Theory of Brownian motion of a particle in a viscoelastic Jeffreys fluid is extended for the case of rotational motion. The employed rheological model combines two viscous mechanisms (instantaneous and retarded) and, in contrast to the Maxwell model, does not produce artifacts and works robustly when applied to diffusion of tracer particles in real complex fluids. With the aid of it, specific features of the dynamic susceptibility of a magnetic Jeffreys suspension and the viscous power losses induced by an ac field are analyzed deriving the conclusions valid for active microrheology and magnetic hyperthermia. In general aspect, it is shown that the developed phenomenology provides an archetypal "frame" for a number of mesoscopic models used to describe confined random walk transport processes in a variety of systems of both biological and inorganic origin.

2014 Les coefficients du développement de l'énergie libre de Landau-de Gennes des polymères nématiques sont évalués dans le cadre d'un modèle moléculaire statistique. Nous montrons que le développement obtenu foumit une description qualitative fiable de la transition de phase vers l'état nématique pour des polymères de rigidité et de longueur moléculaire arbitraires. Cette transition est toujours du premier ordre. Le saut du paramètre d'ordre ne dépend que faiblement de la longueur et de la rigidité des molécules. Abstract. 2014 The Landau-de Gennes free energy expansion coefficients for nematic polymers are evaluated on the basis of a molecular-statistical model. It is proved that the obtained expansion provides a reliable qualitative description of the phase transition to the nematic state for polymers with arbitrary molecular length and rigidity. It is demonstrated that this transition is always of the first-order type and the jump of the order parameter is weakly dependent on the molecular length and rigidity.

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A phenomenological model is proposed to describe the plasticity of soft magnetic elastomer composites under an external field. Along with high elasticity, internal dry friction is introduced whose origin is attributed to the dipole-dipole interaction of the embedded particles. Numerical estimates of the model parameters are obtained from comparison with the measurements performed on iron carbonyl dispersions in soft silicon-rubber matrices.