Precisely Tailoring Bubble Morphology in Microchannel by

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Precisely Tailoring Bubble Morphology in Microchannel by Nanoparticles Self-assembly Yining Wu, Ruoyu Wang, Caili Dai, Yan Xu, Tongtao Yue, and Mingwei Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06057 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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Precisely Tailoring Bubble Morphology in Microchannel by Nanoparticles Self-assembly

Yining Wu[a,b], Ruoyu Wang[a.b], Caili Dai[a,b]*, Yan Xu[c], Tongtao Yue[c]*, Mingwei Zhao[a,b]* a.

School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, People’s Republic of China

b

Key Laboratory of Unconventional Oil & Gas development (China University of Petroleum (East China)), Ministry of Education, Qingdao 266580, P. R. China c.

College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, People’s Republic of China

ABSTRACT Precisely tailoring bubble morphology is always a long-standing great challenge. In this work, a facile and scalable method to generate nonspherical bubbles with long-term stability is proposed. Taking advantage of the electrostatic interaction between silica nanoparticles (SNPs) and cationic surfactants, the SNPs are decorated with surfactants and endowed with interfacial activity. Due to the rearrangement of surfactants, the decorated SNPs transform to a kind of Janus particles at the gas-liquid interface. By precisely manipulating the surface activity, packing density and jamming of Janus SNPs at the bubble surface, four different shapes such as oblaten-like, bullet-like, tadpole-like and worm-like bubble were obtained continuously in the microchannel. Herein, our method to generate bubbles with a 1


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prescribed shape poses opportunities for gas microreactor, cavity material, gas storage and provide a platform to study the applicable scope of the Young-Laplace equation.

KEYWORDS: nanoparticle; surfactant; bubble; non-equilibrium; jamming

1 INTRODUCTION In recent years, bubbles have attracted considerable attention due to their widespread applications in chemical engineering, biochemical analysis, mining, food and oilfields. 1-5

As known, traditional isolated bubbles are spherical. Whether the particle-laden

nonspherical bubbles own special properties, such as long-term stability, foam flow characteristics, oil resistivity and gas-liquid mass transfer, has been a conundrum. Furthermore, whether the nonspherical bubbles with two or more radius of curvature follow the Young-Laplace equation remains in doubt. To explore these questions, the preparation of long-term stable nonspherical bubbles is requisite. However, the generation of nonspherical bubbles is a long-standing great challenge. Compared with nonspherical bubbles, the droplets with non-equilibrium morphologies have been extensively studied. deformed droplets.

6

6-11

Russel’s group reported a route to generate

Spherical droplet was stretched to ellipsoidal shapes under

electrical field. In the meantime, solid particles with surfactancy adsorb onto the interface. Solid particles with interfacial activity self-assembled at the interface to reduce the free energy of the biphasic system.

8,9,12

In addition, the particles

introduced solid properties to the interface, which was reflected in the dilatational elastic modulus of the interface.

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When the packing density of particles was low,

they formed disordered gas-like or liquid-like assembles. 7,14 As the packing density 2



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increased to a critical value, particles formed ordered or solid-like structure.

7,14

Consequently, upon removing the field, the interfacial area shrank and the jammed nanoparticles at the interface arrested further shape change, generating highly non-equilibrium shapes. While for nonspherical bubbles, Stone’s group made the first contribution.

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By

fusing two bubbles decorated by fluorescent polystyrene beads, the areal density of particles increased due to the decrease of the total surface area. When the jamming occurred, coalescence was arrested, resulting in the generation of saddle-like bubbles. However, the bubble shape could not be controlled precisely, limiting its further investigations and applications. No major breakthrough in this field was reported afterwards. According to previous studies, to control the interfacial jamming is the key point in tailoring and preserving the bubble shape. In this work, a method to precisely manipulate bubble shape is firstly proposed. How to control the interfacial jamming is the key point in regulating the bubble shape. At the jamming state, a monolayer of closely-packed particles possesses certain mechanical strength and is capable of bearing load, arresting further reduction of interface area. Thus, interfacial jamming of particles affords a promising route to tailor bubble shape. We expect to shed light on the regulation mechanism and give a platform to study the interfacial jamming and the properties of non-equilibrium bubble through this work.

