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.

Thermally induced nematic to isotropic (N–I) phase transition and dewetting of 5CB liquid crystal thin films on flat and topographically patterned substrates.

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We report how the texture and stability of a nematic liquid-crystal (LC) thin film of 5CB vary as a function of UV−ozone (UVO) exposure of the underlying poly(methyl methacrylate) (PMMA) substrate. UVO exposure of the PMMA substrate not only increases its surface energy, making it more wettable, but also results in the generation of oxygen-containing polar functional groups on the PMMA surface due to photolysis of ester. While the stability of the 5CB films is expectedly enhanced on UVO-exposed PMMA substrates against thermally induced dewetting, the texture of the film also changes as a function of the UV exposure time (t E ). We show that the films continue to exhibit the nematic Schlieren texture for t E ≤ 20 min, although the disclination point density (|m|) gradually reduces with an increase in t E . However, the texture changes completely to a spherulite or fanlike texture in t E ≥ 20 min due to enhanced anchoring of the 5CB molecules on the substrates. In addition, enhanced wettability and stronger anchoring by the UVO-exposed PMMA substrates also suppress the tendency of spin dewetting of the 5CB films due to spontaneous rupture of the dispensed solution layer during spin coating, particularly when the solute concentration (C n ) is very low. The latter observation allows possible creation of thinner LC films, which are otherwise difficult to form by spin coating due to enhanced cohesive interactions between the anisotropic LC molecules. Finally, we show that in continuous films, the nematic-to-isotropic (N → I) and I → N phase transitions with gradual heating and cooling remain completely reversible, irrespective of the texture of the film and wettability of the substrate.

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Self-powered active micro/nanomotors have gained significant research attention in the scientific community due to their unique dynamics and emergent behaviors in response to various external stimuli. In recent years, different prototypes of micro/nanomotors have been investigated extensively for various fundamental studies and useful applications. Enzyme powered motors have emerged as ideal platforms for realizing various biological applications due to their multifunctionality and specificity in operation under complex conditions. However, most enzyme powered motors developed so far suffer from issues pertaining to their integration with biological systems due to retention of their synthetic components. With an aim to design a micromotor completely devoid of non-biological components, the present study reports the fabrication of a catalase driven, bovine serum albumin shelled microbubble motor. In substrate-rich environments, these ‘active’ microbubbles have been found to undergo substrate concentration dependent enhanced diffusion, just like their single enzyme counterparts. Brownian dynamics simulations have also been carried out to estimate the average force generated per catalytic turnover over the motor surface. Interestingly, it was found that these active microbubbles also possess the ability to transfer energy to their surroundings. The experiments were carried out within a three-dimensional setup that offered an advantage over the commonly used quasi two-dimensional systems, as it could enable researchers to probe micro/nanomotor dynamics and associated energy transfer profiles in more in vivo like settings.