Shear Indxuced order of Cncenirated Dispersions
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SHEAR INDXUCED CRDER OF CNCENIRATED DISPERSIONS
=CE J. ACKRSC1 AND T. A. MMRRIS Department of Physics, Oklahoma State University, Stillwater, OK 74078
ABSACT The microstructure or interparticle ordering in concentrated dispersions of colloidal R4MA (polymethylnethacrylate) particles in a mixture of tetralin and decalin have been monitored using light scattering techniques. These sterically stabilized, uniformly sized, nearly hard, colloidal spheres are observed to exhibit an equilibrium phase transition frcm a liquid-like ordering to a crystal-like ordering of suspended particles as the volume fraction of solids increases. The crystals have a close packed - random stacked structure. At the largest volume fractions a ncnequilibrium glassy phase results. Samples at different volume fractions are subjected to steady and oscillatory shear flow. Four basic structures are observed to exist: liquid or distorted liquid-like, string-like, sliding or randomly stacked layers, and face centered cubic (FCC) structures. Oscillatory shear studies will be reported here and are made as a function of strain amplitude and shear history, in addition to volume fraction. Generally, oscillatory shear is effective in ordering samples. For example, an unstable FC ordering can be induced in an equilibrium liquid-like sample.
INTRODUCTIOc The microstructure of particles in a packed powder is an important factor in determining the strength of a ceramic body. Defects in an otherwise uniform microstructure concentrate stress and lead to mechanical failure. Therefore, a large amount of work has been directed at understanding and cmnrolling microstructure. Many years ago Bernal and Scott investigated the microstructural properties of an ideal "powder" made from uniformly sized ball bearings. It was found in repeated experiments that, when ball bearings are poured iqto a container, the volume fraction occupied by the spheres is q%= 0.637 . This is less than the maximum volume fraction of 0.74 when these hard spheres are arranged in a cubic close packed crystalline structure (either face centered cubic FC or hexagonal close packed HCP'). Investigation of the ball bearing microstructure revealed a random arrangemn of the spheres Vimilar to that found for the atomic arreuq nt in pure liquids and glasses . Thus this structure is now referred to as the Bernal close packed glass or Bernal glass. These Bernal glasses have been subjected to oscillatory shear deformations in rectangular containers having hinged corners 4 . When the shear deformation is near unity [wall motion parallel to itself equal to the wall separation], the volume fraction of spheres was observed to increase to 0.66. For smaller or larger shear deformations the volume fraction remained near the Bernal glass value. Investigation of the microstructure of sheared glagses revealed local islands of cubic close packed crystalline structures'. Evidently, the shear oscillations are most effective at moving the spheres into a more close packed structure at a shear deformation of unity. Smal
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