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When subjected to a compressive strain along the columns, a tubular discotic liquid crystal must present a buckling instability, where compressive elastic energy is transferred into bend energy of columns. We present the first observation of this column buckling instability in hexa-rt-octyloxytriphenylene at 7= 75 °C. From the instability threshold, the curvature elastic constant is found to be 6 orders of magnitude larger than in usual liquid crystals, i.e., the correlation length extends on a quasimacroscopic range of 10 3 molecules.PACS numbers: 61.30.-v, 62.10.+s A number of new liquid crystalline mesophases, consisting of disklike molecules, have been synthesized during recent years. 1 Their structure is now well understood 2 : These phases are formed by a regular packing of parallel and independent columns of molecules and the columns are arranged in a two-dimensional network. Their mechanical properties were predicted from their symmetry 3,4 : For instance, longitudinal compression of the columns is that of a one-dimensional solid, but with a thickness-dependent elastic constant, because of the coupling with the two-dimensional network when the thickness is larger than a critical value. 5 As in other mesomorphic smectic materials, observation of mechanical instabilities is a way to obtain the elastic properties. A column buckling instability under compression has indeed been predicted 4 : It transforms a part of the compression energy into bend energy and is similar to the undulation of smectic liquid-crystal layers under dilation. 6 In the present work, we present the first observation of this column buckling instability in a discotic liquid crystal. We derive from the deformation threshold the ratio of the bend to compressive elastic moduli of the material.The sample we use is hexa-fl-octyloxytriphenylene 7 (HET) which exhibits a columnar discotic mesophase between 67 and 886 °C; the columns are arranged in a hexagonal network and the disklike molecules are "ordered" within each column. The sample is oriented with the columns perpendicular to the surface between two glass plates treated with mellitic acid as a surfactant. Because of the absence of a nematic phase, a good orientation in a large sample is difficult to obtain; the best result is obtained when cooling the sample very slowly (0.02 °C per min) from the isotropic to the discotic phase; we generally obtain a sample with grains, about 1 mm in size, some of them slightly tilted from the perfect homeotropic direction. The glass plates are cemented to piezoelectric ceramics in a holder previously described. 8 The piezoelectric ceramics allow one to create an imposed displacement 8 at one of the glass plates and to measure the transmitted force at the other plate through the sample of thickness d. The applied compressive strain e = 8/rfhas a step-function form with a displacement amplitude between 0.4 and 100 A and a typical duration of 10 sec. The applied voltage on one ceramic and the output voltage due to the transmitted stress on the second ceram...

We present the first observation of an undulation instability of the columns in a tubular discotic liquid crystal when submitted to a dilation normal to the columns. This effect is very similar to the layer undulation in smectic materials. From the instability threshold and from the undulation wavelength, we derive a value, anomalously large, of the column curvature elastic constant, very close to the value obtained from observation of the column buckling under longitudinal compression. The static or dynamic origin of such a large curvature elastic constant is not understood