Quantitative Evaluation of Surface Wettability Effect on Drop Propulsion on Heated Surfaces

Abstract

Drops propel in a particular direction on a heated surface if the surface has a fore-aft asymmetrical geometry. We have developed a new passive cooling technology using the drop propulsion. We, therefore, focus more on the propulsion at lower temperatures at which the boiling occurs at the heated surface. This study aims to evaluate the propulsion behavior with boiling by controlling the area of the hydrophilic region on the surface. In this study, we changed the area of the hydrophilic region with good reproducibility and high accuracy by using either a toothpick scratching or a plasma cleaning. As a result, we clarified that the acceleration of the drop marked the highest value with the width of the hydrophilic surface of about 0.3 mm because the propulsion force due to boiling overcame the force on the contact line the most with the width. We also clarified that the maximum climbing angle of the drop coincided with the angle at which the drop stayed in place.