Article pubs.acs.org/JPCB
Direct Observation of Individual Particle Armored Bubble Interaction, Stability, and Coalescence Dynamics Sin-Ying Tan,† Seher Ata,‡ and Erica J. Wanless*,† †
Priority Research Centre for Advanced Particle Processing and Transport, University of Newcastle, Callaghan NSW 2308, Australia School of Mining Engineering, University of New South Wales, Sydney NSW 2052, Australia
‡
S Supporting Information *
ABSTRACT: The interactions between two individual particle-stabilized bubbles were investigated, in the absence of surfactant, using a combination of coalescence rig and highspeed video camera. This combination allows the visualization of bubble coalescence dynamics which provide information on bubble stability. Experimental data suggested that bubble stability is enhanced by both the adsorption of particles at the interface as indicated by the long induction time and the increase in damping coefficient at high surface coverage. The interaction between an armored bubble and a bare bubble (asymmetric interaction) can be destabilized through the addition of a small amount of salt, which suggested that electrostatic interactions play a significant role in bubble stability. Interestingly, the DLVO theory cannot be used to describe the bubble stability in the case of a symmetric interaction as coalescence was inhibited at 0.1 M KCl in both the absence and presence of particles at the interfaces. Furthermore, bubbles can also be destabilized by increasing the particle hydrophobicity. This behavior is due to thinner liquid films between bubbles and an increase in film drainage rate. The fraction of particles detached from the bubble surface after film rupture was found to be very similar within the range of solution ionic strength, surface coverage, and particle hydrophobicity studied. This lack of dependence implies that the kinetic energy generated by the coalescing bubbles is larger than the attachment energy of the particles and dominates the detachment process. This study illuminates the stability behavior of individual particle-stabilized bubbles and has potential impact on processes which involve their interaction.
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INTRODUCTION The use of particles to stabilize bubbles has attracted great interest in recent years due to their excellent stability. It is wellaccepted that particle-stabilized systems offer an advantage over surfactant-stabilized systems due to the high detachment energies of particles which allow them to adsorb irreversibly at the interfaces.1 Surfactant molecules, in contrast, are able to adsorb and desorb from an interface (dynamic behavior) which can complicate the system behavior. Particle-stabilized bubbles can be found in the use of everyday products, for example whipped cream, shampoo, and medicines, and in industrial processes such as mineral recovery, wastewater treatment, and paper recycling. The ability of particles to stabilize bubbles has been studied intensively.2−5 It has been established that physicochemical properties such as particle wettability, interparticle interactions, and solution conditions have significant impacts on the physical, structural, and mechanical properties of the bubbles. The majority of these studies investigated systems where a large population of bubbles were present.6 This approach has undeniably allowed us to gain an understanding of the overall stabilization mechanisms where particles and bubbles are involved. Sadly, it does not provide information on an individual bubble which is crucial to develop a detailed understanding of these systems. © 2013 American Chemical Society
While the stability of bubbles is essential in some applications, it is sometimes necessary to destabilize them to achieve successful process operation. In the recovery of valuable minerals by flotation, for example, bubbles have to be stable enough to lift the particles, but destabilization of the bubble is required to recover the floated particles once they have reached the top of the cell. In general, these bubbles are stabilized or destabilized by the addition of surfactants or salt to the system. Previous studies have shown that bubble stability is indeed enhanced by the presence of surfactants7 and at high solution ionic strength.3,8,9 However, the use of surfactants has the disadvantages that they can be expensive and toxic to the environment. In general, the instability of bubbles (of similar sizes) is due to the draining and rupturing of the liquid film which results in the coalescence of neighboring bubbles. A thin liquid film is formed when two bubbles are in close proximity to each other, and the time taken for liquid to be removed from the gap between the two interfaces and then the rupturing of the liquid film is known as the induction time. As the film drainage rate is the key determining step in bubble interaction stability,10 a long Received: February 27, 2013 Revised: June 21, 2013 Published: June 24, 2013 8579
dx.doi.org/10.1021/jp402052f | J. Phys. Chem. B 2013, 117, 8579−8588
The Journal of Physical Chemistry B
Article
techniques have been widely adopted to modify particle hydrophobicity.20−23 Wettability Measurements. It has been repeatedly shown in the literature that the wettability of glass slides and particles treated under similar conditions are in good agreement with each other.15 Hence, the particle contact angles, for this study, were estimated by measuring the advancing contact angle of water on glass slides modified in a similar manner as the particles. The contact angle measurements were acquired using the sessile drop technique on a contact angle goniometer OCA 20 (DataPhysics Instruments) at different areas across the modified glass slides. The average contact angles were found to be 55 ± 5.2° and 73 ± 3.5° for glass slides modified by methylation and esterification, respectively. Stability of Particle Armored Bubbles. The experimental setup used in this work has previously been described elsewhere.11 Briefly, two air bubbles with a diameter of 2 mm (±5%) were produced in aqueous solution at adjacent capillaries (0.69 × 1.07 mm i.d × o.d.) with the aid of two separate microsyringe pumps (Saratoga, FL). The bubbles were then coated with particles by stirring the aqueous suspension at a speed that was slow enough to prevent the detachment of bubbles from the capillaries and fast enough to promote the adhesion of particles on the bubble surfaces. After the desired coating had been attained, as determined by visual inspection, the stirrer was turned off and the free particles in the suspension were left to settle. The bubbles were then moved toward each other, using a linear actuator, until they were just touching (no visible gap between the bubbles) so that a thin liquid film was formed between them. The bubbles would then coalesce to form a single, larger bubble once the intervening film drained and ruptured. The entire process (from before the bubbles were in contact until after coalescence) was recorded using a high-speed video camera (Phantom 5, Vision Research). Videos were recorded at a resolution of 512 × 256 with a frame rate of 10−300 frames/s for induction time calculations and 6024 frames/s for oscillation analysis using the Phantom 6.30 (Nikon) software. Here, the induction time is the time taken for the film to drain and rupture (i.e., for coalescence to occur) as calculated by subtracting the time associated with the frame before film rupture minus that of the frame where the bubbles initially touched. In this study, experiments were conducted in two modes: asymmetric (a coated and a bare bubble) and symmetric (both bubbles were coated). Unlike in symmetric mode, only one bubble was produced initially for the asymmetric mode. Once the bubble was coated with particles to the desired coverage and the free particles had settled, another bubble was produced from the adjacent capillary. All the experiments were conducted in a 100 mL cylindrical glass cell equipped with custom-made baffles. The purpose of the baffles was to minimize turbulence caused by stirring. The vessel was placed inside a rectangular Perspex container filled with water to prevent image distortion from the curvature of the wall of the glass cell. The whole setup was placed on a vibration isolation table. Before each experiment, the cleanliness of the setup was tested by bringing two bare air bubbles together. If the bubbles coalesced within a frame (
