# Critical Review of Techniques for Food Emulsion Characterization
## 4 Emulsion Fractionation and Characterization Methods
### Field-Flow Fractionation (FFF)
The relationship between the retention ratio and the equilibrium position achieved within the channel is indicated by a retention parameter \(\lambda\), defined as the ratio of mean layer thickness (\(l\)) to channel thickness (\(w\)). In normal separation mode, the retention ratio is typically around six times greater than the retention parameter [8]:  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]  
\[\lambda=\frac{l}{w} \tag{4}\]  
The mean layer thickness (\(l\)) can be determined by dividing the thermal energy (\(kT\)) by the force (\(F\)) applied to droplets, with \(k\) representing Boltzmann's constant and \(T\) absolute temperature:  
\[l=\frac{kT}{F} \tag{5}\]  
\[\lambda=\frac{kT}{Fw} \tag{6}\]  
The droplets can be separated under any type of external field that interacts with their physicochemical properties. Based on the nature of the field applied to separate droplets, FFF techniques can be divided into several categories, including flow FFF, sedimentation FFF, and electrical FFF [229]. Flow field-flow fractionation (FIFFF) represents the most widely used technique in the field of FFF and can be utilized for droplets ranging in size from 1 nm to 50 \(\upmu\)m [230]. In this technique, emulsion flowing through the separation channel is subjected to a perpendicular flow, known as cross-flow, which moves transversely across the channel, forcing the droplets toward the accumulation wall (Figure 16). There are three main configurations of FIFFF channels: symmetrical, asymmetrical, and hollow fiber. The first category, symmetrical FIFFF (SFIFF or SF4), features a symmetric configuration [231], with two permeable walls equipped with porous ceramic frits and a cross-flow passing through both walls (Figures 16 and 17, top panel). The second type, known as asymmetric FIFFF (AsIFFF or AF4) [221], has an asymmetric configuration with only one permeable wall and one solid, impermeable wall, as depicted in Figure 17 (bottom panel). The flow in the separation channel is divided into a longitudinal flow carrying the droplets along the channel, and a transverse flow that traverses the accumulation wall, creating the fractionation field [232]. Figure 16: The principle of emulsion droplet separation in symmetrical flow field-flow fractionation (SFIFFF). Emulsion flowing through the separation channel (represented by the light blue area) is subjected to a perpendicular cross-flow across the channel, forcing the droplets toward the accumulation wall. In SFIFFF, the separation channel has two permeable walls equipped with porous ceramic frits. The cross-flow passes through both walls. Purple and red circles represent large and small emulsion droplets, respectively. The green and bold black arrows indicate the direction of the cross-flow and diffusion counterforce, respectively. Reprinted with permission from [230]. Copyright © 2020 Elsevier. Figure 17: The configuration of symmetrical flow field-flow fractionation (SFIFFF) channel (**top**) and asymmetrical flow field-flow fractionation (AsIFFF) channel (**bottom**). The SFIFFF channel comprises two plastic blocks containing porous ceramic frits, while the AsIFFF channel features one porous ceramic frit and one solid, impermeable wall. A channel spacer and an ultrafiltration membrane are placed on the bottom wall between the two blocks. Two blocks are then clamped together. Reprinted with permission from [230]. Copyright © 2020 Elsevier. The conventional FIFFF separation channel typically has a trapezoidal shape and uniform thickness, achieved by placing spacers between the accumulation wall and the upper wall of the channel. Alternatively, the thickness of the channel can decrease along the channel axis such that the flow rate increases down the channel [233]. The thickness-tapered channels have been shown to generate increased hydrodynamic lift forces, which contributed to improved separation speed, recovery, and resolving power.