Abstract
We report thermally induced nematic to isotropic (N–I) phase transition as well as dewetting of 5CB Liquid Crystal (LC) thin films coated on flat and topographically patterned substrates with grating geometry of different line width (lP) and periodicity (λP). On a flat substrate, the nematic to isotropic (N–I) phase transition, which takes place within a temperature range between 31.1 °C and 34.4 °C is fully reversible, with re-appearance of identical Schlieren texture when the sample is cooled down during isotropic to nematic (I–N) transition. Upon further heating beyond N–I transition and annealing at T ≈ 65 °C, the film undergoes nucleated dewetting with formation and growth of holes, which eventually merge to form isolated droplets. The morphology of the dewetted structures remains unaltered when the film is cooled to room temperature from this stage, though the features undergo phase transition to the nematic state. In contrast on a topographically patterned substrate, the phase transition cycle is associated with a change of the texture of the film during cooling to the nematic stage. Interestingly the molecules exhibit homeotropic anchoring when λP ≈ 1.5 μm and planar anchoring when λP large (≈10 μm). When heated further to T ≈ 65 °C, the film dewets on topographically patterned substrates resulting in a collection of droplets, which are aligned to the substrate patterns when λP is large (≈10 μm). In contrast the dewetted droplets are random and not correlated to the patterns when λP is lower (≈1.5 μm).
Highlights
Ultra-thin liquid lms rupture spontaneously on a non-wetting surface due to ampli cation of thermally excited surface capillary waves with the formation of random holes that grow with time.[1,2,3,4,5,6,7] Ever since the pioneering contribution by Reiter in 1992,1 there has been signi cant research attention on the spontaneous instability of thin lms and dewetting
Using a PMMA substrate eliminates the possibility of dewetting of the 5CB lms due to cohesive interaction between the Liquid Crystal (LC) molecules, which is commonly observed on substrates like glass or cross linked PDMS
We have reported phase transition and thermal dewetting of 5CB thin lm on a at as well as topographically patterned PMMA substrates
Summary
Ultra-thin (thickness < 100 nm) liquid lms rupture spontaneously on a non-wetting surface due to ampli cation of thermally excited surface capillary waves with the formation of random holes that grow with time.[1,2,3,4,5,6,7] Ever since the pioneering contribution by Reiter in 1992,1 there has been signi cant research attention on the spontaneous instability of thin lms and dewetting. On a at PMMA substrate, completely reversible N–I and I–N phase transitions is observed as the system is heated from room temperature to T > TC and subsequently cooled to T < TC.
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