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

Abstract

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 has rarely been measured, usually 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, 5CB, doped with 6 nm diameter magnetic and non-magnetic 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 non-monotonic behaviour 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 × 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 non-magnetic nanoparticles. For non-magnetic 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.

Directed assembly of magnetic and semiconducting nanoparticles with tunable and synergistic functionality

“…Following the elemental signatures, we see that the MNPs are present everywhere within the assembly, whereas the QDs are more densely packed closer to the center. It has been observed in prior works 43–46 that MNPs tend to aggregate at ~0.05 wt.% unless their surfaces are modified with mesogenic ligands. The highest fraction of MNPs we use is 0.04 wt.%, below this threshold, which may explain why the MNPs are not prone to as much aggregation as the QDs.

Figure 1Transmission electron microscopy (TEM) images of ( A ) drop cast CdSe/ZnS QD layer, ( B ) drop cast Fe 3 O 4 MNP layer and ( C,D ) QD-MNP dispersion in nematic liquid crystal (LC).

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Impact of Dispersion of Nanoscale Particles on the Properties of Nematic Liquid Crystals

“…In recent years, various kinds of nanoparticles [2][3][4][5][6][7][8][9][10][11] or quantum dots [12][13][14][15][16][17] have been dispersed/doped in the host nematic matrix and their properties have been investigated. Singh et al [2] found that due to dispersion of silver nanoparticles (Ag NPs) (0.2 and 0.5 wt% concentrations) into nematic matrix 4-(trans-4-n-hexylcyclohexyl) isothio-cyanatobenzoate (6CHBT) the nematic-isotropic (NI) transition temperature (T NI ) and the conductivity anisotropy have increased whereas the threshold voltage (V th ) has decreased.…”

“…They have found that the dispersion has led to increased values of birefringence in all the three dispersed systems which depend on the NPs concentration, temperature range, and the chemical nature of the mixtures. We noticed in a few other works [8][9][10][11] that the various properties of LC systems have been changed by the dispersion of NPs in LC matrix. Similarly, it has also been observed that due to the dispersion of specific quantum dots into nematic LCs the alignment of molecules, switching behavior, and dielectric and electro-optic parameters are influenced significantly showing the possibility of use of dispersed systems in nematic LC based devices [12][13][14][15][16][17].…”

Interactions Between Topological Defects and Nanoparticles

“…Composites made of liquid crystals (LCs) and nanoparticles (NPs) are studied a lot nowadays [1][2][3][4][5]. One idea is to allow for controlled modification of the LC properties, which can be photonic properties but also elasticity, conductivity, magnetic properties or phase transition of LC [6][7][8][9][10]. The other idea is to take advantage of the anisotropy of the LC matrix or of its easy activation under external parameters (temperature, electric field) to build original anisotropic NP organizations [11,12] or/and activable NP organizations [13].…”