Formation of disclination lines near a free nematic interface

We have studied the nucleation and the physical properties of a -1/2 wedge disclination line near the free surface of a confined nematic liquid crystal. The position of the disclination line has been related to the material parameters (elastic constants, anchoring energy and favored anchoring angle of the molecules at the free surface). The use of a planar model for the structure of the director field (whose predictions have been contrasted to those of a fully three-dimensional model) has allowed us to relate the experimentally observed position of the disclination line to the relevant properties of the liquid crystals. In particular, we have been able to observe the collapse of the disclination line due to a temperature-induced anchoring angle transition, which has allowed us to rule out the presence of a real disclination line near the nematic/isotropic front in directional growth experiments. 61.30.Jf,61.30.G

Finite-size effects in smectic A droplets: steps, edge dislocations, and angular matching

Small droplets of smectic A liquid crystal have a single facet parallel to the substrate, while larger droplets do not. By considering the elastic energy of various smectic layer configurations, we show that there is a critical droplet volume below which layers are parallel, with facets and steps on the free surface. For intermediate droplet sizes, there can be angular matching between the facet and the macroscopically curved part of the droplet surface. For larger droplets, the top smectic layer bends parallel to the free surface, and there are edge dislocations in the bulk. Estimates of the critical droplet volume are close to the observed value. The theory illustrates how in a liquid crystal, surface effects (steps) may not so neatly be separated from volume effects (edge dislocations) as they can be in ordinary crystals. Thus the crystal-shape problem for a liquid crystal has qualitative differences from that of an ordinary crystal.Les petites gouttelettes de smectique A ont une facette parallèle au substrat contrairement aux grosses gouttelettes. En considérant l'énergie élastique des diverses configurations en couches possible, nous montrons qu'il existe un volume critique de la goutellette en dessous duquel les couches sont parallèlles au substrat, avec une facette et des marches sur la surface libre. Pour des gouttelettes de taille intermédiaire, il peut y avoir un raccordement anguleux entre la facette et la partie courbée de la goutte. Pour des gouttelettes plus grosses, la couche smectique du dessus se courbe parallèment à la surface libre et des dislocations coin apparaissent à l'intérieur de la goutte. Nos estimations du volume critique sont proches des valeurs observées. La théorie illustre pourquoi, dans un cristal liquide, les effets de surface (marches) ne peuvent pas être nettement séparés des effets de volume (dislocations) comme c'est le cas dans les cristaux ordinaires. Ainsi, le problème de la forme d'équilibre d'un cristal est qualitativement différent dans un cristal liquide et dans un cristal ordinaire

Facets of smectic A droplets I. Shape measurements

We have measured profiles of smectic A droplets in air as a function of temperature for several different materials. When small droplets are cooled below the nematic-smectic A transition temperature $T_{\rm AN}$ , they show a single facet whose radius is proportional to $(T_{\rm AN}-T)^\alpha$. The exponent $\alpha$ differs for different materials but is consistent with that measured for the layer compression modulus $B$. In addition, we measure the shape of curved regions of the surface adjacent to the facet. A power-law fit gives an exponent that varies with both temperature and material and in any case is different from the universal value of 3/2. We also study how droplet shapes relax to equilibrium and find that while the relaxation time for shape changes upon cooling is less than one minute, that for heating ranges from hours to days, depending on $T_{\rm AN}$. An estimate of the energy barrier to nucleating now layers suggests that process is forbidden and that another explanation of the relaxation-rate asymmetry must be found.Nous avons mesuré des profils de gouttelettes smectique A en fonction de la température pour des matériaux diffŕents. Quand de petites gouttelettes sont refroidies en dessous de la température de transition smectique A-nématique $T_{\rm AN}$, une facette unique dont le rayon est proportionnal à $(T_{\rm AN}-T)^\alpha$ apparaît. L'exposant $\alpha$ est différent suivant les matériaux mais est compatible avec celui qui est mesuré pour le module de compression des couches B. De plus, nous avons mesuré la forme des régions courbées adjacentes à la facette. Un ajustement avec une loi puissance donne un exposant qui varie avec la température et le matériau et qui, dans tous les cas, est différent de la valeur universelle 3/2. Nous avons aussi étudié comment la forme des gouttelettes relaxe vers l'équilibre et avons trouvé que le temps de relaxation est inférieur à une minute en refroidissant tandis qu'il varie entre quelques heures et quelques jours en chauffant, suivant la valeur de $(T_{\rm AN}-T)$. Une estimation de la barrière d'énergie nécessaire pour nucléer de nouvelles couches suggère que ce processus est interdit et qu'il faut chercher une autre explication à l'asymétrie du taux de relaxation

Formation of disclinations line near a free nematic interface

We have studied the nucleation and the physical properties of a -1/2 wedge disclination line near the free surface of a confined nematic liquid crystal. The position of the disclination line has been related to the material parameters (elastic constants, anchoring energy, and favored anchoring angle of the molecules at the free surface). The use of a planar model for the structure of the director field (whose predictions have been contrasted to those of a fully three-dimensional model) has allowed us to relate the experimentally observed position of the disclination line to the relevant properties of the liquid crystals. In particular, we have been able to observe the collapse of the disclination line due to a temperature-induced anchoring-angle transition, which has allowed us to rule out the presence of a real disclination line near the nematic/isotropic front in directional growth experiments

Alloy hardening of a smectic A liquid crystal doped with gold nanoparticles

International audienceWe measure the mobility of edge dislocations in a smectic A liquid crystal doped with gold nanoparticles. The mobility is found to decrease when the concentration of nanoparticles increases, which leads to a hardening of the smectic phase. It is found that the closer the temperature to the nematic phase, the weaker the hardening. The critical behavior of the mobility near the smectic A-to-nematic transition temperature is also investigated and compared to theoretical predictions