Localización y características

In this chapter we describe the alignment kinetics of an assembly of non- interacting magnetic nanorods suspended in different fluid.

We first studied an assembly of magnetic nanorods suspended in a Newtonian fluid with constant viscosity  and subject to an external magnetic field B. It has been shown that the alignment kinetics is controlled by a single parameter β. We theoretically predicted and experimentally confirmed that one can control the alignment of an assembly of nanorods by choosing the parameter β and time of application of the external field. Experiments with nickel nanorods covered with PVP in a glycerol–water mixture supported the theory.

Next, we describe we theoretically studied the kinetics of ordering of an assembly of nanorods suspended in a solidifying liquid film. The process of solidification was

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modeled by assuming that the film viscosity increases with time exponentially fast and nanorods are randomly distributed in the films prior to the application of external magnetic field. We showed that the nanorods would not always align parallel to the external magnetic field; some of them will be quenched halfway to the equilibrium orientation prior to the film solidification. Different regimes of ordering were revealed and classified and the time required for the nanorods to align along the field direction was analyzed depending on the physical parameters of the liquids and nanorod materials. Fortunately, all physical parameters were collapsed in a single dimensionless parameter

U0 defined by eq.(4.15). This parameter controls the alignment kinetics and limiting distribution of nanorods in the film and resulting magnetization of the film. We introduced a criterion of the nanorod ordering and constructed phase diagrams shown in Figure 4.16. These phase diagrams predict the physical conditions ensuring the complete order of nanorods in the film providing maximum possible magnetization.

4.6 References

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CHAPTER V

5

VISCOSITY CHANGE OF MULLITE PRECURSOR DURING

EVAPORATION

As shown in the previous chapter, the alignment of magnetic nanorods in a solidifying film significantly depends on the fluid viscosity. In the composite processing when the solvent evaporates, viscosity changes significantly. Therefore, in order to control the nanorod alignment during solvent evaporation it is crucial to understand the effect of evaporation on viscosity. In this chapter, we will first study the evaporation kinetics of the mullite precursor droplet and then use the magnetic rotational spectroscopy (MRS) to measure the time-dependent viscosity. A correlation between the viscosity and concentration of mullite in the precursor drop was analized in detail and the Eyiring model was employed to explain the observed dependency.