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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials
Coalescence or Bounce? How Surfactant Adsorption in Milliseconds Affects Bubble Collision Bo Liu, Rogerio Manica, Qingxia Liu, Evert Klaseboer, and Zhenghe Xu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01598 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019
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The Journal of Physical Chemistry Letters
Coalescence or Bounce? How Surfactant Adsorption in Milliseconds Affects Bubble Collision Bo Liu,† Rogerio Manica,† Qingxia Liu,∗,† Evert Klaseboer,‡ and Zhenghe Xu†,¶ †Department of Chemical and Materials Engineering, University of Alberta, Edmonton,T6G 1H9, Canada ‡Institute of High Performance Computing,1 Fusionopolis Way, Singapore 138632 ¶Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China E-mail:
[email protected] 1
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Abstract The coalescence between two colliding bubbles in ultraclean water can be 3 or 4 orders of magnitude faster than coalescence in contaminated solutions. This surprising result can be mostly explained by the mobile or immobile boundary conditions at the air-water interface. In this work, we employ a rising bubble technique to study bubble collisions in aqueous solutions with up to 2 mM surfactant. The experimental results clearly show that freshly-generated bubbles can coalesce within milliseconds if they collide right after generation. However, once the bubbles reside in bulk for tens of milliseconds the coalescence is heavily hindered. Based on these results, we conclude that the clean air-water interface, rather than clean water, is required to achieve the mobile boundary condition that allows quick coalescence. These findings provide fundamental understanding for further improvements in bubble generation that will benefit industrial processes such as mineral flotation, oil extraction, and wastewater treatment.
Graphical TOC Entry
Keywords Bubble Coalescence, Surfactant, Dynamic Adsorption, Surface Mobility
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Two air bubbles colliding in ultraclean water can coalesce within milliseconds, 1 much faster than the time required in contaminated or surfactant systems. Understanding this behavior is essential for a better-controlled bubble generation technique, which is a core component in mineral flotation, 2 oil extraction, 3 and water purification. 4 In such industries, the typical residence time of particles or oil drops in bulk can be over 10 minutes. However, the attachment of particles/drops to the air bubbles is highly influenced by the short bubble generation process that lasts tens of milliseconds. 5,6 During bubble generation, the newly formed air bubbles collide with each other and with the target particles/drops, thereby influencing the bubble size distribution and bubble-particle attachment, two key factors for successful flotation. 6–9 The rapid bubble coalescence within milliseconds provides an opportunity for rational manipulation of the bubble coalescence or bounce in the short bubble generation period, hence improving the flotation recovery and selectivity. To achieve rapid bubble coalescence, the air-water interface needs to be mobile, 1 i.e., there must be negligible tangential resistance at the interface. The tangentially mobile interface enhances the fluid flow inside the thin liquid film trapped between the bubbles. 1,10–12 The lack of resistance at the interface contributes to a film thinning rate that is roughly equal to the bubble collision speed until the film ruptures at around 50 nm; hence, the bubbles can coalesce within milliseconds. 1 However, contamination that changes the air-water interfacial tension on the order 10−4 N/m, or 0.1 %, can be sufficient to inhibit the air-water interface mobility and render the interface immobile (i.e., zero tangential velocity). 1,10,12 Theoretically, a sharp transition occurs between immobile and mobile boundary conditions, with a narrow range for partially mobile interfaces, 12 a result that was also shown experimentally. 1 For decades, most of the experimental data was modeled using the tangentially immobile boundary condition. 10,13–15 In the few experiments where the mobile air-water interface was achieved, the experimental setup had to be carefully cleaned and water purity maintained. 1,16–18 In industry, however, surface active components are unavoidable or even purposely added to reduce the interfacial tension, 2,7,19 resulting in solutions that are much 3
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‘dirtier’ than the ultrapure water used in laboratory experiments. A question that remains unanswered is whether rapid bubble coalescence with the mobile air-water interface can be achieved in contaminated or surfactant solutions. In this letter, we report bubble collision experiments in surfactant solutions using the rising bubble technique. Side view observations with a high speed camera (5000 frames/s) clearly show the dynamic collision in milliseconds between two air bubbles in aqueous solutions. By observing the bounce or coalescence between the bubbles and comparing with theoretical predictions, we found that the freshly generated air-water interface could be mobile even in surfactant solutions, enabling bubble coalescence within a few milliseconds. The mobility could be easily inhibited once the bubble was left in bulk solution for tens of milliseconds, which is fast but on the same order of magnitude as the bubble generation process. The results show that a clean air-water interface is required to obtain the mobile boundary condition; therefore, controlled bubble coalescence or bounce can be achieved by modulating the cleanliness of the interface. A schematic of the rising bubble experimental setup is shown in Fig. 1a. In a square glass vessel (170 x 40 x 60 mm3 ) that avoids optical distortion, we placed a homemade sharp end capillary tube at the bottom through a capillary holder. Air bubbles (radii 300 µm to 600 µm) were generated by pumping air through the capillary using a syringe pump at volumetric flow rates from 0.5 to 4 mL/min. The flat air-water surface was adjusted to be approximately 5 mm above the capillary orifice. This distance allowed the bubble to achieve terminal velocity before hitting the top flat surface. It also enabled the detailed observation of the complete dynamic process (bubble formation, rise, collision, bounce and/or coalescence) using a high-speed camera (Photron SA4) at a ∼15 µm/pixel resolution. Before the experiment, the glass vessel was carefully washed in 1 M NaOH and rinsed thoroughly with Milli-Q purified water. The capillary holder and respective tubing were ultrasonically washed in Milli-Q water for 20 minutes and rinsed thoroughly. Milli-Q water was used to prepare the surfactant solutions with sodium dodecyl sulfate (SDS, Sigma Aldrich, 4
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98.5%) or methyl isobutyl carbinol (MIBC, Fisher Scientific, 99%) at concentrations ranging from 0.01 mM to 2 mM.
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(c) Bubble center of mass, zm (mm)
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Figure 1: (a) Schematic of the rising bubble experimental setup. (b) Snapshots of a rising bubble and its collision with the flat air-water interface in a 0.03 mM SDS solution. Images were pretreated using ImageJ. (c) The extracted bubble trajectory, from which the rise collision and pinch-off collision experiments were designed (see insets) with different aging time (∼50 ms and ∼10 ms, respectively). Time t = 0 is defined as the time the bubble would have ‘touched’ the flat interface if there was no deformation; the flat surface is defined as the position z = 0. The position of the bubble was extracted from the images using Matlab. Snapshots of the rising bubble are shown in Fig. 1b, from which the rising trajectory shown in Fig. 1c was extracted. Initially, the freshly released bubble oscillated in bulk and rose slowly (
