## 1 Introduction  
Emulsions are a disperse system, consisting of two immiscible liquids in which one of the liquids is dispersed in the other as droplets [1]. The liquid that makes up the droplets is referred to as the dispersed phase, and the surrounding is referred to as the continuous phase. The dispersed phase is generally the smaller fraction that presents in an emulsion system [2]. The type of emulsion depends on several parameters such as the nature of emulsifiers, the nature of the oil, and the effect of other materials [3]. In general, emulsions are classified according to the relative spatial distribution of the oil and aqueous phases [4]. Simple emulsions consist of a dispersed phase in a continuous phase. A system that consists of an oil phase dispersed in an aqueous phase is called an oil-in-water (O/W) emulsion, while a system that consists of a water phase dispersed in an oil phase is called a water-in-oil (W/O) emulsion. Mixed emulsions are obtained from two single emulsions of the same type that contain droplets of different compositions. These emulsions are oil-in-water (O\({}_{1}\) + O\({}_{2}\)/W) emulsions or water-in-oil (W\({}_{1}\) + W\({}_{2}\)/O) emulsions. Double emulsions are simple emulsions dispersed in a continuous phase. Double emulsions are complex systems where both W/O and O/W emulsions exist simultaneously. In the case of W/O/W emulsions, the oil droplets have smaller water droplets within them, and the oil droplets themselves are dispersed in a continuous water phase. The O/W/Oemulsions, however, consist of tiny oil droplets entrapped within larger water droplets, which in turn are dispersed in a continuous oil phase [4]. The process of converting oil and water phases to an emulsion system is called emulsification. The emulsion does not form spontaneously--energy input must be applied through shaking, stirring, or agitation [2]. The initial stage of emulsification is breakup and disrupt oil and water phases, so that one phase (of oil or water) becomes dispersed throughout the other. This stage is also called droplets disruption. The droplet disruption depends on the oil-water interfacial tension. The presence of surfactant decreases the interfacial tension and minimizes the effects of interfacial forces so that it facilitates the droplet disruption. Under mechanical agitation, the small droplets are constantly moving and the frequency of collision is very high. These lead to coalescence, then increasing their size, also called droplets coalescence. The presence of surfactant adsorbs to the surface of the droplets, forms a protective membrane to prevent the droplets from coming close together to coalesce. Rapid stabilization of the droplets against coalescence, once they are formed, depends on the time taken for the surfactant to be adsorbed to the surface of the droplets relative to the time between droplet-droplet collisions. This time depends on the flow profile that the droplets experience, as well as the nature of the surfactant used [5]. If the protective membrane between two droplets is broken, the droplets will merge, thereby reducing the total surface of the droplets. The emulsion tends to revert back to the initial state after it has been formed. The stability of the emulsion refers to the ability of an emulsion to keep its properties unchanged for a certain period--the more stable the emulsion, the more slowly its properties change [5]. To form a stable emulsion, it is necessary to ensure that the majority of the droplets fall within the same size ranges [6]. The size of the droplets produced during preparation is important because it determines the stability and rheological characteristics of the emulsions. An emulsion may become unstable due to a number of different types of physical and chemical processes. Physical instability results in an alteration in the spatial distribution, whereas chemical instability results in an alteration in the kind of molecules present.