Deformation dynamics of particle-laden bubbles: The effect of surfactant concentration and particle contact angle

Highlights

• The fast deformation of silica particle-laden bubbles was studied.

• Surface pressure at the deforming particle-laden bubbles was determined.

• 0.2 μM dodecylamine has negligible effect on the surface pressure at the deforming bubbles.

• 20 μM dodecylamine significantly decreases the deformation of the bubbles.

• The change in particle contact angle changes the surface pressure.

Abstract

The pinch-off dynamics of bubbles coated with silica particles in deionised water and in dodecylamine (DDA) solution, at 0.2 μM and 20.0 μM, was studied using high-speed photography. Surface pressure generated at the deforming particle-laden interface during bubble pinch-off was obtained based on the fitting to a pinch-off model. It was observed that the pinch-off dynamics of these particle-laden bubbles remained almost unchanged at the low DDA concentration of 0.2 μM, while the dynamics slowed down significantly at the DDA concentration of 20.0 μM. Notably, both the 0.2 μM and 20.0 μM DDA concentrations have a negligible effect on the surface tension and pinch-off dynamics of uncoated bubbles. The difference in DDA concentration, however, is known to change the contact angle of silica particles from approximately 27° to 45°. It can be concluded that it is the change in particle contact angle that affects the pinch-off dynamics of particles-laden bubbles. Indeed, at a concentration of 0.2 μM DDA there is no significant change in contact angle of the silica particles with respect to that in DI water only, resulting in similar dynamics. It is suggested that the increase in the particle contact angle changes particle interactions, leading to a change in the surface pressure and apparent surface tension of particle-laden bubbles, which in turn slows down the pinch-off process. The findings in this work are relevant to our understanding of fundamental aspects of deforming particle-laden interfaces, such as those in the coalescence of flotation froths.

Introduction

The destabilisation of flotation froths involves the rupture of the thin liquid film, followed by the deformation of the air–liquid interface. The role of particles in froth destabilization is then twofold. First, particles may rupture thin liquid films either by the bridging dewetting mechanism proposed by Dippenaar (1982) or by the contact between the opposite liquid–vapour interfaces (Morris and Cilliers, 2014). The second role of particles in the destabilisation process is their role in hindering the deformation of the interfaces due to particle interactions (Garbin et al., 2015, Wang and Brito-Parada, 2020a). Although the mechanisms of film rupture by particles has been studied in detail (Dippenaar, 1982, Morris and Cilliers, 2014, Morris et al., 2011a, Morris et al., 2011b, Morris et al., 2011c), the effect of particles on the deformation of air–liquid interfaces remains poorly understood.

In flotation, the deformation of a bubble air–liquid interface is a common phenomenon that can be found in the bursting and coalescence of the froth (Neethling and Brito-Parada, 2018) and also in the coalescence (Finch et al., 2008) and oscillations (Kracht and Finch, 2010, Quinn et al., 2014) of bubbles in the pulp phase. Understanding the dynamics of particle-laden interfaces is essential to obtain valuable insight into froth stability and relevant pulp phase phenomena in which the high rate of deformation of the interfaces plays a key role.

One key parameter that dominates the dynamics of particle-laden interfaces is surface tension. It is known that nanoparticles coating an air–liquid interface generate a two-dimensional pressure, i.e. surface pressure, by particle interactions, which decreases the surface tension of the particle-laden interface. The generation of surface pressure generated by nanoparticle interactions occurs at both static and deforming air–liquid interfaces (Garbin et al., 2012). More recently, the surface pressure generated by microparticle interactions at rapidly deforming interfaces has also been discussed (Wang and Brito-Parada, 2020a, Wang and Brito-Parada, 2020b). It has been observed that the effect of bubble surface coverage on the surface pressure during the pinch-off of microparticle-laden bubbles is negligible (Wang and Brito-Parada, 2020a). In addition, the surface pressure has also been found to be affected by the size of microparticles during the deformation of air–liquid interfaces, such as during bubble pinch off (Wang and Brito-Parada, 2020a) and coalescence (Wang and Brito-Parada, 2020b). It is noted, however, that the effect of particle contact angle on bubble pinch-off dynamics has not been the focus of studies in the literature.

The deformation of the air–liquid interface in flotation is usually fast, making it challenging to study the effect of particles on the interface deformation and the particle interactions during the deformation process. The challenges in studying the fast deformation of particle-laden interfaces and particle interactions have been discussed in detail by Huerre et al. (2018b). Previous studies on deforming particle-laden interfaces have been focused on relatively slow deforming interfaces (Garbin et al., 2012) or on fast deforming interfaces during bubble coalescence (Ata, 2008, Wang and Brito-Parada, 2020b) and oscillation (Huerre et al., 2018b).

In this work, an individual bubble was generated from a submerged nozzle and then coated with microparticles following similar procedures to those in the literature (Bournival and Ata, 2010, Ata, 2011, Wang et al., 2020), the particle-laden bubble was used to carry out pinch-off experiments to investigate the fast deformation of microparticle-laden interfaces. The pinch-off dynamics was quantified by the change in the minimum radius of the bubble neck, R, in the pinch-off region using high-speed photography. The experimental data was then used to determine the surface pressure generated by particle interactions.

In particular, the focus of this study is on the effect of the particle contact angle on the surface pressure and pinch-off dynamics. In order to achieve this, silica microparticles were treated with dodecylamine (DDA) to change the particle contact angle, while keeping the surface tension of the bubble unchanged compared to the uncoated bubble. The effect of particle contact angle on the deformation dynamics of microparticle-laden bubbles is presented, which provides insights into the destabilisation phenomenon, which has implications for froth flotation.