2 Experimental section 2.1 Materials and measurements Cetyltrimethyl ammonium bromide (CTAB) was purchased from Fisher Scientific. 3


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The silica nanoparticles (LUDOX HS-30, Sigma-Aldrich) were purchased as an aqueous dispersion (30 wt %) with an average diameter around 13 nm and a density of 1.21 g/mL at 25 ℃. Sodium chloride purchased from Fisher Scientific was used to investigate the effect of salt concentration. Hydrochloric acid (1N solution, Fisher Scientific) and sodium hydroxide (1N solution, Fisher Scientific) were used to adjust the solution pH. Ultra-pure water was prepared by using a reverse osmosis unit (ULUPURE, UPT-II-5T). Malvern Zetasizer HS-3000 instrument was used to measure the diameter and zeta potential of NPs. The surface tensions σg and the interfacial viscoelasticity was measured with a dynamic interfacial oscillatory drop tensiometer, Tracker-H, from Teclis France. Microscopy investigations were performed to directly study the bubble morphologies in expanding chamber (Figure S1a) using inverted microscope (Leica DMi8 C) equipped with a Photron Fastcam SA-Z high speed camera shown in Figure S1b. Pictures were analyzed in post-processing using Matlab to quantify the surface area of the bubbles.

2.2 Simulation system setup To simulate the interfacial behavior of silica nanoparticles (SNPs) decorated with surfactants in adequate temporal and spatial scales, we adopted the widely used Martini coarse-grained force field. The Martini coarse-grained method is based on a four-to-one mapping strategy, i.e., four heavy atoms are represented by a single

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interaction site. The simulation system setup simply includes a water slab in vacuum. Each SNP was constructed by arranging a number of hydrophilic Nda-type beads into a spherical shape of 5 nm in radius (Nda is a parameter in Martini force field). 40% of surface beads for each SNP was set negatively charged by setting the bead charge to -1. To decorate SNPs with cationic surfactants, different numbers of CTAB molecules were physically adsorbed onto the hydrophilic SNP surface. Electrostatic interactions between negatively charged SNP surface and positively charged surfactant head groups help stabilize the complex structure in both gas and liquid phases, albeit in different arrangements due to interactions with water molecules.

2.3 Process of the coarse-grained molecular dynamics (CGMD) simulations All simulations were performed using GROMACS 4.6.7. For all simulations, a cutoff of 1.2 nm was used for van der Waals interactions. The Lennard-Jones potential was smoothly shifted to zero between 0.9 nm and 1.2 nm to reduce the cutoff noise. For electrostatic interactions, the coulombic potential, with a cutoff of 1.2 nm, was smoothly shifted to zero from 0 to 1.2 nm. When conducting simulations of aggregation of multiple SNPs decorated with CTAB molecules, the cutoff of electrostatic interactions was increased to 10 nm to precisely probe competing roles of the hydrophobic interactions between CATB tails and the electrostatic repulsion between charged SNPs. Temperature was kept constant at 25 ℃ using the Berendsen weak coupling algorithm with a time constant of 1.0 ps. The simulation box size was

5


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fixed at 16.6 × 16.6 × 25.8 nm3 and kept constant during all simulations. Periodic boundary conditions were implemented in all three directions. Neighbor list for non-bonded interactions is updated every 10 steps. Snapshots were rendered by VMD.

3 RESULTS AND DISCUSSION 3.1 Synergistic effect between CTAB and SNPs Here, a facile and scalable method based on microfluidic technology 16-18 to precisely manipulate bubble shape is firstly proposed. Using the microfluidic device (Figure S1a), bubbles with narrow size distributions were generated. The generated bubbles stabilized by SNPs alone (Figure 1a), CTAB alone (cetyltrimethyl ammonium bromide, cationic surfactant) (Figure 1b) and composite of CTAB-SNPs (Figure 1c), respectively, presented initially cylinder-shape due to the confinements of the microchannel. Upon bubble entering the expanding chamber, confinement in horizontal direction was removed. Bubbles attempted to revert into the lowest energy morphology - oblaten-like shape - driven by surface tension. For SNPs alone, the surface activity is relatively low13, therefore, SNPs cannot anchor stably at the surface. When the surface area decreases, SNPs tend to detach and barely form jamming at the surface. CTAB as a kind of surfactant owns high surface activity and can adsorb at the interface to reduce interfacial tension. The CTAB molecules adsorbed at the interface and the CTAB molecules in bulk phase are in dynamic balance, namely, the adsorption of CTAB is reversible. CTAB will be extruded from 6



