Flow curves for the concentrated suspension of kaolinite (20 g/30 ml-medium) were obtained in the presence of polyvinylpyrrolidone (PVP) and sodium dodecyl sulphate (SDS) by means of a Couette-type rotary viscometer. The flow curves were Newtonian or non-Newtonian, depending on the concentration of PVP and SDS added. From these curves, the plastic viscosity (r/vL) and Bingham yield value (F0) were obtained. The relative viscosity (Gel) of r/PL with respect to the medium viscosity (r/0) were calculated and the contour lines for Fo and for 17rel obtained as a function of the concentration of SDS and PVP added. The feature of these contour lines was qualitatively similar to that for the mean diameter (din) of the secondary particles in a dilute suspension of kaolinite. It was concluded that both the increase in drain the dilute suspension and the increase in r/to1 and F 0 in the concentrated suspension were brought about by the interpartide bridging effect of PVP. The increase in r/re1 was due to the formation of bulky flow units of large void volume. The increase in F 0 reflected the growth of gel structure caused by the bridging effect. It was shown that PVP behaved as a flocculating agent at lower concentrations, but behaved as a dispersing agent at higher concentrations, while SDS at any concentration behaved as a dispersing agent for kaolinite suspension.
Laight propagation through a gold film with a periodically structured 50 nm width slit array into Cu 2 O has been calculated with a FDTD (Finite Difference Time Domain) method with a 5 nm mesh-grid. Owing to the presence of a guided mode in the slit and a surface plasmon mode at the gold/Cu 2 O interface, optical near field generation in Cu 2 O are efficient in the case where the polarization of the incident light is perpendicular to the slit. The efficiency of the electric quadrupole (E2) transition of the 1s-exciton in Cu 2 O, which is modified by the field gradient of the excitation light, is evaluated for the calculated optical near field. Slit period dependences of the near field generation and the E2 transition efficiency are examined for a 50 nm thick array film. The enhancement of the effective E2 transition efficiency in the volume of 50 nm thickness above the film relative to the efficiency by the plane wave excitation is found to reach a factor 28 at the slit period of 110 nm. 1 Introduction An optical near field gathers much notice for applications in spatially resolved optical measurement beyond the diffraction limit of light [1]. Owing to the localization of the field at a probe of a scanning near field optical microscope, excitonic wave functions were observed [2]. An optical near field has a characteristic wave vector, which originates with the spatial localization or the large gradient of the field. For example, the wave number of the optical near field accompanied with a small aperture has a distribution, the width of which corresponds to the inverse of the aperture diameter [3]. Since an electric quadrupole (E2) interaction between a light field and matter depends explicitly on the wave vector or the field gradient, it is expected that efficiency of an E2 transition is enhanced in an optical near field. The enhancement of the E2 1s-exciton transition in Cu 2 O was examined for the optical near field at a small aperture [4]. This kind of enhancement was experimentally observed by attenuated total reflection spectroscopy for the (s-d) E2 transition of cesium atoms [5].The 1s-exciton in Cu 2 O is well known as a model system to investigate the Bose-Einstein condensation of excitons [6]. Because of weak absorption of the E2 1s-exciton transition, a high density of excitons in Cu 2 O is usually generated by band-gap excitation through inter-and intra-band relaxations [7]. However, a temperature increase in the lattice and exciton systems often takes place at strong excitations, which may be due to the excess energy emission during the inter-and intra-band relaxations. If an efficient method to directly excite 1s-excitons in Cu 2 O is available, it may be useful for this kind of study.
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