| 초록 |
Processing of particle dispersion is frequently encountered in many areas such as paints, composite materials, ceramics, electronic industry, polymers and so on. Generally, the flow of particle dispersions shows highly non-Newtonian behaviors even though they are suspended in Newtonian medium. Moreover, the deviation from the Newtonian flow behaviors becomes pronounced at high volume fractions and under strong flow field. When the particle volume fraction is very low, the suspension behaves like a Newtonian fluid as the medium solvent. However, as the solid concentration increases, non-Newtonian behaviors are observed at high shear rates. Semi-dilute or moderately concentrated suspension displays shear thinning and high-shear-rate limiting viscosity. When the particles are highly concentrated, the suspension undergoes shear thickening in a continuous or discontinuous fashion by formation of three dimensional particle clustering. When the imposed flow becomes strong, a semi-dilute or moderately concentrated suspension displays the high shear-rate limiting viscosity through the alignment to flow direction as a form of string or hexagonally ordered layered structure. Meanwhile, shear thickening occurs in concentrated suspensions at high shear rates under which the ordered layered or string structure transforms into a disordered network structure by forming particle cluster. These phenomena were monitored by advanced experimental techniques such as small angle neutron scattering and dichroism. Although many results on the shear thickening have been reported, comprehensive understanding about the transient state of shear thickening still requires additional researches. However, the microstructure evolutions and related rheological behavior of the particle suspension have not been successfully explained yet especially in terms of the effects of stabilization of the particle dispersion, which is of practical significance. This is the primary thrust of the present study. In the present work, the rheological behaviorsand phase stability are investigated for the silica suspensions. The monodisperse silica particles were synthesized successfully through sol-gel method via hydrolysis and condensation of silicon alkoxide. Moreover, Lee and Yang considered the stabilization effects for two different silane coupling agents such as vinyltriethoxy silane and (-methacryloxypropyl triethoxy silane. The results showed that MPTES was more effective than VTES in enhancing the dispersion stability in tetrahydrofurfuryl alcohol, which was used as a refractive-index matching solvent. Thus, in this study, particles of about 100~300nm were used extensively in measuring rheological behavior of particle suspensions and the particle surface was coated silane coupling agent. The van der Waals dispersion forces between the particles suspended in good solvent, for example, cyclohexane or tetrahydrofurfuryl alcohol, were shown to be negligible. This is in agreement with the index of refraction and thus with the polarizability of the particles which is virtually the same as that of many organic solvents. This feature alone opens up many interesting experimental possibilities(index matching, tracer diffusion) but more importantly it is probably a necessary condition for the intrinsic stability of such model colloidal systems which have a very short-range repulsive pair potential. Therefore, the steric barrier effectively screened the van der Waals dispersion forces and the particles behaved like hard spheres. Then, rheological responses of the silica suspensions under simple shear flow were examined as functions of the particle size, volume fraction.
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