# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Rheological Analysis of Emulsions
The viscosity of emulsions stored under creeping flow conditions, at some point, reaches a plateau known as zero-shear viscosity. Zero-shear viscosity describes the mobility of the droplets or droplet flocs/aggregates within the emulsion. An emulsion with a high zero-shear viscosity exhibits low droplet movement, which translates to a limited incidence of interactions between droplets that may lead to coalescence and reduced separation processes. Therefore, a formulation based on such emulsion would be expected to display a prolonged shelf-life. Zero-shear viscosity of emulsions may be approximated based on Stokes' Law by calculating the terminal velocity of a droplet (\(v\)) moving through the viscous continuous phase. The terminal droplet velocity \(v\) is related to the drag force (\(F_{d}\)) acting on the droplet on the interface between the continuous and the dispersed phase (also known as frictional force or Stokes' drag), continuous phase viscosity (\(\mu\)), and droplet radius (\(R\)), as given in Equation (1):  
\[F_{d}=6\pi\mu Rv \tag{1}\]  
According to [PERSON]'s law, the resistance of the emulsion to instabilities caused by droplet flocculation becomes higher as its viscosity increases [37]. Nevertheless, the application of the Stokes equation to real-life emulsions is limited, as it assumes no interactions between droplets moving through the fluid. The more accurate method to establish emulsion zero-shear viscosity is based on the experimental determination of changes in emulsion viscosity under different levels of shear stress. In this approach, the range of zero-shear stress viscosity is identified based on the curve depicting the correlation between viscosity and shear stress obtained by fitting a set of experimental data. For instance, Figure 11 shows the effect of shear stress on the viscosity of five cosmetic emulsions. Body Lotion A demonstrates a zero-shear viscosity plateau of around \(10^{3}\) Pa-s (1 million centiPoise), whereas Day Cream A exhibits zero-shear viscosity at approximately \(7\times 10^{4}\) Pa-s (70 million centiPoise). This implies that lotion A has around 70 times higher droplet mobility than cream A, which may contribute to its substantially shorter shelf-life. The rheological properties of emulsions are strongly related to their stability. Depending on the nature and mechanism of destabilization processes occurring in the system, emulsion rheology can be affected in numerous ways. Separation of the emulsion through flocculation tends to increase its relative viscosity due to the entrapment of liquid within droplet flocs, and an increase in floc volume fraction compared to single droplets. On the other hand, coalescence and Ostwald ripening may reduce the relative viscosity, especially in high-volume fraction emulsions. Phase inversion leads to a substantial decrease in emulsion viscosity due to the drastically reduced volume of the dispersed phase after inversion. The reduction results from the fact that the volume of the continuous phase (which becomes the dispersed phase after the inversion) is typically lower than the volume of the dispersed phase. However, correlating rheological parameters to emulsion breakdown phenomena has been challenging due to the complexity and overlapping timing of multiple separation processes. Therefore, changes in the rheological behavior of emulsions may not directly relate to stability. As a result, rheological analysis alone cannot provide a reliable prediction of long-term emulsion stability. Another limitation of the rheological analysis of emulsions is related to anomalies exhibited by emulsions during measurements with rheometers and viscometers due to inertia and slip. To avoid false measurements, it is important to ensure that data is collected under conditions of constant shear rate. This requirement is typically met by small (<4 inches) cone-and-plate apparatuses or concentric cylinder geometries with a gap-to-radius ratio \(\geq\) 0.95.