Free growth of a discotic liquid crystal

Collaboration with Patrick Oswald (PhD advisor), John Bechhoefer, Pierre Pelcé, and Dmitrii Temkin.

 

Summary

    We studied the free growth of a discotic liquid crystal (hexaoctyloxytriphenylene) in thin samples and homeotropic anchoring. An efficient quenching of the sample and a new three-dimensional vizualization mehod of the germs (Fig.1) make possible the quantitative study of both the dendritic and cellular (up to absolute restabilization of the growth front) regimes.

Fig.1: 3D-reconstruction of a growing germ (from figure 2, left).

    In the first regime, for supersaturations lower than 0.7, there are two types of dendrites, depending on the ratio R of the impurities diffusion-length to the sample thickness. For R larger than 50, the stationary dendrites are two-dimensional, and their characteristics (Peclet number and stability constant) are in agreement with the 2D-theory (Fig.2, left). For R less than 10, the dendrites are three-dimensional, as a layer of liquid appears between them and the limiting glass plates (Fig.2, right). In this case, the stability constant decreases and tends to a well-defined value when R is less than 1. Nevertheless, the Peclet number never obeys to the three-dimensional Ivantsov's law. That proves that they are confined. For supersaturations between 0.7 and 1, the density of the side-branches increases, the dendritic solution vanishes, and a dense-branching regime results.

Fig.2: 2D (left) and 3D (right) dendrites.

In the kinetic regime, for supersaturation larger than 1, the growth front is first cellular (Fig.3, left) and then restabilizes (Fig.3, right) when the supersaturation passes 4. We have measured the molecular attachment kinetic coefficient and its anisotropy. We found that the easy growth axes at low and large velocities make a 30 degrees-angle between them.

Fig.3: Unstable (left) and stable (right) growth fronts in the kinetic regime.
The pictures show the superimposition of 4 snapshots taken with a constant time interval.

Finally, we present an additional study of the front kinetics in directional growth. In particular, we have observed evidence of an anchoring transition at large velocities which leads to a non-linear kinetic law when the molecular columns are perpendicular to the growth front (Fig.4).

Fig.4: Snapshot of the growth front  below (b) and above (c) the anchoring transition.
The crystal [underneath the front (line)] growths to the top of the pictures.

Related publications

Croissance libre de la phase colonnaire hexagonale d'un cristal liquide discotique,
J.C. Géminard, PhD thesis, Universi Claude Bernard - Lyon I (1993).
(Download the PDF, in French)

On wetting and apparently asymmetric kinetics in free growth,
J.C. Géminard, J. Bechhoefer and P. Oswald, J. Cryst. Growth 114 (1991) 640-646.

Statistical approach for radial fingering in a Hele-Shaw cell,
J.C. Géminard and P. Pelcé, J. Phys. II France 2 (1992) 1931-1940.

Rapid solidification of a columnar hexagonal liquid crystal,
J.C. Géminard, P. Oswald, D. Temkin and J. Malthęte, Europhys. Lett. 22 (1993) 69-75.

An improved directional growth apparatus for liquid crystals:
applications to thermotropic and lyotropic systems,

P. Oswald, M. Moulin, P. Metz, J.C. Géminard, P. Sotta and L. Sallen, J. Phys. III France 3 (1993) 1891-1907.

Kinetic dendrite in the boundary-layer model,
D. Temkin, J.C. Géminard and P. Oswald, J. Phys. I France 4 (1994) 403-409.

Three-dimensional vizualization and physical properties of dendrites
in thin samples of a hexagonal columnar liquid crystal,

J.C. Géminard and P. Oswald, J. Phys. II France, 4 (1994) 959-974.

Equilibrium shape of an anisotropic crystal confined between two planar surfaces,
J. C. Géminard and P. Oswald, Phys. Rev. E., 55 (1997) 4442.