Enhanced y-component droplet motion via droplet-wall-contact induced explosive boiling on a convex surface

Abstract

Inspired by the light-diverging effect induced by convex mirrors, we propose a novel droplet actuator for potential application in mixing/reaction. We use a rectangle formed by an open tunnel and two convex surfaces. When a droplet contacts a convex surface, a sufficiently large thrust force is generated due to contact-induced explosive boiling, generating self-sustained droplet oscillation. The convex surface enhances the droplet motion in the width direction. A thermal insulation technique is applied to reach hotter hydrophilic convex surfaces but a colder superhydrophobic tunnel. The former generates thrust and scatters the droplet trajectories, but the latter is used for rapid droplet transportation. The collision frequency with the tunnel surface when using the convex side surface (CSS) with θ = 15.0° is three times that obtained from the flat side surface (FSS), where θ is the half-curvature angle of CSS. Holding CSS constant, the enhanced collision with the tunnel surface creates fold line trajectories for the droplet. Meanwhile, the statistical droplet velocity in the y-component when using the CSS is two times that obtained using the FSS. Incident angles and reflection angles are analyzed for collisions with the contact boiling surfaces. We conclude that the range of reflection angles becomes larger when using the CSS. Droplet rotation further increases the reflection angle. The enlarged reflection angle is the key mechanism for forming the fold line trajectory and enhancing the motion in the width direction. This work provides a new principle for creating perturbations to enhance mixing/reaction for chemical and biological applications.