## 1 Introduction  
Emulsions are dispersions of two immiscible liquids. They are of interest in many important practical applications in the food, cosmetic, oil production, agriculture, and several other process industries [1; 2; 3; 4; 5; 6]. There are two types of emulsions: Oil-in-water (O/W) type, where oil forms the dispersed (droplets) phase and water forms the continuous phase; and water-in-oil (W/O) type, where water forms the dispersed (droplets) phase and oil forms the continuous phase. Without an effective interfacial stabilizer, emulsions are unstable systems and they readily separate out into oil and water phases as the mechanical agitation is stopped. As emulsions are created by increasing the interfacial area between the two immiscible phases, they possess a high interface free energy. Given the chance, the droplets tend to coalesce to decrease the interfacial area and, hence, the interfacial free energy. There are a number of methods available to improve the thermodynamic and kinetic stabilities of emulsions. One such method is to add a surfactant to the system. The presence of a surfactant at the interface lowers the interfacial tension between the two phases and hence improves the thermodynamic stability. The surfactant also provides a steric barrier to intimate contact between two approaching droplets and thus improves the kinetic stability. Surface-active polymers and amphiphilic solid nanoparticles can also improve the thermodynamic and kinetic stabilities of emulsions by lowering the interfacial tension and creating a steric barrier between the approaching droplets. The kinetic stability of emulsions can further be enhanced by reducing the rate of sedimentation or creaming of droplets by decreasing the average droplet size of the emulsion and by increasing the viscosity of the continuous phase. Figure 1 shows a schematic diagram of the surfactant-stabilized and nanoparticles-stabilized oil droplets in an O/W emulsion. Emulsions stabilized with solid nanoparticles, referred to as Pickering emulsions, are becoming increasingly important due to their widespread applications [3]. The physical properties of Pickering emulsions can be easily manipulated by changing the nature and number of nanoparticles added to the emulsion. Furthermore, the solid nanoparticles used in the Pickering emulsions are environment-friendly, natural materials such as starch nanoparticles. In the formulation, mixing, and pumping of emulsions, the most important physical property to be considered is viscosity. Thus, accurate measurement of the viscosity of emulsions is important. The key factors affecting the viscosity of oil-water emulsions are the viscosity of the continuous phase, the ratio of dispersed phase viscosity to continuous phase viscosity, and the volume fraction of the dispersed phase [7]. The droplet size and droplet size distribution also affect the viscosity, especially at high-volume fractions of the dispersed phase [8]. Rheological characteristics of emulsions can be determined using off-line and on-line methods. In the off-line methods, samples of emulsion are extracted from the process and sent to the laboratory for analysis and characterization. As emulsions are heterogeneous fluids, sampling of emulsion from the process itself may pose problems. As discussed by [PERSON] [9], even when the emulsion upstream of the sampling device is well dispersed (pseudo-homogeneous), the collected sample may not be representative of the actual emulsion in the process. Thus, special care is required to ensure representative sampling. As emulsions are prone to creaming, sedimentation, and coalescence of droplets, it is important to develop an in-situ method of measuring the viscosity of emulsions under mixing conditions. The in-situ measurement of the viscosity of emulsions under mixing conditions is especially important when emulsions are unstable in nature.