Rod-like nanometric objects such as molecules, polymers, aggregates and particles, either in pure form or dispersed in a solvent, can form liquid-crystalline phases of various symmetries [1]. The transitions between these phases generally occur because of a change of temperature or concentration. Recently, at the ESRF, we discovered a case where the transition between two liquid-crystalline phases can actually be induced by applying a magnetic field. We were investigating the magnetic-field influence on suspensions of goethite (-FeOOH) lath-shaped nanorods of average dimensions 150 x 25 x 10 nm3. In zero-field, these aqueous colloidal suspensions display three different phases, depending on volume fraction. Dilute suspensions form the usual isotropic liquid phase; upon increasing concentration, a transition is reached, leading to a nematic phase that shows long-range orientational order but only short-range (liquid-like) positional order. A further concentration increase leads to a transition to a columnar liquid-crystalline phase that has 2-dimensional long-range positional order. All these phase transitions are first-order (i.e. with phase coexistence).

Fig. 68: a) Small-angle X-ray scattering pattern of a single domain of the nematic phase showing two diffuse spots. The goethite nanorods are aligned in the horizontal direction, perpendicular to the X-ray beam. b) Small-angle X-ray scattering pattern of the same sample submitted to a strong magnetic field (B = 1.5 T), in the columnar phase (The (20) reflections are overexposed). The nanorods are aligned parallel to the X-ray beam.

The small-angle X-ray scattering (SAXS) pattern of a single domain of the nematic phase displays two diffuse spots that arise from interferences between the nanorods in planes perpendicular to their common alignment direction, called the nematic director (Figure 68a). The position of these diffuse spots is related to the average distance (~ 50 nm) between particles in planes perpendicular to the director while the width of the spots (along a radius) is related to the distance over which the short-range positional order extends [2]. Interestingly, when the magnetic-field intensity was raised beyond about 1 Tesla, the SAXS pattern displayed sharp reflections that point to the onset of long-range positional order (Figure 68b). Goethite nanorods then assemble on a 2-dimensional lattice perpendicular to the director, which is typical of the columnar phase. Moreover, instead of a “powder” (i.e. random) distribution of small crystallites, a large single domain grew and gave rise to the pattern in Figure 68b. This large domain has grown in the so-called homeotropic orientation, i.e. with the nanorods main axis parallel to the beam and perpendicular to the surfaces of the flat glass capillary that held the sample. The production of a single domain allowed us to determine the 2-dimensional space group (c2mm) of the columnar phase and to understand its organisation (Figure 69). Due to their magnetic anisotropy, the nanorods orient their main axis perpendicular to the magnetic field and their thickness parallel to it. This orientation was confirmed by optical measurements. The 2-dimensional unit-cell parameters naturally depend on volume fraction, with typical values of a ~ 100 nm and b ~ 70 nm. There are two nanoparticles per unit cell. Since the particles cross-section is not circular, the columnar phase has rectangular rather than hexagonal symmetry. As expected for a liquid-crystalline phase, no periodic order could be detected in the direction perpendicular to the 2-dimensional lattice. The phase transition is actually first-order since SAXS patterns showing both the nematic diffuse spots and the columnar sharp reflections could be observed.

Fig. 69: Schematic representation of the organisation of the nanorods in the columnar phase, left: in the plane perpendicular to the rods main axis, right: in a plane parallel to the rods main axis.

This magnetic-field-induced phase transition is fully reversible (possibly with some field hysteresis) as the sharp reflections disappear and the diffuse spots reappear when the magnetic field is suppressed. Moreover, these experiments could be reproduced many times both with the same and with different goethite samples. The critical field required to achieve the nematic to columnar transition is a decreasing function of the volume fraction. In other words, in a volume fraction - field intensity phase diagram, the transition line is clearly tilted towards lower volume fractions, which is very unusual for liquid crystals. This result means that, quite unexpectedly, the magnetic field affects not only the orientational degrees of freedom but also the translational ones. To this date, we have no complete explanation for this unusual transition and understanding its mechanism in detail remains a tantalising but challenging theoretical question.

 

References

[1] P.G. De Gennes, “The Physics of Liquid Crystals”, Clarendon press, Oxford (1979).
[2] B.J. Lemaire et al., Eur. Phys. J. E, 13, 309 (2004).

Principal Publication and Authors

B.J. Lemaire (a), P. Davidson (a), P. Panine (b), J.P. Jolivet (c), Phys. Rev. Lett., 93, 267801 (2004).
(a) UMR 8502 CNRS, Orsay (France)
(b) ESRF
(c) UMR 7574 CNRS, Paris (France)