VOLUME 77, NUMBER 10 PHYSICAL REVIEW LETTERS 2SEPTEMBER 1996 Wetting and Anchoring of a Nematic Liquid Crystal on a Rough Surface M. P. Valignat, 1 S. Villette, 1 J. Li, 2 R. Barberi, 2 R. Bartolino, 1,2 E. Dubois-Violette, 3 and A. M. Cazabat 1 1 Physique de la matière condensée, Collège de France, 11 place Marcelin Berthelot, 75231 Paris, France 2 Istituto Nazionale di Fisica della Materia, Unità de Cosenza, co Dip. di Fisica, UNICAL, 87036 Rende (CS), Italy 3 Laboratoire de physique des solides, Bâtiment 510, Université de Paris-sud, 91405 Orsay, France (Received 21 December 1995) Ellipsometric measurements of a nematic drop spreading on a rough surface are reported. The thickness profile of the drop shows two typical lengths belonging to the microscopic and to the mesoscopic scales (a few tens and some hundreds of Å, respectively). No intermediate thicknesses are observed during the spreading. A simple model involving the balance among elastic, anchoring, and spreading energies is used to interpret the results. [S0031-9007(96)00975-1] PACS numbers: 61.30. – v, 64.70. – Md, 68.45.Gd Nematic liquid crystals (NLC) are uniaxial liquids. In the bulk, molecules tend to be parallel. The director n (unitary vector) gives an average orientation of the molecules. Without external strains n is degenerated in space while if the NLC is in contact with a solid surface the degeneracy breaks down. This distortion of the director orientation gives rise to an elastic torque [1]. Surfaces are usually treated in order to define specific orientation; n perpendicular to the surface is called homeotropic orientation u  0, and n in the surface plane is called planar orientation u  p 2. Any orientation can be achieved [2]: for example, a clean flat glass leads to planar or homeotropic alignments, depending on the NLC nature; oblique evaporation of SiO x gives planar, tilted, or bistable states, depending on the parameters of the evaporation (angle, SiO x thickness, evaporation rate [3]). Those orientations are defined for “macroscopic” thick film (at least few mm), but do not give any information on the tilting of the molecules close to the surface. Very few wetting studies have been related to the structure of LC at the solid interface either in the case of smectics [4] or in the case of nematics [5,6]. In this paper we report microscopic studies of 5CB 4-n-pentyl-4 0 -cyanobiphenyl spreading over a controlled surface of amorphous silicon wafers coated with SiO x evaporation. Samples of 5CB were purchased from BDH Ltd., the purity of this compound announced by the furnisher is better than 99%. All the results are presented at ambient temperature close to 23 ± C at which the 5CB is in a nematic phase. The 5CB molecule has a molecular dipole associated to the cyano group lying roughly parallel to the molecular long axis. In bulk, it is well known that molecules form a quadrupole pair with their polar head facing each other. The apparent length of this bilayer is estimated to be 25 Å [7] while the molecule length is 18.7 Å. We also used other compounds of the same nCB series containing n methylene/methyl groups n  5 11. Silicon substrates (type N , dopant PH, orientation 111 purchased from Siltronix) are coated with silicon monoxide ground evaporated at the rate of 4 Ås in a vacuum of 10 26 Torr for times ranging from 2 to 60 s. Oblique evaporation (0 ± to 75 ± from the surface normal) leads to anisotropic surfaces where roughness can be controlled. Those surfaces are well known to induce strong molecular alignment, but the mechanism is not fully understood. All substrates are kept at room temperature in N 2 atmosphere before use. A very small drop of 5CB V  10 27 mm 3  is de- posited on the solid surface, and the thickness profiles of the drop are recorded by spatially resolved 30 3 300 mm 2  ellipsometry at successive times. The setup has already been described elsewhere [8], and we shall emphasize a few points. The ellipsometer is phase modu- lated l  6328 Å, working at the Brewster angle with a typical time constant of 20 mspoint. We took as the optical index for the 5CB an average bulk value of 1.6, which is also the value assumed for the SiO x layer. Each measurement gives the thickness of the layer on top of the silicon (complex optical index n  3.88 2 0.019j ; the profile of the drop is deduced by subtracting the base line. Notice that this apparatus is very sensitive to changes in thickness (0.2 Å). The theoretical model used to extract the thickness from the ellipsometric parameters is very simple (one layer sandwiched between two semi-infinite media of known optical index) and does not take into ac- count either the anisotropy of the liquid or the roughness of the substrate; this device gives good accuracy in spe- cific details appearing in the shape of the NLC film (one has to take care of the enormous distortion in scales on the figures), while the error in the absolute measurement of thickness is estimated to 15%. As the lateral resolution is larger than the typical wavelength of the roughness, it is completely smoothed out and cannot be seen on the profile [9]. In this paper, only the static shape of the drop is studied and no reports on the dynamic are done. The time evolution of a drop in our experiment conditions T  23 ± C, relative humidity  50% is of the order of a few days. Even if spreading is a dynamic process, we may consider each measurement as a stable state for molecules 1994 0031-9007 96 77(10) 1994(4)$10.00 © 1996 The American Physical Society