# Critical Review of Techniques for Food Emulsion Characterization
## 4 Emulsion Fractionation and Characterization Methods
### Capillary Hydrodynamic Fractionation (CHDF)
Increased interaction with the capillary walls improves peak resolution in CHDF by minimizing band-broadening effects such as diffusion and dispersion. Additionally, longer capillaries slow down droplet migration and increase their elution time. The eluant ionic strength is another parameter that impacts the separation resolution significantly. A decrease in eluant ionic strength leads to reduced droplet elution times and increased separation factor. This influence is linked to the increase in thickness of the ionic double layer around the droplets as the eluant ionic strength decreases. Thicker double layers contribute to a greater repulsive force between the droplets and the capillary wall, causing droplets to migrate towards the center of the capillary, where velocity is higher, and move faster [246]. The velocity of the mobile phase also affects the separation efficiency in CHDF. As the velocity of the eluant increases above the critical velocity dependent on the capillary internal diameter, the separation factor and resolution increase. Increasing eluant velocity contributes to higher radial lift force exerted by the fluid on the droplets, contributing to more rapid radial migration. Understanding the impact of the above parameters is crucial for optimizing the performance of CHDF and achieving desired separation outcomes. By tailoring capillary diameter and length, as well as eluant ionic strength and velocity, the desired level of separation efficiency and resolution can be achieved. CHDF offers several advantages compared to other commonly used separation techniques. Firstly, it is a relatively fast, simple, and gentle separation technique that does not require harsh conditions or complex sample preparation [248]. This makes it suitable for analyzing samples prone to denaturation or degradation. Secondly, CHDF has the flexibility to separate analytes in their native conditions, preserving their original structures. Thirdly, CHDF allows droplet size determination even in emulsions with high droplet concentrations, which can resolve one of the key limitations of DLS. CHDF has been successfully applied to establish droplet size distribution and to monitor droplet size changes over time as a measure of emulsion stability [223; 245]. Nevertheless, CHDF has several significant limitations. Firstly, it is a relative technique that requires the establishment of a diameter calibration curve based on elution times of standards of known diameters. In addition, the particle refractive index needs to be known. Another key issue arises from instrumental broadening (IB) [249], which stems from factors such as finite injection volume, detection cell volume, non-uniform parabolic velocity profile in the capillary, and Brownian motion affecting droplet axial displacements. Due to IB distortion, a single species with diameter \(D\) will exhibit a range of elution times instead of one specific time. Errors in instrumental broadening correction can lead to inaccuracies in estimating droplet size distribution, thereby impacting resolution accuracy. To sum up, the applicability of emulsion characterization methods described in this review for different types of emulsion samples along with their key advantages and limitations have been summarized in Table S1.