Gazeau F, Dubois E, Bacri J C, Boué F, Cebers A, Perzynski R
Laboratoire des Milieux Désordonnés et Hétérogènes, UPMC Case 78, 4, Place Jussieu, 75252 Paris Cedex 05, France.
Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Mar;65(3 Pt 1):031403. doi: 10.1103/PhysRevE.65.031403. Epub 2002 Feb 15.
Small-angle neutron scattering is used to measure the two-dimensional diffraction pattern of a monophasic magnetic colloid, under an applied magnetic field. This dipolar system presents in zero field a fluidlike structure. It is well characterized by an interaction parameter K(0)(T) proportional to the second virial coefficient, which is here positive, expressing a repulsion of characteristic length kappa-10. Under the field a strong anisotropy is observed at the lowest q vectors. The length kappa-10 remains isotropic, but the interaction parameter K(T) becomes anisotropic due to the long-range dipolar interaction. However, the system remains stable, the interaction being repulsive in all directions. Thus we do not observe any chaining of the nanoparticles under magnetic field. On the contrary, the revealed structure of our anisotropic colloid is a lowering of the concentration fluctuations along the field while the fluidlike structure, observed without field, is roughly preserved perpendicularly to the field. It expresses a strong anisotropy of the Brownian motion of the nanoparticles in the solution under applied field.
小角中子散射用于测量单相磁性胶体在施加磁场下的二维衍射图案。这个偶极系统在零场时呈现出类似流体的结构。它可以通过与第二维里系数成正比的相互作用参数K(0)(T)很好地表征，这里该参数为正，表示特征长度κ - 10的排斥作用。在磁场下，在最低的q矢量处观察到强烈的各向异性。长度κ - 10保持各向同性，但由于长程偶极相互作用，相互作用参数K(T)变得各向异性。然而，系统仍然稳定，相互作用在所有方向上都是排斥的。因此，我们在磁场下没有观察到纳米颗粒的任何链化现象。相反，我们的各向异性胶体所揭示的结构是沿磁场方向浓度涨落的降低，而在无场时观察到的类似流体的结构在垂直于磁场方向上大致保持。这表明在施加磁场的溶液中纳米颗粒的布朗运动具有强烈的各向异性。
