 3. On the basis of the flow rate and the geometry of the fluid channel, we estimated that the concentration of ethanol had increased by a comparable ratio for the given time. Spatial Arrangement of Microbubbles on Microcavity Arrays. The time course snapshots in Figure 5 show several notable features of microbubbles on the closely spaced array. (1) Enhanced bubble growth at the corners of the array: The bubble growth did not follow one bubble on one cavity, but started from the four corners of the array. At t = 18 s, bubbles on the corners were already large enough to be clearly resolved from the images, whereas bubbles at other locations were barely distinguishable from the cavities themselves. (2) Enhanced bubble growth on the edges of the array: The bubbles on the edge (04 and 06) were then clearly larger after 26 s, and the latest were those in the inner area (34 and 26) around 32 s. Remarkably, the second bubble never appeared next to the first bubble. (3) Dissolution of smaller bubbles: At t = 52 s, bubbles located in the inner area of the array started shrinking, whereas some larger bubbles on the corners kept growing and the total number of bubbles decreased with time. The dissolution of the smaller bubbles followed the sequence in the location of the inner area, the center, and the edges of the array. (4) Symmetrical arrangement of bubbles: Bubbles on approximately symmetric locations in the array (corners, center of edges, and center of the array) last longer than bubbles in the inner area. The remaining bubbles acquired symmetrical
⎞ ⎟ ⎠
3
(1)
On the basis of the bubble volume plotted in Figure 4(g), we calculated the oversaturation level by the solvent exchange. The volume gain (dV/dt) of the bubble follows the expression generalized by Lohse and Zhang7 ⎡⎛ ⎤ π dV 4σ sin θ ⎟⎞ cs = − LD⎢⎜P0 + − c∞⎥f (θ ) dt 2ρ ⎣⎝ L ⎠ P0 ⎦
(2)
where D is the diffusion coefficient, L is the diameter of the microcavity, cs and c∞ are the solubility and the concentration of gas in the liquid, ρ is the gas density, and θ is the contact angle. f(θ), a geometry factor, is plotted in Figure 4(c) as a function of the contact angle of the bubble. f (θ) =
sin θ +4 1 + cos θ
∫0
∞
1 + cosh 2θξ tanh[(π − θ)ξ]dξ sinh 2πξ (3)
Because the size of the bubbles is large enough (R ≫ 2σ/P0, which is ∼1.4 μm for air in water at ambient pressure), the gas pressure Pg inside the bubble is dominated by the ambient pressure.1 The contribution from the surface tension can be neglected. dV π = − LD(cs − c∞)f (θ ) dt 2ρ
(5)
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microcavities in the array. (4) The number of bubbles decreased mainly by coalescence with neighboring bubbles. There was also a significant difference in terms of the number and the total volume of bubbles on the two arrays. As shown in Figure 7, the bubble number N on the closely spaced array
Figure 7. Number (a) and total volume (b) of all microbubbles in a closely spaced (D = 13 μm) and a loosely spaced array (D = 40 μm). There are more microbubbles with a larger total volume on a loosely spaced array under the same conditions of solvent exchange. Figure 5. Time course snapshots of growing bubbles during the solvent exchange on a closely spaced microcavity array with D = 13 μm. The collective interactions are revealed by the sequence of the growth and dissolution of bubbles at different locations and the symmetrical patterns of microbubbles.
increased to 25 at t = 20 s and then decreased to 4 (the corner ones) at t = 122 s. For the same duration of time, more bubbles were formed on the cavity surface with a larger spacing, showing the number of bubbles N of 100 at t = 20 s and 17 at t = 122 s. The total volume of bubbles on the two arrays increased rapidly during the first 30 s and then slowed gradually, suggesting that the system was oversaturated all the time. Growth Rate of Microbubbles on the Arrays. We analyze the growth rate of representative bubbles located on the corners, edges, and inner cavities on the top-right quarter of the array labeled in Figure 8(a). The time window was the first 100
arrangements at 70 and 122 s. The last four bubbles occupied the four corners. The above key features are general for all of the arrays on the surface. The video of bubble growth over a large surface area is provided in the Supporting Information. As a comparison, we followed bubble growth on the loosely spaced array where 10 μm microcavities were separated with a spacing of D = 40 μm. The top-view snapshots of growing bubbles on the array are given in Figure 6. Several features were
Figure 8. (a) Plots of the bubble radius as a function of time on the closely spaced array. The location of the bubbles is the inner area (upper), edge (middle), and corner (lower) of the array. (b) The oversaturation (ΔC) as a function time of these bubbles, calculated from eq 5. The oversaturation averaged for the same period of time is the highest for the bubble on the corner (01) and lowest in the inner area (34).
