Shear-induced thickening/thinning phenomena of aqueous rodlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were investigated by means of simultaneous measurements of rheology and small-angle neutron scattering (SANS), the so-called Rheo-SANS. The aqueous CTAB/NapTS solutions were classified into five different categories dependent on their flow behavior and micellar structure. By increasing salt concentration and/or shear rates, the micelles underwent morphological transition from (i) spherical or short rodlike micelles to (ii) long rodlike micelles without entanglements, followed by (iii) those with entanglements. These transitions were recognized as changes in flow behavior from Newtonian to shear-thickening and shear-thinning flow, respectively. In the latter two cases, anisotropic SANS patterns appeared around these critical shear rates. The physical meaning of the anisotropic SANS patterns accompanied by shear-thickening flow behavior is discussed in conjunction with other shear-thickening systems.
The microscopic structures of clay-polymer aqueous solutions under shear deformation were investigated by means of small-angle neutron scattering (SANS). Laponite (clay) platelet dispersions in poly(ethylene oxide) (PEO) aqueous solutions with various deuterious/hydrogeneous water compositions were prepared, and contrast-variation SANS experiments were carried out. Reversible shear-thinning behaviors were observed in both clay dispersions and PEO solutions. No anisotropy was observed in SANS patterns even though clay platelets are highly anisotropic, indicating rotation of platelets in a shear flow. In the presence of PEO, on the other hand, a strong anisotropic SANS pattern was observed in the shear flow. This suggests that clay platelets are embedded in a cloud of PEO chains and the cloud is elongated along the shear flow direction by shearing, resulting in a strong clay orientation. Contrast-variation SANS clearly exhibited shear-induced adsorption-desorption of polymer chains on the clay surface.
The structure evolution of network structure in clay-polymer aqueous solutions after shear thickening under shear deformation was investigated by means of small-angle neutron scattering (SANS; Rheo-SANS). Laponite (clay; C) platelet dispersions in poly(ethylene oxide) (PEO; P) aqueous solutions with various deuterious/hydrogeneous water compositions were prepared, and contrast-variation SANS experiments were carried out under flow field. The two-dimensional scattering contour patterns changed suddenly from isotropic to anisotropic patterns at the shear thickening transition point. The orientations of clay platelets and PEO chains were individually characterized by the partial structure factors, S CC (q), S CP (q), and S PP (q), which were obtained by decomposition of the SANS intensity functions. The SANS results show that PEO chains are adsorbed on clay platelets even in a flow field and are peeled off at the shear thickening transition point. Flow birefringence results suggest that the shear thickening is a corporation of clay and polymer chains, i.e., bridging of clays by PEO chains. Aging of the clay-PEO solutions leads to lowering the threshold of the critical shear rate at which shear thickening occurs.
A powdery sample of CaH2 heated to ∼730–790 K in a readily attainable high vacuum (∼1 ×10−7 Torr) was found to emit thermal electrons (∼ 10−7–10−5 A) and H− ions (∼ 10−15–10−12 A, mass analyzed) from its surface (∼ 0.1 cm2). The work function of the sample and the desorption energy of H− were 4.8 ± 0.1 and 7.4 ± 0.2 eV, respectively. The latter fairly agreed with the value (7.1 ± 0.1 eV) derived theoretically by our simple model for the thermal negative ion emission.
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