Supercooled condensation droplets sliding on frost: Origin and anti-frosting effect observed on Cotinus coggygria leaf

Fascinated by the purple color, water-repellent, and self-cleaning properties of <i>Cotinus coggygria Scop.</i> leaves, we studied their morphology, wetting, and condensation frosting. Wax nanotubules confer high contact angles, enabling coalescence-induced condensation droplet (out-of-plane) jumping, which, as known, contributes to slowing down frost. Another type of movement-this time in-plane-becomes predominant in reducing the frosting velocity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>v</mml:mi> <mml:mrow><mml:mi>f</mml:mi> <mml:mi>r</mml:mi> <mml:mi>o</mml:mi> <mml:mi>s</mml:mi> <mml:mi>t</mml:mi></mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) within a sub-cooling temperature range. Specifically, supercooled droplets slide toward the frost bridges upon contact, moving in the opposite direction to frost propagation. Between -11 and -2°C, Sliding on Frost (SoF) shifts from being rare to very frequent, reducing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>v</mml:mi> <mml:mrow><mml:mi>f</mml:mi> <mml:mi>r</mml:mi> <mml:mi>o</mml:mi> <mml:mi>s</mml:mi> <mml:mi>t</mml:mi></mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub><mml:mi>v</mml:mi> <mml:mrow><mml:mi>b</mml:mi> <mml:mi>r</mml:mi> <mml:mi>i</mml:mi> <mml:mi>d</mml:mi> <mml:mi>g</mml:mi> <mml:mi>e</mml:mi></mml:mrow> </mml:msub> </mml:mrow> </mml:math> from approximately 4 to 1, respectively. Using high-speed microscopy, we observed that the advancing contact angle of supercooled water on ice decreases with temperature. We describe the primary role of this behavior in SoF with a model that accounts for the forces involved and explains the observed transition.