# Critical Review of Techniques for Food Emulsion Characterization
## 4 Emulsion Fractionation and Characterization Methods
### Field-Flow Fractionation (FFF)
Such channels can be beneficial for eluting long-retained particles without reducing the entire channel thickness or for applications involving additional field programming [233]. The third FIFFF category is hollow fiber FIFFF (HIFHIFF or HF5) [232], which employs a porous cylindrical tube consisting of either a hollow fiber ultrafiltration membrane or a ceramic hollow fiber [234]. The underlying force driving the separation is a cross-flow applied in the hollow fiber. The sample is introduced into the inlet of the hollow fiber. The flow of the sample through the fiber is referred to as the \"channel flow\" or \"axial flow\". \"Focusing flow\" is introduced in the opposite direction to the channel flow from the fiber outlet (Figure 18). The difference in flow rates establishes a focusing point, compelling the droplets to settle into their equilibrium positions. After relaxation, the focusing flow ceases and the channel continues to convey carrier liquid into the fiber. The split flow from the fiber inlet comprises both axial and radial components: axial flow transports droplets along the fiber and exits through an outlet, while radial flow passes through membrane/fiber pores [224]. Compared to asymmetric FIFFF, the HfFIFFF method generally shows higher or similar effectiveness and enhanced detectability with minimal sample dilution during separation [235]. Additionally, the separation channel is cost-efficient and can be used in a disposable manner to avoid cross-contamination. These qualities are crucial for further analysis of collected fractions in studies involving cells or drugs that need to be performed  
Figure 18: The separation principle in hollow fiber-flow field-flow fractionation (HFFIFF). Red and purple circles represent small and large emulsion droplets, respectively. The blue ring represents the hollow fiber wall. Green arrows indicate the direction of radial flow. Reprinted with permission from [230]. Copyright © 2020 Elsevier. under sterile conditions (e.g., in pharmaceutical quality control) [236]. However, the HfFIFFF technique is not designed for high-throughput analyses or large sample volumes. Throughput can be improved by using multiple parallel HF5 modules, like in the MxHF5 system [237]. The equilibrium position and retention time of the droplets in FIFFF are correlated to their diffusivity and hydrodynamic radius. Droplet retention in SFIFFF can be predicted based on the force acting on the droplets, which corresponds to the transverse velocity (\(U\)) and friction coefficient (\(f\)):  
\[F=fU \tag{7}\]  
The transverse velocity (\(U\)) can be described by an equation involving the volumetric cross-flow rate (\(V_{C}\)) divided by the cross-sectional area:  
\[U=\frac{V_{C}w}{V_{0}} \tag{8}\]  
where \(V_{0}\) is the void volume or volume of the channel. The friction coefficient \(f\) is defined by Stokes' Law:  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]0  
where \(\eta\) is the viscosity and \(d_{h}\) is the hydrodynamic diameter of the droplet. The retention parameter in SFIFFF can be expressed as:  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]1  
The hydrodynamic diameter of the droplet is described as:  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]2  
Since predicting droplet retention parameters in AsIFFF and HfFIFFF configurations is more complex, it exceeds the scope of this review. A more detailed discussion on this matter can be found in the following excellent papers [238; 239]. FIFFF techniques come with a set of constraints. Interactions between droplets and membranes can result in droplet loss, emulsion dilution, and diminished recovery. Further analysis may necessitate preconcentration of the collected droplet fractions. Another subtype of FFF is sedimentation field-flow fractionation (SedFFF), known also as centrifugal FFF (CFFF). SedFFF is a high-resolution technique that utilizes a centrifugal field to separate emulsion droplets according to their density and sedimentation rate [222].