Measurement of low yield-stress materials

A special method has been devised to measure yield stress of biosolids material or cake. A material with yield stress is placed in a container initially resting on its base. By slowly rotating the container 90 degrees incrementally under "quasi-equilibrium", the profile of the material is reformed in which the height is deeper on one end and shallower on the opposite end. To measure material with low yield stress, an immiscible lighter liquid is introduced with the material fully immersed in the pool of the lighter liquid and the above procedure is repeated. Also the dimension of the side of the container should be much greater that of the base. Despite the yield stress of the material is small the interface profile between the lighter liquid and the material can be established under reduced weight due to buoyancy force of the lighter liquid. The measured material profile is compared with the theoretical prediction based on a model assuming the material establishes a hydrostatic equilibrium in its final position with the container resting on its side. The effective density used in the test is the density difference between the lighter liquid and the material, which can be made small, catered for measuring material with very low yield stress. The new method was used to measure xanthan gum with low yield stress with magnitude less than 1 Pa. The cross sectional shape of the container affects the measurement and the subsequent interpretation. Both rectangular and cylindrical geometries have been investigated and both give comparable yield stress for the xanthan gum being tested. The proposed method is attractive for providing reasonable and accurate measurement despite its simplicity. (c) 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.