Thin Nematic Films: Metastability And Spinodal Dewetting

Abstract

We present new results about the stability of 5CB nematic films spun cast onto silicon wafers. We observe experimentally the dewetting of thin films while thick ones remain stable. We interpret this behavior as a competition between elasticity and van der Waals forces. At later stages, the experimentally observed dewetting instability leads to the formation of structures (islands) which grow and finally merge to form a film of uniform thickness. We show that the islands' characteristic size L͑t͒ scales as t 1͞3 as expected from late stage growth theories. [S0031-9007(99)08781-5] 64.70.Md, 68.45.Gd Dewetting may occur in thin liquid films because of the high deformability of the liquid-air interface. This has been extensively studied for mainly two reasons. Dewetting may hinder technological applications such as coatings or lubrication. From a more fundamental perspective, a better knowledge of specific interactions involved during (de-)wetting processes is necessary. Two main mechanisms have been identified as inducing dewetting [1-3]: dry patches nucleation due to material heterogeneity (dust, defects), or amplification of thermal fluctuations at the free surface of the liquid film. The latter dewetting mechanism is called spinodal dewetting. While the stability of thin polymeric liquid films has been largely studied [4,5], little is actually known about the dewetting of liquid crystals [6]. Spinodal dewetting has been only recently observed in "soft solid" liquid crystal films [7]. As far as liquid crystals are concerned, a significant effort has been carried out, for the last two decades, to achieve a precise understanding of the solid/liquid crystal interface: anchoring mechanisms [8], surface-induced orientational order [9], and orientational wetting [10] have been and still are the objects of intensive research.In previous Letters, we have reported on a detailed investigation of the spreading of 5CB nematic droplets on silicon wafers [11,12]. The anchoring conditions are antagonist (strong homeotropic anchoring at the free nematic-air interface and weak planar anchoring close to the solid surface) and thus induce an elastic distortion in the film. Ellipsometric profiles of the drops reveal two characteristic thicknesses. The first, belonging to the molecular scale, corresponds to a trilayer structure (tilted monolayer bearing a smecticlike bilayer). The second belongs to the mesoscopic scale (tens of nanometers) and corresponds to the equilibrium thickness of the drop. That previous study allowed us to identify the basic interactions which account for the structure of wetting nematic droplets on silicon wafers. Now, we focus our attention on the dewetting of thin 5CB nematic films spun cast onto the same surfaces. The inner structure of the film, induced by the boundary conditions and resulting from the strong coupling of both interfaces, significantly affects the dewetting behavior of such films. We report in this Letter on spinodal dewetting occurring in 5CB nematic films and propose a first quantitat...

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