Batch-Arrangement of Droplets on Silica Surface Based on Laser Wettability Modification

The laser wettability modification method for a silica surface was applied to the liquid batch-arrangement process. The droplets on the contact angle boundary produced by the modification moved to the more hydrophilic modified area because of the driving force of the surface energy hysteresis. Based on this phenomenon, a substrate with an array of 1-mm square modified areas was dip-coated with different concentrations of glycerin solution to align the droplets in batches. The experimental and analytical results showed that when the amount of liquid adhering to the hydrophilic area was small, the droplet shape became round owing to surface tension, and the reproduction of the modified rectangular shape was difficult. To increase the amount of the adhered liquid, the arrangement was conducted under conditions that increased the capillary number, such as high viscosity and high pull-out speed. The reproducibility of the intended shape was found to increase with an increase in the capillary number. Although the spatial resolution of the arrangement was limited by the laser spot size, it was demonstrated that the proposed method could align droplets onto a silica surface in a batch with the intended geometry.