Droplet/particle size distribution

Farahzadi H, Shahrokhi M. Dynamic evolution of droplet/ particle size distribution in suspension polymerization of styrene. Iran J Chem Eng 2010 7 49-60. 

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

Where the polyurethane comprises <30% of the blend, the polyurethane remains in discrete droplets within the polyacetal matrix. In this range the particle size and particle size distribution of the elastomer particles are of importance. Where the elastomer component is in excess of 30%, interpenetrating polymer networks exist in the sense that there are two interpenetrating continuous phases (as opposed to two cross-linked interpenetrating polymer systems). 

Particle Size Measurement. The best way to evaluate an emulsion s stability is probably to measure its particle size distribution. A number of methods are available for droplet size determination (see Sec. VIII.A). Optical microscopy, although a time-consuming technique, is a direct way of measuring droplets larger than 1 pm. Nowadays, laser lightscattering, diffraction, and transmission methods are becoming popular for routine determination of particle size [151, 152],... 

For parenteral emulsions, the formulation scientist must be particularly aware of changes in particle size distribution of the oil phase. Droplet coalescence results in increased droplet size. As a general rule, average droplet size should be less than 1 pm. Droplet sizes of more than 6 pm can cause blockage of capillaries (capillary emboli). 

In the case of droplets and bubbles, particle size and number density may respond to variations in shear or energy dissipation rate. Such variations are abundantly present in turbulent-stirred vessels. In fact, the explicit role of the revolving impeller is to produce small bubbles or drops, while in substantial parts of the vessel bubble or drop size may increase again due to locally lower turbulence levels. Particle size distributions and their spatial variations are therefore commonplace and unavoidable in industrial mixing equipment. This seriously limits the applicability of common Euler-Euler models exploiting just a single value for particle size. A way out is to adopt a multifluid or multiphase approach in which various particle size classes are distinguished, with mutual transition paths due to particle break-up and coalescence. Such models will be discussed further on. 

The phase-Doppler method is capable of accurately measuring particle size distribution and velocity J655] The most recent models ofphase-Doppler particle analyzer (PDPA) can generate data of droplet size and velocity simultaneously as a function of time, from that droplet drag can be calculated and clustering phenomenon can... 

M. Loewenstein, G. V. Ferry, K. R. Chan, and B. L. Gary, Particle Size Distributions in Arctic Polar Stratospheric Clouds, Growth, and Freezing of Sulfuric Acid Droplets, and Implications for Cloud Formation, J. Geophys. Res., 97, 8015-8034 (1992). 

Nasal Sprays Clarity, microbial bioburden, pH, particulate matter, unit spray medication content uniformity, droplet and/or particle size distribution, weight loss, pump delivery, microscopic evaluation of suspensions, particulate matter, extractables, leachables from plastic and elastomeric components of container closure and pump. 

Particle size distribution (oil droplets) using a light scattering technique was measured on a Leeds and Northrop Microtrak particle size analyzer as described by Weiss and Frock (18). Replicate samples of the encapsulated products were dissolved in water. The beta-cyclodextrin sample was only partially soluble therefore, the insolubles were allowed to separate out before removing the supernatent for evaluation. 

All these ideas have been put into one equation, called Stokes law. Nothing against Sir Frederick Stokes, but vapor viscosities are almost always so small that they do not affect settling rates. Also, we never know the particle size distribution of the droplets. There is a more fruitful way to look at the settling tendency of droplets of liquid in an upflowing vapor stream, as shown in Fig. 26.1. The method states... 

Laser diffraction is the most commonly used instrumental method for determining the droplet size distribution of emulsions. The possibility of using laser diffraction for this purpose was realized many years ago (van der Hulst, 1957 Kerker, 1969 Bohren and Huffman, 1983). Nevertheless, it is only the rapid advances in electronic components and computers that have occurred during the past decade or so that has led to the development of commercial analytical instruments that are specifically designed for particle size characterization. These instruments are simple to use, generate precise data, and rapidly provide full particle size distributions. It is for this reason that they have largely replaced the more time-consuming and laborious optical and electron microscopy techniques. 

Figure D3.4.7 Change in cumulative particle size distribution of a 20% (w/v) oil-in-water emulsion stabilized by 2% (w/v) Tween 20 at the lower port (A) and upper port (B). (C) Change in mean droplet diameter and volume fraction of the emulsions as a function of time.

Dye, J.E., Baumgardner, D., Gandrud, B.M., Kawa, S.R., Kelly, K.K., Loewenstein, M., Ferry, G.V., Chan. K.R., and Gary, B.L. (1992) Particle size distributions in Arctic polar stratospheric clouds, growth and freezing of sulfuric arid droplets, and implications for cloud fotmation, J. Geophys. Res. 97,8015-8034. 

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