the surface to the bulk phase as surface area decreases. Thus, for CTAB alone or SNPs alone, the surface area of bubble decreased gradually which was reflected in the decrease of the surface area retention rate St / So (Figure 1d; where St and So stand for the surface areas at the real time and the surface areas of the initial cylinder bubble, respectively) and finally the bubbles turned into oblaten-like shape (Figure 1a; Figure 1b) with St / So of circa 0.6 (Figure 1d, SNPs alone and surfactant alone). When CTAB molecules and SNPs were added to aqueous phase simultaneously, the adsorption of CTAB molecules on SNP surface19 endowed the SNPs with surface activity. Hence, these decorated SNPs are able to anchor and assemble at bubble surface (Figure 2a). Upon the bubble entering the chamber, the packing density of decorated SNPs increased due to the decrease of surface area (Figure 2b). Then the assembled layer of decorated SNPs jammed to form a 2 dimensional structured framework and arrest the change of bubble shape. This process was also reflected in the evolution of surface area retention rate which decreased firstly and stopped at late time (Figure 1d, NPs-surfactant). At last, bubble was locked in a non-equilibrium shape (Figure 1c, T = 55 ms) due to the surface jamming of decorated SNPs. The above mentioned experiments indicate the composite of CTAB-SNPs is capable to enhance the surface activity and consequently affect the surface jamming of SNPs. This discovery enlightens us that the bubble shape can be precisely tailored by changing the surfactancy of the decorated SNPs.

3.2 Bubble morphology regulation 7


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The surface activity of SNPs was adjusted by varying the CTAB concentration in the aqueous phase (ranging from 0 to 0.18 mmol/L, with 2 wt % SNPs). In the expanding chamber, four different bubbles including oblaten-like (Figure 3a), bullet-like (Figure 3b), tadpole-like (Figure 3c, 3d) and worm-like (Figure 3e) bubbles were obtained with the increase of CTAB concentration. Although these bubbles possessed high aspect ratio, they resisted the breakup caused by Plateau–Rayleigh instability. In addition, these non-equilibrium bubbles showed extraordinary stability without noticeable shape changes in a week, and the coarsening process was alleviated. Taking advantage of microfluidic technology, a simple route to continuously generate long-stable non-equilibrium bubble was established (Figure 3f). To reveal the mechanism underlying manipulation of the bubble morphology by SNPs decorated with CTAB molecules requires figuring out the effect of CTAB on both the surface activity and the packing density of SNPs at the gas-liquid interface. We thus performed coarse-grained molecular dynamics (CGMD) simulations on the equilibrium states of SNPs decorated with CTAB molecules of different densities at the gas-liquid interface. In the absence of CTAB, the bare SNP preferred to enter the water phase (Figure 4a), indicating a low surface activity. When 10 % of the SNP surface area was decorated with CTAB molecules, strikingly, the modified SNP steadily stayed at the gas-liquid interface (Figure 4b). Furthermore, once reaching the gas-liquid interface, CTAB molecules gradually migrated along the SNP surface to avoid unfavourable interactions between hydrophobic tails and water. Then CTAB molecules formed a beret-like top with the SNP hanged underneath. These two parts 8



formed an asymmetric Janus-structure. As increase of the CTAB density, the top shape of CTAB molecules transformed from beret-like to hat-like. Besides, the hydrophobic top formed by CTAB increased gradually as increase of the density of CTAB molecules. To avoid unfavourable interactions between hydrophobic tails of CTAB and water and to decrease the system free energy, the larger hydrophobic top will exert greater forces to pull the SNP into the gas phase. Consequently, the steady state position of the SNP was found to increase, as reflected by evolutions of the distance between each SNP and centre of the water layer (Figure 4e). To further examine the interfacial stability of SNPs decorated with CTAB molecules, we then performed steered MD simulations, i.e. an external spring force was exerted on the centre of each SNP to pull it from the interface to the water phase (Figure 5a, 5b, 5c). It was found that pulling each SNP toward the water phase first generated a striking increase of the resistance force (Figure 5d), indicating that the CTAB decorated SNPs preferentially located at the gas-liquid interface. Moving downwards lead to unfavourable contacts of hydrophobic tails of CTAB with water, thus increasing the total system free energy in the first stage. As further decrease of the SNP location with respect to the interface, it caused a contrarily decrease of the resistance force, which is in accordance with the separation between the SNP and CTAB molecules being also reflected by a continuous increase of the interaction energy between the SNP and CTAB molecules (Figure 5e). As the increase of the decoration ratio, the resistance force derived from electrostatic interactions between CTAB and SNP was further increased, indicating that the surface stability of 9


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decorated SNPs increased as the SNP was decorated with more CTAB molecules. Our simulations suggested that the CTAB decorated SNPs always evolved into a kind of Janus particle at the bubble surface. This Janus particle has a significantly higher interfacial activity than its homogeneous counterpart.

