Influence of a dispersion of magnetic and nonmagnetic nanoparticles on the magnetic Fredericksz transition of the liquid crystal 5CB

A long time ago, Brochard and de Gennes predicted the possibility of significantly decreasing the critical magnetic field of the Fredericksz transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid crystals and ferromagnetic particles, the so-called ferronematics. This phenomenon is rarely measured to be large, due to soft homeotropic anchoring induced at the nanoparticle surface. Here we present an optical study of the magnetic Fredericksz transition combined with a light scattering study of the classical nematic liquid crystal: the pentylcyanobiphenyl (5CB), doped with 6 nm diameter magnetic and nonmagnetic nanoparticles. Surprisingly, for both nanoparticles, we observe at room temperature a net decrease of the threshold field of the Fredericksz transition at low nanoparticle concentrations, which appears associated with a coating of the nanoparticles by a brush of polydimethylsiloxane copolymer chains inducing planar anchoring of the director on the nanoparticle surface. Moreover, the magnetic Fredericksz threshold exhibits nonmonotonic behavior as a function of the nanoparticle concentration for both types of nanoparticles, first decreasing down to a value from 23% to 31% below that of pure 5CB, then increasing with a further increase of nanoparticle concentration. This is interpreted as an aggregation starting at around 0.02 weight fraction that consumes more isolated nanoparticles than those introduced when the concentration is increased above $c=0.05$ weight fraction (volume fraction $3.5\times {10}^{-2}$). This shows the larger effect of isolated nanoparticles on the threshold with respect to aggregates. From dynamic light scattering measurements we deduced that, if the decrease of the magnetic threshold when the nanoparticle concentration increases is similar for both kinds of nanoparticles, the origin of this decrease is different for magnetic and nonmagnetic nanoparticles. For nonmagnetic nanoparticles, the behavior may be associated with a decrease of the elastic constant due to weak planar anchoring. For magnetic nanoparticles there are non-negligible local magnetic interactions between liquid crystal molecules and magnetic nanoparticles, leading to an increase of the average order parameter. This magnetic interaction thus favors an easier liquid crystal director rotation in the presence of external magnetic field, able to reorient the magnetic moments of the nanoparticles along with the molecules.

Figure 1

TEM images of cerium oxide (a) and iron oxide (c) nanoparticles; size distributions from TEM analysis (b).

Figure 2

(a) Experimental setup: a linearly polarized red laser beam passes through the sample submitted to an in-plane magnetic field and is analyzed by a polarizer and a photodiode; (b) sketch of a sample cell submitted to a magnetic field $H$ showing the competition between homeotropic anchoring on the cell walls and the magnetic alignment force. (c) the laser beam propagate along the polarizer axis ($P$) which is at 45° from the magnetic field direction ($H$), the initial direction of the analyzer ($A$) is perpendicular to ($P$).

Figure 3

Typical signal collected by the photodiode. The red dots correspond to the extrema where the phase lag equals $m\pi /2$, where $m$ is an integer.

Figure 4

Integrated birefringence (represented by the phase lag) vs field for the pure 5CB and 0.05wt. fraction doped 5CB with nonmagnetic nanoparticles (5CBNP) and magnetic nanoparticles (5CBNPM).

Figure 5

Evolution of the magnetic threshold field as a function of the nanoparticle weight concentration for nonmagnetic and magnetic nanoparticles.

Figure 6

(a) Experimental setup of the dynamic light scattering (DLS). (b) The scattering geometry, where the wave vector $k_s$ of the scattered light is quasisymmetric to the wave vector $k_i$ of the incident light with respect to the optical axis (exactly symmetric in air), with an ordinary polarization for the incident light and an extraordinary polarization for the scattered light.

Figure 7

Evolution of the diffusion coefficient as a function of the nanoparticle weight concentration for nonmagnetic and magnetic nanoparticles.

Figure 8

Optical microscopy picture between crossed polarizer of a composite film 5CB-${\text{Fe}}_2{\text{O}}_3$ with (a) 0.0115 weight fraction and (b) 0.023 weight fraction.

Figure 9

Scheme of the expected liquid crystal geometry around (a) iron oxide ($\gamma {\text{Fe}}_2{\text{O}}_3$) nanoparticles with a magnetic moment inducing a planar anchoring with a director orientation parallel to the magnetic moment of the nanoparticle in its vicinity. This is possible due to the spherical shape of the nanoparticle, here represented in side view; (b) cerium oxide (${\text{CeO}}_2$) nanoparticles without magnetic moment and with planar anchoring of the director. We expect formation of defects roughly indicated by the red points.

Figure 10

Evolution of the magnetic threshold field as a function of the nanoparticle volume fraction, $f$, for magnetic nanoparticles. The dashed lines represent the best fit of the function $H_c^2=H_{co}^2-A\times f$ in the range of volume fraction $f$ belonging to [0; 0.01].