A novel experimental procedure to measure interfacial tension based on dynamic behavior of rising bubble through interface of two immiscible liquids

Highlights

• The theoretical aspects on evaluating the interface interaction are modified.

• An experimental procedure presented to produce a single bubble of precise volume.

• High-quality pictures are provided illustrating the interfacial phenomena.

• A novel volume-based method for measuring interfacial tension is developed.

Abstract

Bubble passing through interface of immiscible liquids has great importance in many industrial applications. In this paper, theoretical aspects on evaluating the bubble and liquid–liquid interface interaction are discussed and a novel experimental procedure is implemented to produce a single bubble of precise volume. In this procedure, unlike the previous experimental studies, unwanted randomly rotational forces on the bubble surface is eliminated and repeatability of the experiments is guaranteed. According to the experiments, meaningful dimensionless numbers are introduced and their exact threshold values are reported for the initiation/termination of specific interfacial phenomena. The effect of asymmetry in bubble shape and its rising pathway is also studied. Finally, based on experimental results of the bubble production method, the theoretical equations are modified and a novel volume-based method for measuring interfacial tension is developed.

Introduction

Gas-liquid–liquid multiphase systems are involved in many industrial processes and technological applications. The dynamic behavior of bubbles and their interfacial effects must be controlled based on the fields of presence and application. In some chemical processes, the injection of gas bubbles into a quiescent pool of two immiscible liquids is used to improve the surface transport phenomena like heat and mass transfer rate. Gas injection treatment in metal purification and production processes has been employed to enhance liquid–liquid surface area and surface reaction rates (Li et al., 2000, Song et al., 2012, Song et al., 2010). Gas floatation is a water treatment process that removes suspended oil droplets and solid particles by injecting bubbles into the water phase (Eder and Preißinger, 2020, Fu and Wang, 2011, Mahmoud et al., 2015, Rubio et al., 2002). In order to separate hydrophobic particles, rising bubbles should be covered by a stable thin layer of hydrophobic liquid (Fuerstenau et al., 2007, Liu et al., 2002, Luo et al., 2019, Sis and Chander, 2003, Tripathi et al., 2015, Wang et al., 2018, Zhou et al., 2015, Zhou et al., 2014). Bitumen recovery (Al-Otoom et al., 2010, Zhou et al., 2004) and paper deinking (Gübitz et al., 1998, Pesman et al., 2014) are some other examples of gas floatation process. Moreover, surface of seas and oceans is usually occupied by organic phases such as proteins, oils and hydrocarbons and rising bubbles collide with organic phase layer (Cunliffe et al., 2013). Therefore, the dynamic behavior of rising bubbles through immiscible liquids layers is of interest to further environmental and biological studies as well, especially in risk assessment of offshore oil spills.

The rupture or formation of the heavy liquid film around a rising bubble are two opposite subjects intended in different industrial applications. In the case that film around rising bubble ruptures, the entrained heavy liquid is mostly separated from the bubble which improves phase separation efficiency and reduces the time required for heavy liquid recovery. Furthermore, the formation of numerous micro-droplets increases liquid–liquid surface area and consequently surface transport phenomena, especially in metal purification and production processes. However, the presence of a thin and stable film layer around a rising bubble is necessary for selective floatation systems.

The inherent asymmetry of the bubble production system and non-adjustable size of produced bubbles are two main weaknesses of previous experimental studies (Bonhomme et al., 2012a, Emery et al., 2018, Singh et al., 2017) that make their results unrepeatable and doubtful (boundaries of various interfacial phenomena are not clear). At symmetric condition, a rising bubble can have spherical, elliptical, and spherical cap or skirt shape depending on its size (Cao and Macián-Juan, 2020, Tripathi et al., 2015) while its dynamic behavior can be predicted by dimensionless numbers of Galilei and Eotvos (Sahu, 2017, Sharaf et al., 2017). However, in bubble injection systems, it is very difficult to produce complete symmetric bubble. An asymmetric bubble rises in a zigzag or spiral path which affects the stability of bubble and has not been discussed so far. Moreover, previous studies of bubble and liquid–liquid interface interaction have focused on only one specific interfacial phenomena such as bouncing/passing a bubble at/through liquid–liquid interface (Bonhomme et al., 2012b, Singh et al., 2017), satellite formation (Li et al., 2014) and generation of micro droplets (from heavy liquid film rupture) (Rabenjafimanantsoa and Time, 2013). A comprehensive study of all possible interfacial phenomena has not been conducted.

In this paper, first the previous theories for the presence/passage of bubble at/through liquid–liquid interface are reviewed. Then, a novel experimental procedure is developed that guarantees experiment repeatability to produce a single bubble of a desired size. In a system of air–water-hexane, all possible interfacial phenomena are observed and their exact boundary are clarified using meaningful dimensionless numbers. Finally, the previous equations related to bubble size criterion are modified and a novel route is proposed for measuring interfacial tension.