Effects of surface-active agents on bubble growth and detachment from submerged orifice
Introduction
Formation of gas bubbles in liquid and the interactions of the bubbles with submerged surfaces influence the performance of many engineering systems (Kulkarni and Joshi, 2005). For instance, in many electrochemical devices, bubbles, formed at the electrode/electrolyte interface, reduces the active electrode-area and thus limits the device performance. While designing electrochemical systems, special techniques are often employed to remove the bubbles from the electrode surfaces. Such techniques include the use of ultrasound field (Zadeh, 2014), magnetic field (Matsushima et al., 2009), as well as surface-active agents (Lee et al., 2005). Leong et al. (2011) studied the role of surfactant head group, chain length, and cavitation microstreaming on the growth of bubbles. Influence of acoustic fields on the growth and collapse of bubbles has been studied (Ashokkumar et al., 2007, Leong et al., 2010) in detail. Efficient implementation of the above techniques, however, requires careful investigation of bubble nucleation, growth and detachment on submerged surfaces.
Studies on bubble growth and detachment usually follows two distinct approaches. In the first approach, many bubbles grow on a submerged surface and diagnostics are focused on the collective behavior of the bubbles in liquid. Second method uses single or very few bubbles and focus on their evolution. Single bubble studies are particularly effective in isolating the influences of various factors impacting the gas bubble evolution in liquid. Among such influencing factors, use of surfactants has emerged as a useful means to control the bubble dynamics (Allen and Deutsch, 2014). In the present study, effects of surfactant are investigated on single bubble evolution from submerged orifice.
Growth and departure characteristics of single gas bubble in liquid has been studied using a variety of computational and experimental techniques. Computational analysis on bubble growth characteristics typically uses volume-of-fluid (Albadawi et al., 2013, Chakraborty et al., 2009, Valencia et al., 2002) or particle-based methods (Das and Das, 2009). Large number of computational studies have successfully captured the fluid dynamics of bubble motion in for a variety of gas-liquid environments(Chakraborty et al., 2009, Eshraghi et al., 2015). Deodhar (2012) simulated the influence of surfactant during bubble growth in aqueous solutions. It was observed that the dynamic surface tension influences the flow field due to non-uniform adsorption and desorption of surfactant molecules around the bubble interface.
Hsu et al. (2000) conducted a series of experiments to identify the variations in bubble shape and volume due to surfactant addition. Using high-speed imaging, Kalaikadal (2012) showed that the dynamic surface tension plays primary role in dictating the shape and volume of a gas bubble in a surfactant. Loubière and Hébrard (2004) studied the kinetics of adsorption and desorption of cationic, anionic and non-ionic surfactants and investigated the effects of the surfactants on bubble volume and frequency. The study emphasizes the role of dynamic surface tension and the need of further research for better understanding the interactions between the bubbles and the surfactants.
Using particle image velocimetry (PIV) as well as numerical techniques, King and Sadhal (2014) studied the effects of Sodium dodecyl sulfate on the growth of air bubble in water. The investigation shows that the surface tension gradient assists in detachment and formation time up to the critical micelle concentration (CMC) of the surfactant. The PIV results indicated enhanced liquid-velocity at the gas-liquid interface induced possibly by the surface tension gradient. In contrary, the PIV measurement by Kurimoto et al. (2016) showed that the surfactant Triton X-100 creates smaller bubbles while keeping the velocity field largely unaffected.
The above review of literature clearly indicates that, for the sub-micellar concentration of the surfactant, the dynamic surface tension and the surface tension gradient play vital role in bubble growth and detachment. Studies also show that, based on the flow rate, the bubble formation dynamics may be grouped under three different categories: static, dynamic and turbulent (McCann and Prince, 1971). In the quasi-static regime, the bubble detachment volume does not depend on the gas flow rate and the volume can be calculated using the bubble orifice and surface tension value as given by Tate’s law (Kulkarni and Joshi, 2005):
In the present experiments, the gas flow rates are controlled to maintain the constant-volume bubble growth regime. The constant-volume regime is ensured by limiting the Reynolds number, defined in Eq. (1.2), below 100 (Xiao, 2004).where Q represents the discharge rate, orifice diameter and, gas viscosity.
The effect of time dependant surfactant properties on the bubble detachment can be deciphered clearly in quasi static regime. Such observation necessitates the study of the combined effects of dynamic surface tension and surface tension gradient in quasi-static bubble growth regime. Different surfactant concentrations are used to ascertain the effects on detachment of bubble and the flow field around the growing bubble. The present work, therefore, focuses on the quantitative understanding of the bubble growth in the quasi static regime where only buoyancy and surface tension forces are dominant. Present study uses PIV and high-speed imaging techniques for careful measurement of the transient velocity field as well as the volume and shape of the quasi-static bubbles. The ensuing discussion outlines on the experimental setup design, parameters that are controlled and techniques that have been employed to study the characteristics of bubble growth.
Section snippets
Experimental
The schematic diagram of experimental set up is shown in Fig. 1. The bubble growth experiments are conducted on a water column fabricated of transparent Poly (methyl methacrylate) (PMMA) acrylic glass sheets of size 100 mm × 100 mm × 500 mm. The size of the experimental setup is quite large compared to the bubble size such that the wall effects remain negligibly small. The orifices used are of 1 mm and 1.6 mm diameter and connected to a New Era (model number NE-300) syringe pump through a long
Results and discussions
The results and discussions section has been divided into two parts. While the first part covers the bubble growth, detachment and shape analysis for various flow rates and surfactant concentrations obtained from the high-speed imaging, the second part discusses the instantaneous velocity fields at various stage of bubble growth and surfactant concentrations.
Conclusions
Experiments are conducted to identify the effects of surfactant concentration on the dynamics of quasi-static gas bubble growth in a liquid. While the various stages of bubble growth of the gas bubble are captured using high-speed imaging, the velocity field is quantified using particle image velocimetry. The major conclusions from the present study are as follows:
- 1.
For quasi-static bubble growth in DI water, the bubble detachment diameter is independent of flow rate. For the surfactant
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