Droplet size comminution

The situation in the case of emulsions prepared by comminution techniques can be partly illustrated with the help of the characteristic quantities defined above. These data are summarized in Table 8.7 for two emulsions, namely one with an average drop size of 50 pm (e.g. prepared by a common rotor/stator device) and one with an average droplet size of 100 nm (e.g. prepared by ultrasonification). 

Comminution, or particle size reduction of solids, is considerably different from that of the breakup of one liquid by dispersal as small droplets in another. Particle size reduction is generally achieved by one of four mechanisms (1) compression, (2) impact, (3) attrition and (4) cutting or shear. Equipment for particle size reduction or milling includes crushers (which operate by compression, e.g., crushing rolls), grinders (which operate principally by impact and attrition, although some compression may be involved, e.g.,... 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

A surfactant used in froth flotation to adsorb onto solid particles, make them hydrophobic, and thus facilitate their attachment to gas bubbles. See also Froth Flotation. A high-shear mixing device used to prepare colloidal dispersions of particles or droplets by size reduction (comminution). Also termed dispersion mill . 

Colloid Mill A high-shear mixing device used to prepare colloidal dispersions of particles or droplets by size reduction (comminution). Also termed dispersion mill. 

The fabrication of encapsulation systems may be carried through mechanical processes, in general based on a top-down approach to the disruption of larger systems into homogeneously sized particles or droplets with desired properties, or by physicochemical processes, based on a bottom-up approach to the assembling of molecular building blocks into structured systems (i.e., micelles, microemulsions, and some biopolymeric nanoparticles), as well as by a mixed approach, where molecular assembly and comminution processes are combined together (i.e., liposomes, multilayer emulsions, and some biopolymeric nanoparticles). 

Finally, under well-defined conditions, it is possible to polymerize performed emulsion droplets. This is especially true for emulsions prepared by condensation methods where the conditions can be controlled in such a way that both secondary nucleation can be avoided and droplet or particle stability can be maintained during the entire polymerization. In the case of emulsions prepared by comminution techniques, suspension polymerization is a good example of a system where the (conditions) properties of emulsions can be converted into the corresponding properties of sols/suspensions. For smaller drop sizes, the solubility of the monomer in water is crucial, but unfortunately, very hydrophobic monomers are technically unimportant, at least nowadays. The addition of hydrophobic molecules needs tailored emulsification procedures regarding and DSD, and a certain maturation time to result in stable emulsions. Miniemulsion polymerization is a promising way, although the question as to what extent a 1 1 copy of an emulsion is possible is still waiting for an answer. 

To overcome these problems, another comminution technique—atomization—was developed. Atomization produces solid or aerosol particles with reduced sizes by spraying molten material or material solution or suspension under conditions such that it breaks down and then solidifies as fine powder [32]. In a typical atomization process, a molten material passing through a nozzle scatters into fine droplets by a highspeed medium (e.g., gas or water) and then the droplets solidify to powder. Obviously, the atomization technique is highly efficient for preparing micron and submicron powders at industrial scales and recent development has enabled atomization to produce nanoparticles of sizes down to 20nm [33]. 

Particulate products, such as those from comminution, crystallization, precipitation etc., are distinguished by distributions of the state characteristics of the system, which are not only function of time and space but also some properties of states themselves known as internal variables. Internal variables could include size and shape if particles are formed or diameter for liquid droplets. The mathematical description encompassing internal co-ordinate inevitably results in an integro-partial differential equation called the population balance which has to be solved along with mass and energy balances to describe such processes. 

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