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With the increase of the

CTAB concentration, the beret-like hydrophobic top composed of CTAB enlarged as shown in Figure 4, leading to the hydrophobic moiety and hydrophilic moiety became more balanced. Thus, with the increase of CTAB concentration, the decorated SNPs possessed a higher interfacial stability, a higher surface activity and could effectively lower the surface tension. This is consistent with the experimental results (Figure 6). Besides the surface activity of decorated SNPs, the surface coverage of SNPs greatly relies on the packing density or concentration of SNPs lining at the gas-liquid interface. To reveal how the coverage density of CTAB molecules on the SNP surface influence packing of SNPs at the interface, four SNPs decorated with CTAB molecules of two different densities were positioned at the air-water interface. The initial distance between two adjacent SNPs was set to 6.8 nm. Our unbiased MD simulation results showed that the steady packing density of SNPs at the interface was controlled by the competition between the electrostatic interaction energy and the hydrophobic interaction energy. When the electrostatic repulsion dominated, the adjacent SNPs were repelled with each other to reach a lower packing density at the surface. Otherwise, decorated SNPs reached a higher packing density. For SNPs with 10 % surface decorated with CTAB molecules, they were found to be repelled with each other to reach a steady distance of 11.0 nm (Figure 7a). For SNPs with 40 % 10



surface decorated with CTAB molecules, the electrical repulsion between decorated SNPs decreased. This is also in accordance with the Zeta potential measurements (Figure 7c). As expected, we found a rapid decrease of the distance between SNPs at the interface (Figure 7b), which indicates the increase of CTAB concentration was conducive to increase the packing density. These results also showed that the bubble shape evolution from oblaten-like to worm-like as shown in Figure 3a-e (CTAB concentration increased from 0.04 to 0.18 mmol/L) was accompanied with the increase of packing density of SNPs. We use Sf / So to represent surface coverage of the decorated SNPs and hereby to reflect the packing density of SNPs and the degree of bubble reverting to the lowest energy shape, where Sf and So are surface areas for the final state and the initial state, respectively. When CTAB concentration was low (less than 0.04 mmol/L), no surface jamming was observed and the bubble always returned back to oblaten-like shape to minimize the interfacial area. The surface coverage of the decorated SNPs was under 0.68 as shown in Figure 7d. With the increase of CTAB, the surface coverage of decorated SNPs increased continuously. At CTAB concentration of 0.07 mmol/L, the surface jamming occurred when the surface area decreased by circa 25%. The bubble was locked in a bullet-like shape. When the CTAB concentration further increased to 0.12 mmol/L, the surface coverage reached up to 80% resulting in a classical tadpole-like bubble. For worm-like bubble, the surface coverage reached up to circa 96% with the surface area remains almost unchanged. These results demonstrate the surface coverage or the surface jamming can be precisely regulated by manipulating 11


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the electrical interaction between CTAB and SNP.

3.3 Effect of ionic strengths and pH values Given that the electrical interaction plays a key role in tailoring surface jamming, 21 it is not difficult to imagine the effects of salt and pH. When the salt concentration is low, the addition of salt can screen the double-layer electrical repulsion between decorated SNPs.

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The packing density or the surface coverage increased with the

increase of salt concentration, which is reflected by the decrease of surface tension and the increase of surface elastic modulus (Figure 8). The bubble changed from bullet-like to tadpole-like shape (Figure 8b). Nevertheless, when the salt concentration was high, the electrical interaction between CTAB and SNP was suppressed. The abundance of ions competed with CTAB on the adsorption to SNP surface. Therefore, part of CTAB molecules desorbed from SNP turning into free surfactant, which was reflected on the further decrease of surface tension and surface elastic modulus (Figure 8a). The decrease of surface activity and packing density of decorated SNPs went against surface jamming resulting in ultimately recovering of the bubble shape. In addition, pH of aqueous phase can also be used to regular bubble morphology. The mechanism is analogous to salt. The aqueous phase pH affects the surface charge of SNP. At high pH, the SNP surface became highly deprotonated. A mass of surface charge result in strong electrical repulsion between SNP, which leads to a low surface coverage (pH=9.9, Sf / So =0.77 as shown in Figure.9a). At low pH, the surface charge 12



of SNP decreased resulting in a weak interaction between CTAB and SNP. Consequently, the surfactancy of decorated SNP is poor which leads to an extremely low surface coverage of SNP (pH=4.1, Sf / So =0.63 as shown in Figure.9a). When the aqueous phase is close to neutral, the surfactancy of decorated SNP is high and the repulsion between SNP is low. This is documented by a relative low surface tension and a relative high elasticity modulus as shown in Figure. 9b. Thus, the surface coverage of SNP (pH=7.1, Sf / So =0.83) is high in neutral solution. Taken together, the mechanism of pH and salt concentration bear similarity.The pH and salt concentration dependence of the surface jamming was quite evident (Figure 8 and Figure 9).

