Using Sound To Study the Effect of Frothers on the Breakaway of Air Bubbles at an Underwater Capillary Pengbo Chu,* ,† Randolph Pax, ‡ Ronghao Li, † Ray Langlois, † and James A. Finch † † Department of Mining and Materials Engineering, McGill University, 3610 Rue University, Montre ́ al, Que ́ bec H3A 0C5, Canada ‡ RAP Innovation and Development Proprietary Limited, Post Office Box 559, Indooroopilly, Queensland 4068, Australia ABSTRACT: Frothers, a class of surfactants, are widely employed in froth flotation to aid the generation of small bubbles. Their action is commonly explained by their ability to hinder coalescence. There are occasional references suggesting that the frother may also play a role in the initial breakup of the injected air mass. This work investigates the possible effect of the frother on breakup by monitoring air bubbles produced quasi-statically at an underwater capillary. Under this condition, breakup is isolated from coalescence and an impact of frothers on the detached bubble can be ascribed to an impact on breakup. The breakaway process was monitored by an acoustic technique along with high-speed cinematography. The results showed that the presence of frothers did influence the breakaway process and that the acoustic technique was able to detect the impact. It was demonstrated that the acoustic frequency and acoustic damping ratio depend upon the frother type and concentration and that they are associated with a liquid jet, which initially excites the bubble and then decays to form a surface wave. The addition of the frother did not influence the formation of the jet but did increase its decay rate, hence, dampening the surface wave. It is postulated that the action of the frother is related to an effect on the magnitude of surface tension gradients. ■ INTRODUCTION Bubbles are ubiquitous in nature and have found many significant industrial applications. One in particular is froth flotation. In the flotation process, hydrophobic particles are collected by attachment to air bubbles and carried (floated) to the overflow, while hydrophilic particles remain in suspension and exit as an underflow. Usually, the hydrophobic particles, natural or induced, comprise the valuable component, with the overflow being the concentrate and the underflow being the tailings. Bubbles clearly play the central role determining the available interfacial area for collecting and transporting the hydrophobic particles. For a given volumetric air flow rate, the smaller the bubbles, the greater the interfacial area but the bubbles must have sufficient buoyancy to levitate the collected particles. A compromise size is ca. 1 mm diameter, with bubbles in practice ranging from ca. 0.5 to 2.5 mm. It is, however, not easy to generate such size bubbles in pure water because the high surface tension drives bubbles to coalesce. 1-3 To aid small bubble production, surfactants known as frothers are employed. The impact of these solutes on reducing bubble size is well- known, but their action remains obscure. 4 Reduction in surface tension has been speculated as the cause, 5,6 but others have pointed out the deficiencies of that notion. 7-10 A common explanation of the role of frothers in reducing bubble size recognizes their ability to hinder coalescence, 11,12 a phenomenon that can be readily demonstrated. For instance, in the work of Ata 13 and Bournival et al., 14 the authors used high- speed cinematography (up to 6000 frames per second) to monitor the events when two bubbles each held at a capillary tip are brought together. It was demonstrated that coalescence was increasingly delayed as the frother concentration was increased. In addition, the oscillation of the resultant bubble (i.e., after coalescence) could be characterized by a damping ratio derived by fitting the variation of the projected bubble area, and the damping ratio increased (i.e., oscillation decreased) as the frother concentration was increased. With it being noted that the bulk of the literature emphasizes the role of frothers in coalescence prevention, there are occasional references suggesting that frothers may also have an impact on another mechanism, namely, enhanced breakup. 7,15 The work of Kracht and Finch 16 is relevant in this regard. They investigated the role of frothers on the breakup of a monosize distribution of bubbles exposed to a turbulent field. In addition to suppressing coalescence, they surmised an effect on breakup by noting an increase in the fraction of bubbles at 90% of the original volume. Javor et al. 17 adapted the same technique and came to a similar conclusion, with their results indicating that the minimum observed bubble size was smaller with long-chain frothers than with short-chain frothers. A drawback of these Received: January 13, 2017 Revised: March 17, 2017 Published: March 20, 2017 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.7b00114 Langmuir XXXX, XXX, XXX-XXX