Abstract
Spin dewetting refers to spontaneous rupture of the dispensed solution layer during spin coating, resulting in isolated but periodic, regular sized domains of the solute and is pre-dominant when the solute concentration (Cn) is very low. In this article we report how the morphology of liquid crystal (LC) 5CB thin films coated on flat and patterned PMMA substrate transform from spin dewetted droplets to continuous films with increase in Cn. We further show that within the spin dewetted regime, with gradual increase in the solute concentration, periodicity of the isotropic droplets (λD) as well as their mean diameter (dD), gradually decreases, till the film becomes continuous at a critical concentration (Cn*). Interestingly, the trend that λD reduces with increase in Cn is exact opposite to what is observed in thermal/solvent vapor induced dewetting of a thin film. The spin dewetted droplets exhibit transient Radial texture, in contrast to Schlieren texture observed in elongated threads and continuous films of 5CB, which remains in the Nematic phase at room temperature. Finally we show that by casting the film on a grating patterned substrate it becomes possible to align the spin dewetted droplets along the contours substrate patterns.
Highlights
Thin films coated on solid surfaces find wide applications as adhesives[1], biological membranes[2], organic photovoltaics[3], modulation of optical properties of surfaces and so on[4]
The morphology changes from an array of isolated spin dewetted droplets for Cn upto 1.0%, a transition morphology comprising collection of droplets and elongated threads for Cn = 1.25% and a continuous film for Cn ≥ 1.5%
In this article we have looked into the fundamental aspect of spin coating a dilute solution of liquid crystal 5CB in ethanol on a PMMA substrate
Summary
Thin films coated on solid surfaces find wide applications as adhesives[1], biological membranes[2], organic photovoltaics[3], modulation of optical properties of surfaces and so on[4]. Disjoining pressure arising out of interfacial van der Waals’ interaction within the film leads to the growth of thermally excited surface capillary waves with time, leading to spontaneous rupture and dewetting of ultra-thin films, on non wettable surfaces[6] While such instability is undesirable from the standpoint of coatings[11], pattern formation associated with it is gaining popularity as a viable non lithographic alternative for creating meso and nano structures, on a topographically or chemically patterned substrate, which results in perfect ordering of the features[5,12,13,14,15,16,17]. The alignment of liquid crystal molecules strongly depends on the boundary conditions whether the LC film is confined between two plates or with a free surface on a substrates. Very recently by Bandyopadhyay et al utilized spin dewetting to obtain an ordered array of LC droplets on a physico-chemically patterned substrate[70]
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