Interrelationships of Particle Structure and Flow in Concentrated Suspensions
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work, published by Hoffman 2 in the early 1970s, heralded the discovery of a number of particle structures in flowing suspensions, as well as the interdependence of flow and structure. 3 " 6 This work showed thaf changes in flow and structure can be graduai in some cases and sudden in others. One of the most dramatic examplés is a discontinuous j u m p in v i s c o s i t y w h i c h is coupled w i t h a n a b r u p t c h a n g e in p a r t i c l e structure. 2 Abrupt changes in flow behavior such as this become an important considération for engineers and materials scientists; and knowing how to control or alter thèse changes is a major c o n c e r n in m a n y p r o c e s s i n g situations. With this in rhind, the article that follows has been written to acquaint t h o s e w o r k i n g in o t h e r fields w i t h some of the récent d e v e l o p m e n t s in concentrated suspension rheology. The intent is to p r o v i d e an overview of some of t h e salient a s p e c t s of t h i s w o r k . E m p h a s i s w i l l be p l a c e d on s t r u c t u r e formation in concentrated suspensions and on the interrelationships of structure formation and flow. Those interested in further information can use the références to broaden their understanding of this intriguing area of study. Particle Structure Particulate dispersions which show the highest degree of structure formation generally meet twb requirements. First, they contain particles small
e n o u g h to be strongly influenced by the colloid chemical forces of attraction and r e p u l s i o n , and second, the particles are stabilized against flocculation. Generally speaking, this means that the particles will be about 10 micrometers or less in size, and they will be stabilized either by a double layer of c h a r g e , w h i c h causes r e p u l s i o n between the particles, or by adsorbed (or attached) steric chains, 7 which keep the particles from getting close enough to f locculate through the attractive forces generated by van der Waals-London interactions. In thèse Systems, particle ordering is rigorous e n o u g h to be characterized by light or neutron diffraction when t h e p a r t i c l e s are m o n o s i z e d . 2 , 5 The techniques used for characterization are directly a n a l o g o u s to the procédures used to quantify atomic structures in crystallme solids. 8 That is, light directed through a f lowing suspension has a spëctrum of wavelengths nominally equal to the distance b e t w e e n the regularly ordered particles, while x-rays directed t h r o u g h a crystalline solid h â v e w a v e l e n g t h s n o m i n a l l y equal to the interatomic distances of the regularly ordered atoms. As a resuit, both kinds of ordered arrays diffract t h e r a d i a t i o n i n t o g é o m é t r i e patterns which can be used to infer the placement of the particles or atoms. Examples of some of the diffraction p a t t e r n s obtained from flowing suspension are shown in Figure 1. Patterns like thèse hâve been used to elucidate five basic s t r u c t u r e s .
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