Orientational Order in the Liquid-Crystal Surface Induced by the Evaporation of Water Droplets

The evaporation of a droplet induces a variety of ordered patterns near the contact line between the droplet and a substrate. This pattern formation involves both the behavior of colloidal suspensions and interactions between a droplet and a substrate. Although studies on the effect of the behavior of colloidal suspensions on the deposition process have been actively conducted, studies on the effect of the interaction between a droplet and a substrate have not been actively conducted because the observation of the interaction is difficult due to the lack of structural changes in the substrate by the interaction. In this study, we investigated the solvent-substrate interactions using a water droplet and a liquid-crystal droplet, where the liquid crystals were placed on a glass slide. The orientation patterns of the liquid crystals were induced at the contact line between the water droplet and the liquid crystal and were observed by using a polarized optical microscope. Pinning and depinning events were observed at a certain anchoring strength of the liquid crystal on the glass slide and formed linear patterns on the surface of the liquid crystal. Additionally, the interfacial tension between the water and liquid crystal slowly varied at this anchoring strength owing to the variation in the orientation of the liquid crystals. These results indicated that the evaporation-induced patterns appeared when the strength of the solvent-substrate interactions was neutral. The interaction between a droplet and a substrate is important for the fabrication of the desired ordered patterns.