## 1 Introduction  
It is well-known that oil-in-water (O/W) emulsions are thermodynamically unstable systems composed of oil droplets dispersed in an aqueous continuous phase. However, kinetically stable emulsions can be produced using optimized formulation and processing techniques [1]. Emulsions are subject to a number of destabilization processes such as creaming, phase inversion, coalescence and Ostwald ripening (OR). Coalescence and OR both promote the same phenomenon in emulsions, i.e., an increase in droplet size with aging time. However, the mechanisms of the two processes are very different. Ostwald ripening involves a diffusive transfer of the dispersed phase from smaller to larger droplets; coalescence is due to the rupture of the thin film between droplets, which causes them to fuse [2]. It is important to distinguish between these two mechanisms because their causes--and possible solutions--are different. According to the Lifshitz-Slyozov-Wagner theory, Ostwald ripening depends on the interfacial tension between the two phases and on the solubility of the oil in the continuous phase. Interfacial tension and solubility both vary with the nature and concentration of the surfactant [3]. The choice of surfactant is therefore crucial when seeking to stabilize this type of system. The Ostwald ripening rate is also influenced by the size of the droplets, so that the rate increases as droplet size decreases. One way to characterize the surfactants used in emulsions is by using the hydrophilic-lipophilic balance (HLB) number. This parameter is a semi-empirical scale used for selecting surfactants and developing emulsions. For example, a low HLB number means that a surfactant is soluble in oil; a high HLB number indicates water solubility. Sometimes, a single surfactant is not enough to obtain the stability required [4] and so a mixture of surfactants can be used to achieve synergic effects [5,6]. Interestingly, this may be a consequence of non-ideal mixing effects in the aggregates, which leads to a reduction in interfacial tension. An optimum HLB value for a selected oil phase can be obtained by using an appropriate mixture of surfactants. For example, it is common to use mixtures of Span 20 and Tween 80 to obtain different HLB numbers. Span 20 is a non-ionic surfactant which is also known as sorbitan laurate. It is used as a food additive (E493) and has an HLB number of 8.6. Tween 80 is also a food additive (E433), but it has an HLB number of 15. The use of these two surfactants in combination has been reported in several studies on food products [7,8]. Essential oils (EOs) are aromatic compounds, some of which are associated with biomedical benefits due to their anti-inflammatory, anticancer, antifungal and antioxidant properties. Lemon essential oil (LEO) is extracted from lemons mainly by cold pressing or steam distillation. LEO is known to exhibit anticancer, antioxidant, antiviral, anti-inflammatory and bactericidal properties [9,10], and has been used as a safe additive in foods, medicines and nutritional supplements [10,11]. However, LEO is very sensitive to light, temperature and oxidation. To overcome these limitations, researchers are increasingly investigating the use of LEO in nanoemulsions. Finally, the physical stability of emulsions depends not only on their formulation, but also on the emulsification method used. However, the emulsification process is, itself, affected by the formulation used. For example, to create droplets via a shearing process, the ratio between the dispersed phase viscosity and the continuous phase viscosity cannot have a value greater than five, and the optimum ratio is approximately one [12]. Many methods are used to develop nanoemulsions using high-energy approaches, such as microfluidization. The Microfluidizer(r) device is based on microchannels through which a coarse emulsion is forced to pass using high pressure (up to 150 MPa). This results in a very high shearing action, producing fine emulsions.