Bubbles entrapment for drops impinging on polymer surfaces: The roughness effect

Abstract

A study on the influence of the substrate roughness on the occurrence and the formation mechanisms of air bubbles entrapped into water drops impinging on polymeric surfaces, typified by distinctive superficial grinding (15⩽Rq⩽1159nm), was conducted by digitizing silhouettes of the impacting droplets. The images were processed to determine the drop outer profile, the morphology of an air cavity formed at the center of the droplet during the droplet deformations, and the shape of the enclosed bubble. The fluid dynamic processes leading to the entrapment of the bubble were meticulously analyzed, and three new formation mechanisms were observed and detailed. The investigation was conducted for different droplet sizes (2.13⩽D0⩽3.87mm), and at a wide range of impact velocities (0.14⩽Vi⩽0.59m/s). The entrapped bubbles were more likely found into drops falling on more rugged surfaces, while were rare or totally absent into drops impinging on less rough substrates. However, considering just impacts on more rugged surfaces, the bubble occurrence appeared to be minimally influenced by the substrate roughness. At fixed roughness, the bubbles were caught into droplets impinging at lower velocities with increasing the drop size. For fixed drop size and substrate roughness, the range of the bubble existence was often split in two distinctive intervals of impact velocities, phenomenon especially noted for drops impacting on more rugged surfaces. Finally, we suggested a relation between the peculiar shapes that the liquid assumed in the course of the impact and the occurrence/absence of the entrapped bubble.