Clumping of particles

Viscosity. Because a clump of particles contains occluded Hquid, the effective volume fraction of a suspension of clumps is larger than the volume fraction of the individual particles that is, there is less free Hquid available to faciHtate the flow than if the clumps were deagglomerated. The viscosity of a suspension containing clumps decreases as the system becomes deagglomerated. This method is not very sensitive in the final stages of deagglomeration when there are only a few small clumps left. 

Initial studies were made with the Rank Bros, electrophoresis unit, using the dilute supernatant suspension over a dispersion of 3.33g of carbon black per liter of dodecane equilibrated for 24 hours with the added 0L0A-1200. The electrophoretic mobility (u) of 1-3 pm clumps of particles was observed at a field of 100 volts per centimeter. The zeta-potentials ( ) were calculated... 

Macroscopically, a liquid has a definite volume but no definite shape. It conforms to the shape of its container. Microscopically, the particles are moving much more than in the solid. There are usually clumps of particles moving relatively freely among other clumps. 

At the microscopic level, the liquid particles are in constant motion. The particles may exhibit short-range areas of order, but these usually do not last very long. Clumps of particles may form and then break apart. At the macroscopic level, a liquid has a specific volume but no fixed shape. Three additional macroscopic properties deserve discussion surface tension, viscosity, and capillary action. 

Figure 10.3 The effect of ram pressure acting on a clump of particles in a gas disk. The six panels show the positions of the particles after 1.75 orbital periods. Each point in the plots is a superparticle representing a large number of millimetersized objects. The clump is initially 10 model units in diameter, equivalent to 104 km. Upper panels (front, above, and side views) without any cohesion, the clump is soon shredded. Lower panels when gravitational forces between particles are included, the clump remains mostly intact and becomes more compact. From Cuzzi et al. (2007).

Clumps of particle may contain occluded air that is difficult to displace this is best dealt with by adding the liquid whilst the powder is under vacuum. 

The problem of how adhesion affects structuring is an old one, going back to the early theories ofnucleation of a droplet from a vapor, a crystal from a gas," or a crystal from a melt or glass The arguments about such phase separation have carried on over many years, as summarized by Zettlemoyer and others." " A clump of particles, i.e. a nucleus, has to appear within the random gas by a nucleation process, and this clump can then grow as further particles stick onto it, as shown in Fig. 8.27. 

This is like putting a clump of particles inside a bag, sealing it shut, and then trying to get the particles apart. While it is possible to break the agglomerate within the bag, all pieces of that agglomerate still remain within the bag until the bag is broken. 

Dilute latex emulsions, with a glass transition above room temperature, can be atomized or sprayed using a fine pipette attached to a standard can of Freon gas. Difficulties arise if the latex is not dilute enough, as clumps of particles are then observed. With low glass transition latexes, cryogenic or chemical hardening methods must be used to ensure the mechanical stability of the polymer particles. 

These surmised functionalities of clump size with liquid velocity and particle size can then be used to analyse the behavior of catalyst particles in a trickle bed using a modified catalyst Weiz and Prater modulus, (f) . For a clump of particles in the bed, ... 

Coagulation. The clumping of particles in order to settle out impurities often induced by chemicals such as lime or alum. 

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