Figure 6. (a) Time course snapshots of growing bubbles during the solvent exchange on a loosely spaced microcavity array with D = 40 μm. Scale bar: 200 μm. (b) Close-up images in the area of the green square. Scale bar: 80 μm.
s after the growth of the first bubble on the corner (01). The plot shows a smooth increase in the radius of all bubbles for their initial growth. Some jumps occurred with the larger bubbles as a result of the coalescence with the bubble on the adjacent microcavity. The size of those bubbles could not be determined from their radius in the top view because their base area was unknown. We could obtain the growth rate of bubbles from the initial smooth expansion of the bubble radius based on the radius in the top view and the constant contact area growth mode
observed. (1) Edge effects: In the early stage, it can be clearly seen that the bubbles at the four edges are larger than the bubbles in the center of the pattern. (2) No obvious corner effect: Bubbles on the four corners did not show significantly enhanced growth, compared to other bubbles on the same edge. (3) No symmetrical locations: Bubbles grew on all of the 11269
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four corners of the square array could receive the most flux along the surface because they are surrounded by fewer neighbors. The gas diffusion horizontally differentiates the corners from other locations, leading to the obviously larger bubbles on the corners. The lateral diffusion of oversatured gas from neighboring bubbles caused less interference in the loosely spaced array; therefore, the enhanced growth on the corners was less pronounced. Given more gas through diffusion in the lateral direction from the liquid phase, more bubbles could grow, and the total volume of bubbles was significantly larger on the loosely spaced array. Recently, corner and edge effects have been reported for water droplet nucleation and condensation during cooling, where the planar substrate is divided into a hydrophobic area and a hydrophilic area. The enhanced droplet growth at the boundary between two areas is attributed to a different local vapor concentration profile.25 Because each of the microcavities in the array was identical, our experiments show clearly that the local concentration profile at corners and edges can also contribute to the formation and growth of microbubbles at a very low oversaturation level. As soon as the bubbles on the corners gained a marginally larger size, their growth rate could be enhanced by a positive feedback mechanism. As the surface of the larger bubbles expands outward, they are closer to other smaller bubbles, creating even stronger depletion effects. Meanwhile, the Laplace pressure inside a bubble with a larger radius is smaller, and near their surface the gas solubility becomes smaller. The Laplace pressure depends on the bubble size, making the solubility of the gas in a diffusive boundary layer near a bubble also dependent on the bubble size. As a result smaller bubbles release more gas, which larger bubbles take in. Therefore, they could grow even faster than smaller bubbles in the oversatured liquid. Ostwald ripening may also make a contribution to the enhanced growth of larger bubbles. Minimization of the interfacial energy drives the gas flow from the small bubbles to their large neighbors through gas diffusion. Although it usually takes time for the consequence of Ostwald ripening to be observed, it may not be too slow, compared to the growth of bubbles in our experiments. Here the growth of bubbles was at an extremely low rate (less than a micrometer per second) even for the bubbles on the corner. This is different from many other rapid phase-transition processes, such as crystallization, nanoparticle synthesis, and microemulsions. In those cases, the effects from Ostwald ripening become dominant only after the formation of the new dispersed phase. Following the above logic, we might expect that only a pattern of four bubbles on the corners forms on the closely spaced array. However, the diffusive interactions among the bubbles are closely associated with the external concentration gradient. The oversaturation level increased with the continuous increase in ethanol fraction during solvent exchange. At higher oversaturation levels, the depleted volume from the growth of a larger bubble becomes smaller. Hence, the oversaturation pulse led to the number of bubbles increasing. Because the symmetric arrangement did not happen on the largely spaced arrays, the formation of their symmetric patterns was determined by the cooperative interactions among neighboring bubbles under the oversaturation pulse. The scheme of the gas concentration gradient for bubbles on two different cavity surfaces is shown in Figure 10. The sequence of the growth and dissolution of bubbles at different locations is consistent with the depletion effects
sketched in Figure 4. According to eq 5, the oversaturation level is calculated from the growth rates of bubbles at different locations. As plotted in Figure 8(b), the averaged oversaturation (× 10−3 kg/m3) was 2.36 on the corner, 1.74 on the edge, and 1.01 in the inner area. For the same period of time, the oversaturation supplied by solvent exchange was assumed to be constant because the microcavities were close to each other. The oversaturation on the corner is more than twice as large as in the inner area. The difference in the oversaturation for the bubble growth at different locations is attributed to the collaborative interactions. The inner area was depleted by the bubbles on the four corners. The obtained results provide a quantitative measure of the depletion effects, which is the strongest in the inner area near the bubble on the corner. For the loosely spaced array, the radius of the bubbles and the calculated oversaturations are shown in Figures 9. Three
Figure 9. (a) Plots of the bubble radius as a function of time on the loosely spaced array. The location of the bubbles is the center (55), middle of the left edge (51), and corner (91) of the array. (b) Oversaturation (ΔC) as a function time of these bubbles, calculated from eq 5. The oversaturation level averaged for the same period of time is similar for the bubbles at three locations.
bubbles (91, 51, and 55) are selected to represent the bubbles on the corner, edge, and inner area of the array. These three bubbles start to grow at a slightly different time, possibly because of the time required for the flow to reach those locations. The bubble located in the corner (91) grew slightly faster than the one on the edge (51) and on the center (55). The oversaturation for the corner was 30% higher than that for the edge and the center of the array.
