Water droplet alignment film fabricated by patterning hydrophilic and hydrophobic regions using a moth-eye structure

Organisms naturally develop various physiological properties over time. For example, rose petals exhibit spherical repellence to water droplets, preventing them from falling even when inverted. In our previous study, we reproduced this adhesive hydrophobicity by creating microholes in a hydrophobic nanostructured film. However, this microstructure caused light scattering, which reduced the transmittance of the film. To address this, we focused our attention on an insect called the tenebrionid beetle, which collects water from fog using a two-region structure consisting of hydrophilic regions on a hydrophobic surface background. In a previous study, we combined this structure with a moth-eye structure to fabricate an adhesive hydrophobic surface with high permeability. We hypothesized that by reducing the size of the hydrophilic region within the two-region structure, it would be possible to align the water droplets within the hydrophilic regions. In future study, we aim to use these films as pipettes by transferring water droplets aligned on the hydrophilic regions onto a substrate. The optical transmittance of the film is important because it adjusts from the back of the film where the water droplets are being transferred. In this experiment, we fabricated hydrophilic regions using photolithography on a moth-eye mold, applying a hydrophilic photoresist. Subsequently, UV nanoimprint lithography was employed, utilizing a hydrophobic resist to form a two-region moth-eye-structured surface. When the hydrophilic regions had diameters ranging from 100 μm to 750 μm, water droplets preferentially aligned on the hydrophilic regions upon mist exposure using a humidifier. Notably, even when the film is inverted, the water droplets remain adhered, and due to the transmittance of the film of ∼90 %, they are visible from the reverse side. In addition, a 7 μL water droplet placed on the film demonstrated a contact angle of 148.4 degrees, confirming strong adhesive hydrophobicity. • A two-region structured film with an entirely moth-eye structure was fabricated. • The water was aligned in a circular pattern from 750 μm to 100 μm in diameter. • The aligned water droplets showed adhesive hydrophobicity. • The increase in the pitch disturbed the uniformity of the water volume. • The adhered water droplets were visible from the back side.