Droplet Freezing and Solidification

This number is conceptually an energy ratio, but independent of the interface heat extraction rate and thus the contact area. Since the interface heat transfer is assumed to control the solidification process of an impacting droplet, the choice of a dimensionless number should involve an evaluation of the influence exerted by this key factor. Therefore, the use of this newly defined dimensionless number is limited to an initial decision on which of the Impact number and the Freezing number is most appropriate for the application to a given material system at a know impact velocity. 

The first step of fieeze drying is atomization, which was discussed in Section 8.2. The next step is solidification, which is discussed in the book Transport Phenomena in Metallurgy [9]. The solidification or freezing time for a droplet of volume, V, and surfece area. A, is given by the time, t, required to transfer the heat of fusion from the fluid to the freezing droplet ... 

Many pure substances, water in particular, experience supercooling before the solidification process starts. Of particular interest is the freezing process of a droplet, which, according to Hindmarsh et al. [6], can be described in the following four stages, and is illustrated in Fig. 16.1. 

Simpler dripping methods have been reported where the solidification starts immediately after contact of the drops with the liquid. An example represents freezing of slurry droplets in liquid nitrogen and subsequent freeze-drying and calcination of the bodies [50]. However, such methods produce particles of anisotropic shape, and the size distributions are broad because many droplets break apart upon bouncing against the liquid surface. 

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