## 4 Microencapsulation Techniques
### Physical Process
#### 4.2.2 Electrohydrodynamic Encapsulation  
The electrospray process is one-step electrohydrodynamic atomization. The polymeric solution is polarized by applying electrical forces. Then the charged liquid is stretched and accelerated into a jet of charged droplets by the electrostatic force generated at the droplet surface. Then, the charged jet breaks up into tinier charged droplets by coulomb repulsion forces. Finally, the micro/nanoparticles are collected after the evaporation of the solvent.  
Electrospraying has become a promising technique of PCMs microencapsulation due to its advantages, such as high encapsulation efficiency, high core loading content, and easy handling. Moreover, this method does not include any toxic chemical additives or surfactants; thus, it is considered a sustainable method.  
Furthermore, the morphology of particles could be controlled by optimizing different parameters and choosing nozzle geometry. The single nozzle is employed in the preparation of simple core/shell microcapsules. In contrast, using a coaxial double or tri-capillary nozzle allowed double-layered shell microcapsules to enhance properties (Figure 9) [141].  
Figure 8: Schematic representation of the principle of spray-drying process [139].  
Figure 9: Schematic representation of experimental setup for simple electrospray (**left**) and coaxial electrospray (**right**) [141].  
Electrospinning is widely used with the phase change materials to prepare core-sheath structure nanofibers via a coaxial system as an electro atomization method. For instance, a coaxial spineret was performed by [PERSON] et al. to prepare polyethylene glycol and cellulose acetate as PCMs and shell solutions for the fabrication of nanofibers [142].  
Nevertheless, the encapsulation of PCMs by electrospraying is scarcely reported in the literature. [PERSON] et al. synthesized _n_-nonadecane/sodium alginate by coaxial electrospraying process [143]. They observed that the size distribution increased with the increase in alginate concentration and the increase in the volumetric flow rate of the core/shell solution. The diameter of microcapsules was lower than 100 \(\upmu\)m, and the loading of contained _n_-nonadecane was about 56 wt%. Following the same method, they encapsulated _n_-nonadecane using sodium alginate and calcium chloride as shell materials. They concluded that the mean diameter of microcapsules increased with alginate concentration, feeding rate, needle-to-collector distance, and stirrer rate, and decreased with needle gauge and needle-to-tip collector length. Nonetheless, they indicated that optimizing operating conditions leads to alginate/PCMs in the nanoscale (80-350 nm) [144].  
[PERSON] et al. encapsulated _n_-hexadecane and _n_-eicosane in a polycaprolactone matrix [145]. Ethyl acetate (EA) and chloroform (Chl) were employed as solvents. They denoted that the mean diameter depends on the nature of core materials and the type of solvent. The morphology of the microcapsules and the loading content were affected by the solvent evaporation and the phase separation between the PCM and poly(caprolactone) (PCL) matrix. They also used the coaxial system to prepare microparticles using _n_-hexadecane/polycaprolactone (PCL) [146]. They demonstrated that the increase in _n_-hexadecane and PCL concentration led to a change in the particle size distribution from a poly-disperse to monodisperse size distribution and a modification in the surface state from porous to non-porous. This study leads to the preparation of spherical microcapsules with a mean diameter of 10-20 \(\upmu\)m and high encapsulation efficiency (96%).