Wetting and Anchoring of a Nematic Liquid Crystal on a Rough Surface

Abstract

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] 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 0͒, and n in the surface plane is called planar orientation ͑u p͞2͒. 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 -cyanobiphenyl͒ 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 11͒.Silicon substrates (type N, dopant PH, orientation 111 purchased from Siltronix) are coated with silicon monoxide ground evaporated at the rate of 4 Å͞s 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...

Scanning Polarization Force Microscopy: a Technique for Studies of Wetting Phenomena At the Nanometer Scale

“…This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances (Valignat et al, 1996). Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors.…”

“…In addition, these interesting liquids provide a good testing ground for fundamental theories of wetting. Films of 8CB (4'-n-octyl-4-cyanobiphenyl) have been studied by optical microscopy, X-ray reflectivity (Bardon et al, 1999;Pershan, 1989), ellipsometry (Valignat et al, 1996;Bardon et al, 1999), second-harmonic generation (Feller et al, 1991), scanning tunneling microscopy (STM) (Stevens et al, 1998), AFM (Fang et al, 1997), and SFA (Artsyukhovich et al, 1999).…”

Thin film dynamics: theory and applications

“…The competition of attractive van der Waals forces and short range repulsive forces, like Born repulsion, can produce complex instabilities in layers of thin liquid films on solid substrates. Experimental studies have shown this behavior for polymer films [66,81,87,88,101], liquid crystal films [58,93,94], liquid metals [29,58] and evaporating films [41,76]. During these dewetting processes, large droplets are formed which are connected by ultra-thin films.…”

Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

“…The LC droplet thereby experiences anchoring at two separate interfaces: LC-solid interface and LC-air interface. Given that the nematic anchoring at LC-air interface is typically homeotropic [177][178][179], the overall director field within the droplet is essentially determined by the boundary conditions at the solid substrate. For instance, if the solid substrate possesses uniform planar anchoring, then the director field is expected to equilibrate to a hybrid-aligned configuration (Section 4.…”