The observation of air bubble entrapment for water droplets impinging on parafilm surface

Abstract

The formation of an entrapped air bubble for a water droplet impinging on a parafilm surface was studied by using a high-speed video camera system. The images were taken sequentially immediately after the droplet impingement. The relaxation of impinging droplets from both the top and side views was monitored, and a formation mechanism of the entrapped air bubble was proposed in this work. An investigation of droplets with different sizes (D0) and impact velocities (Vi) was performed, and it was found that the formation of the entrapped air bubble was observed only at (i) Vi=0.35–0.56m/s for droplets with D0=2.14mm, (ii) D0=1.93–2.14mm for Vi=0.57±0.02m/s, and (iii) D0=2.30–2.46mm for Vi=0.52m/s.The fluid flow inside the droplet during the formation of air bubble entrapment was examined by performing two sets of droplet impact experiments: (i) droplets with various Vi at a fixed droplet size and (ii) droplets with various volumes at a fixed Vi. Once the water droplet touched the parafilm surface, the drop spread, and the central cone of the impinging droplet descended. After a few milliseconds, the central cone disappeared, an air cavity formed, and the droplet started to retract. The retraction generated an inward, radial flow mainly to the top and bottom of the central air cavity. A crucial condition for the formation of an entrapped air bubble in this work is the inward flow over the top of the air cavity closes earlier than the upward movement of the fluid below the central cavity.