# Critical Review of Techniques for Food Emulsion Characterization
## 4 Emulsion Fractionation and Characterization Methods
### Field-Flow Fractionation (FFF)
The analyzed emulsion is introduced into a circular separation channel, which is subjected to rotation at a high rate (up to 2500 rpm) around its axis [8]. Rotation generates a centrifugal force acting radially to the rotation axis and perpendicularly to the flow in the separation channel [224]. As the emulsion passes through the rotating channel, droplets of different sizes migrate at various rates, as shown in \[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]3 Droplets flowing through the separation channel are distributed across various axial flow vectors based on the equilibrium between the applied centrifugal field and diffusion. The centrifugal force leads to the sedimentation of the droplets (without affecting their shape or causing aggregation [8]) at a rate dependent on their effective volume, inertial mass, and the difference in density between dispersed and continuous phases [220]. As a result, larger and denser droplets migrate towards slower spinning streamlines near the outer channel wall and are collected later from the channel, while smaller droplets travel towards faster rotating inner streamlines closer to the inner channel wall and are eluted earlier [225]. To increase separation resolution, spinning gradients, and various emulsion flow rates can be applied [8]. The retention parameter (\(\lambda\)) in this subtype of FFF can be determined based on the droplet diameter (\(d\)), the difference in density between dispersed (\(\rho_{p}\)) and the continuous phase (\(\rho\)), the radius of the centrifuge rotor (\(r\)), angular rotation frequency (\(\omega\)), the channel thickness (\(w\)), and the thermal energy (\(kT\)):  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]4  
In SedFFF, small droplets with a density closely aligned with that of the continuous phase may encounter challenges in effective separation, requiring specific operational conditions, such as a high centrifugal field and low flow rate. However, employing a high centrifugation force may extend elution times and diminish detectability for larger droplets, leading to their accumulation on the channel wall. Distortions in the shapes of peaks representing individual droplet fractions are also common at low sample flow rates. The third category of FFF methods used for emulsion separation and characterization is electrical FFF (EIFF) [240]. In EIFF, an electrical field is generated across the separation channel by two charged blocks with high electrical conductivity, which serve both as electrodes and channel walls [241]. The force driving droplet separation depends on the electrical voltage applied and the electrophoretic mobility of the droplets, which is directly related to their charge. The retention behavior of the droplets is determined by their diameter (\(d\)), electrophoretic mobility (\(\mu\)), the strength of the effective field (\(E_{\textit{eff}}\)), and the viscosity of the continuous phase (\(\eta\)), according to Equation (13):  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]5  
\[R=\frac{t_{0}}{t_{r}}=6\lambda \tag{3}\]6 Schematic presentation of droplet separation by sedimentation field-flow fractionation (SedFF). The emulsion is introduced into a circular separation channel, which is subjected to rotation at a high rate. As the emulsion passes through the rotating channel, droplets of different sizes migrate at various rates. Larger and denser droplets (represented by black circles) migrate towards slower spinning streamlines near the outer channel wall, while smaller droplets (represented by yellow circles) move towards faster rotating inner streamlines closer to the inner channel wall. Orange arrows indicate the direction of sample flow through the channel. The green arrow indicates the direction of channel rotation. EIFF systems offer several benefits compared to other FFF techniques. They provide a gentle perpendicular separation field that decreases the reliance on field strength for achieving resolution.