The influence of particle shape (aspect ratio) on the intrinsic viscosity is investigated, taking three Czech kaolin products (floated kaolins) as paradigmatic examples. An average aspect ratio is obtained for each kaolin from a comparison of particle size measurements using sedimentation and laser diffraction. The intrinsic viscosity is obtained by a multistep procedure: firstly, flow curves are recorded for each kaolin with the optimum deflocculant concentration, secondly, the (apparent) relative viscosities read off from the flow curves are plotted against the kaolin volume fraction and, thirdly, these data are fitted using the Krieger relation to obtain the intrinsic viscosity in the asymptotic dilute limit. It is shown that the data determined with the method proposed are within the Jeffery and Brenner bounds and that an average aspect ratio of about 20 (17-22) results in an intrinsic viscosity of about 10 (7-13), compared to 2.5 for spherical particles. Although currently th e measurement precision is not suffi cient to seriously assess the influence of Brownian motion, the method can principally be used to predict the intrinsic viscosity when the average aspect ratio of the system (and its particle size distribution) is known, and vice versa., Eva Gregorová, Willi Pabst and Jean-Baptiste Bouchet., and Obsahuje bibliografické odkazy
The prehistory of clay mineralogy is highlighted from the beginnings in ancient Greece to the mineralogical works of Agricola, in particular his famous handbook of mineralogy, entitled De natura fossilium (1546). Starting with a few scattered hints in the works of Archaic and Classic Greek authors, including Aristotle, the first treatment of clays as a part of mineralogy is by Theophrastus. This basic tradition was further supplemented by Roman agricultural writers (Cato, Columella), Hellenistic authors (the ge ographer Strabo and the physicians Diosco rides and Galen), the Roman engineer-architect Vitruvius, and finally summarized in Pliny’s encyclopedia Naturalis historia, which has become the main source for later authors, including Agricola. It is shown to what extent Agricola’s work is just a great summary of this traditional knowledge and to what extent Agricola’s work must be considered as original. In pa rticular, Agricola’s attempt to a rational, combinatorical classification of "earths" is recalled, and aplausible explanation is given for his effort to include additional information on Central European clay depos its and argillaceous raw material occurre nces. However, it is shown that - in contrast to common belief - Agricola was not the first to include "earths" in a mineralogical system. This had been done almost one thousand years earlier by Isidore of Seville., Willi Pabst and Renata Kořánová., and Obsahuje bibliografické odkazy