Structure of strongly dipolar fluids at low densities

Abstract

We propose a simple statistical mechanical theory for a strongly dipolar fluid at low densities, based on the analogy between a polymer chain and a chain formed by strongly polar particles. The general methods developed in the theory of semiflexible polymers enable one to obtain simple expressions for the energy and conformational entropy of a long dipole chain. We then consider the equilibrium between chains of different lengths and derive a general expression for the free energy as a functional of the chain length distribution. Both steric and dipolar interactions between long chains are shown to be weak and as a result the rarefied fluid of strongly dipolar spheres resembles the ideal gas of noninteracting polydisperse chains. It is shown that the chain length distributions found in simulations are compatible with the assumption of very weak interchain interactions if strong finite-size effects are taken into account. We also investigate whether sufficiently strong attractive van der Waals forces between particles can cause dissociation of the chains. Finally, we discuss the case of a dipolar fluid in an applied field and argue that the coexistence between two aligned phases of chains, as observed by computer simulation, is unlikely to occur in an infinite system.

Magnetic Properties of Colloidal Suspensions of Interacting Magnetic Particles

“…For both dipolar soft spheres and Stockmayer particles they show that λ c is getting smaller in applied fields whereas the value φ c ≈ 0.017 remains mainly unchanged. In [173] however it is argued that this observed phase transition for soft spheres may be just an artifact of the insufficient system size.…”

“…The most detailed model was proposed by Osipov, Teixeira, and Telo da Gama [173,174] who also take into account chain-chain interaction. They compare with results from [161] and [162].…”

Effect of nanoparticle chain formation on dielectric anisotropy of nematic composites

“…It has been shown [13] that the equilibrium chain length strongly depends on the contact interaction potential normalized by the temperature. Long chains of NPs may occur only if the contact interaction is of the order of 10k B T [15] which is satisfied, for example, for ferroelectric NPs [9,13]. Such long polar chains should make a significant contribution to the dielectric anisotropy of nematic composites.…”

Microstructure and magnetic properties of magnetic fluids consisting of shifted dipole particles under the influence of an external magnetic field

“…The investigation of ferrofluid magnetic properties started more than 50 years ago, and comprises an extreme variety of experimental, 2-10 theoretic, [11][12][13][14][15][16][17][18][19][20][21][22][23] and computer simulations [24][25][26][27][28][29][30][31][32][33][34][35] studies. The findings of these studies can be summarised as follows: (1) there are various mechanism with which the magnetic particle might respond to an external magnetic field, namely, Neél (the dipole rotate within the particle to coalign with an applied field, inherent to smaller particles, grows with particle size and depends on the particle material) and Brownian (the particle rotates as a whole to adjust its dipole moment to the direction of the field, a) weeber@icp.uni-stuttgart.de b) marco.klinkigt@me.com c) sofia.kantorovich@univie.ac.at d) holm@icp.uni-stuttgart.de inherent to bigger particles, grows with the volume of the particle and carrier viscosity); (2) the characteristic magnetic relaxation time of the ferrofluid depends on the magnetic particle size distribution; (3) there is no ferrofluid which exhibits an initial susceptibility lower than the one of the Langevin law; 36 (4) the stronger the inter-particle interaction is, the higher is the magnetic susceptibility of the system; (5) an external magnetic field strongly enhances the chain formation, and for higher fields even magnetic fluids with moderately interacting particles become strongly aggregated.…”