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DISCUSSION On the closely spaced array, bubbles do not grow independently from each other. The far-reaching consequence of collective effects among microbubbles is (1) enhanced growth of bubbles on the corners and edges and (2) the emergence of symmetrical patterns during the increase in the total volume of all bubbles in the array. The enhanced growth on the corners is explained as follows. The air pockets are in the same initial condition for all of the cavities in the array, fed by the oversaturated gas from solvent exchange. A single bubble far from others takes in the oversaturated gas from a volume of liquid centered around it, limited only by the geometrical constraint from the shape of the bubble. Although bubbles in a crowded array still take in the gas in the direction vertical to the substrate, the gas diffusion horizontally is shared by the neighbors. The microcavities in the 11270
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ethanol can reach 0.3 early (where the oversaturation level is at its peak). Afterward, the nanobubbles grow further at a reduced oversaturation level. On the other hand, the positive feedback for the microbubble pattern formation in this work may not hold for surface nanobubbles on a homogeneous surface if the growth of surface nanobubbles is in constant contact area mode with a shape smaller than a hemisphere.
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CONCLUSIONS We have experimentally investigated the growth of bubbles on highly ordered hydrophobic microcavities by solvent exchange. The gas pockets in water act as initial sites for bubble growth when displacing water by ethanol from the surface. The oversaturation level increased first and then decreased with time. The collective interactions among neighboring bubbles have a significant impact on the size and location of microbubbles. On closely spaced arrays, we observed an enhanced growth of bubbles on corners and edges of the arrays, and several symmetric patterns emerged as the bubbles on the edges and inner area formed and then dissolved. The analysis of the growth rate on different locations suggest that the local oversaturation level for the bubbles on the corners was more than twice the level for the bubble in the inner area of the array. No enhanced growth was observed for bubbles on the corners or the edges of the largely spaced array where the number of microbubbles was greater than in a closely spaced array. The findings in this work have important implications for the formation of surface nanobubbles by solvent exchange. Those regular microbubble patterns on closely spaced arrays may be useful for the deposition of nanoparticles guided by microbubble templates.
Figure 10. Sketches of gas diffusion around growing bubbles. A side view (a, b) and a top view (c, d) on a closely spaced (a, c) and loosely spaced (b, d) microcavity array. The arrows represent the concentration gradient of the dissolved gas.
mainly from the four bubbles on the corners. Near the middle of the edges, the bubbles were the furthest from both corner bubbles and were surrounded by fewer small neighbors. Therefore, they grew slightly slower than the bubbles on the corners. At an even higher oversaturation level, the bubbles in the inner area, closer to the corner bubbles, also grew larger with less symmetry in size and location. Because of stronger depletion effects from the largest bubbles on the corners, the oversaturation possibly peaked with the concentration of ethanol, and as a result, the bubbles in the inner area started dissolving. The smaller the bubbles were, the faster they dissolved. For those bubbles with similar sizes, the one closer to a large neighbor dissolved earlier because of the depletion of dissolved gas by the continuous growth of the larger bubble. The symmetric patterns emerged from the collective interactions among the bubbles, consisting of bubbles on the corners and in the middle site of the four edges. By the end, the largest bubbles on the four corners remain after the dissolution of all other bubbles in the inner area and on edges. Compared to the evolution of the microbubble patterns, it was not clear whether the pulselike oversaturation had been essential for the formation of nanodroplet patterns in the recent report.23 Those surface nanodroplets at the rim of a microcap self-organized into symmetrical patterns of quadruplets, triplets, or doublets, whereas the sizes of all of the droplets in the patterns were similar. The number of surface nanodroplets in the patterns always decreased during droplet growth.23 Ostwald ripening was proposed to be the main reason for the pattern formation.23 The findings in this work have several important implications in the formation of surface nanobubbles. First, the oversaturation level by solvent exchange is a pulse regardless of the sequence of the two solvents. For many experimental studies of nanobubbles, the substrate is flat without pre-existing air pockets. In this case, the energy barrier for the heterogeneous nucleation of the nanobubbles must be overcome at the oversaturation level above a certain threshold before the diffusive growth of the nanobubbles. Nanobubble formation may be enhanced if the solution composition during solvent exchange can be controlled in such a way that the fraction of
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02066. Bubble growth over a large surface area (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
Seattle, Washington 98115, United States.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS X.Z. acknowledges support from the Australian Research Council (LP140100594) and Revalesio Corporation (Washington), and S.P. acknowledges support from an ARENA Research Fellowship. We thank Dr. Shuying Wu for technical assistance with SEM imaging and Martin Klein Schaarsberg for assistance with data analysis. We also acknowledge the use of facilities and the associated technical support at the RMIT MicroNano Reserach Facility (MNRF) and the Microscopy and Microanalysis Facility (RMMF).
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b02066 Langmuir 2016, 32, 11265−11272
