Drying droplet as a template for solid formation: A review

Unraveling Particle Formation: From Single Droplet Drying to Spray Drying and Electrospraying

“…The need for more sophisticated particles for pharmaceuticals, food products, ceramics, and catalysis has increased in recent years [ 1 ]. The variety of particles that can be produced inquires particle engineering approaches to control the particle morphology, size distribution, porosity, density, composition, and purity of the particles [ 2 ].…”

“…The difference between both techniques is that, in the case of spray drying, drying occurs at an increased temperature, while in the case of electrospraying the application of a high voltage leading to small droplets makes it possible to dry the droplets at room temperature. Although these are established drying techniques, there is still a lack of knowledge on the particle formation process [ 1 ].…”

“…It should be kept in mind that the droplet in SDD investigations will undergo a slightly different drying process due to a generally increased droplet diameter (i.e., mm instead of µm) and drying time (i.e., minutes instead of seconds). Next to this, the droplet will not experience droplet–droplet interactions, droplet-wet particle interactions or droplet-wall collisions [ 1 , 6 ]. Despite these differences, SDD methods have an added value to better understand and optimize pharmaceutical drying processes, as discussed further in this article in more detail.…”

“…Eventually, the diffusional motion of the solutes towards the center of the droplet becomes lower than the reduction rate of the droplet diameter due to the constant rate solvent loss. At this point, crust formation will occur due to solute enrichment at the droplet surface, leading to decreased drying rate and introducing the second drying stage [ 1 ]. This point is referred to as the critical point or locking point.…”

“…The drying rate is now controlled by the diffusion or capillary flow rate of the solvent from the wet core through (the pores of) the crust. The slowed solvent evaporation still results in shrinkage of the wet core and substantial growth of the crust towards the center of the droplet [ 1 , 11 ]. The thickening of the crust will lead to an increasing resistance for mass transfer and thereby a gradual reduction of the drying rate.…”