4 CONCLUSION In conclusion, we have combined experimental and simulation results to report surface activity, assembly and jamming of CTAB decorated SNPs at the gas-liquid surface under different CTAB concentrations, ionic strengths and pH values. By manipulating the occurrence of surface jamming in the microfluidic device, the bubble morphology can be precisely tailored. Molecular dynamics simulation results suggest that CTAB molecules migrate along the SNP surface to spread at that gas-liquid interface, forming Janus-like structures to stably anchor SNPs at the interface for enhancing surface activity. Multiple SNPs can form aggregates at the interface via competition between the electrostatic interaction energy and the

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hydrophobic interaction, which can be modulated by the number of CTAB molecules adsorbing onto the SNP surface. These results open a new platform for studying the properties of non-equilibrium bubble and the surface jamming of Janus particles in gas-liquid surface. SUPPORTING INFORMATION Schematic diagram of the microfluidic device. Schematic diagram of the experimental setup. ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (51704313, U1663206), the Chang Jiang Scholars Program (No. T2014152), the Climb Taishan Scholar Program in Shandong Province (tspd20161004) and the Fundamental Research Funds for the Central Universities (18CX02028A).

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; [email protected]

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(d) Figure 1 (a-c) Time-lapse sequence of the deformation of a bubble in expanding chamber. Bubbles were stabilized by (a) 2 wt % SNPs, (b) 0.13mmol/L CTAB and (c) mixture of 0.13mmol/L CTAB and 2 wt % SNPs. (d) Time-evolution of the surface area retention rate of bubbles in different solutions. The scale bar is of 400 μm.

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Figure 2 The schematic of the packing density of decorated SNPs (a) before and (b) after bubble penetrates into expanding chamber.

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Figure 3 (a-e) Bubble morphologies in aqueous phase with different CTAB concentration and 2 wt % SNPs. (f) The continuous generation of non-equilibrium bubbles in expanding chamber. The scale bar is of 400 μm.

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Figure 4 Molecular dynamics analysis for the gas-liquid interfacial behaviour of SNPs decorated with CTAB molecules. (a-d) Steady states of the interfacial interactions of SNPs decorated with CTAB molecules of different ratios, including (a) 0%, (b) 10%, (c) 20% and (d) 40%. Light blue, dark blue, purple and green stand for water, nanoparticle, salt and

surfactant, respectively. Each snapshot was displayed from both side and top views for clarity. (e) Time evolutions of the distance between SNPs and centre of the water layer.

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Figure 5 Time sequences of typical snapshots during pulling SNPs decorated with CTAB molecules from the gas-liquid interface to enter the water phase. Three different coating densities were considered and compared, including (a) 10%, (b) 20% and (c) 40%. Light blue, dark blue, purple and green stand for water, nanoparticle, salt and surfactant,

respectively. (d) Time evolutions of the resistance force pulling each SNP from the interface to enter the water phase. (e) Time evolutions of the interaction energy between SNP and CTAB molecules during pulling simulations.

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Figure 6 The effect of CTAB concentration on surface tension and elasticity modulus. The content of SNPs kept constant at 2 wt %.

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Figure 7 Time sequences of typical snapshots depicting different packing density of SNPs modulated by different CTAB concentration. (a) SNPs decorated lower density of CTAB molecules reach a lower packing density, (b) whereas SNPs decorated higher density of CTAB molecules form aggregate. (c) Zeta potential of silica nano-particles in solutions as a function of CTAB concentration. (d) Surface coverage variation of the bubble with increasing of CTAB concentration. The content of SNPs kept constant at 2 wt %.

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(a)

(b) Figure 8 (a) The effect of Salt concentration on surface tension and elasticity modulus. (b) Surface coverage variation of the bubble with increasing of salt concentration.

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Sf / So =0.63

Sf / So =0.83 (a)

Sf / So =0.77

Figure 9 (a) The morphologies of bubbles in aqueous phase with different pH. (b) The effect of pH on surface tension and elasticity modulus. The scale bar is of 400 μm.

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Precisely Tailoring Bubble Morphology in Microchannel by

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