Dynamics of drop impact on heated metal wires: Thermally induced transition from tail to splash to jumping modes

Abstract

A drop impact on high-curvature structures at high temperatures is observable in various engineering platforms and understanding its dynamics about the heat transfer mechanism will significantly advance existing thermal applications. However, most studies on the drop impact on complex structures have focused solely on the dynamics of the fluidic motion at ambient temperatures. Herein, we report experimental and theoretical analyses of the drop impact on heated metal wires within a wide temperature range and demonstrate the transition modes depending on the contact surface temperatures as functions of the physicochemical properties (ethanol, water, and acetone), the impact velocity, and the dimensions of the liquid drops. According to an increase in the wire temperature from ambient temperature to beyond the Leidenfrost point, the impacting drops revealed three distinct impact regimes, which can be classified as tail, splash, and jumping modes. The onset of splash mode, in which tiny mono-dispersed drops are circularly sprayed, is observed at a higher temperature than the nucleate boiling temperatures of all liquids used, whereas the drops in tail mode show a wrapping of the nichrome wire and a consecutive free-fall. At over the Leidenfrost temperature, a jumping mode indicating the presence of bouncing daughter drops appears around the wire in a half-ring shape owing to the instant formation of the vapor cushion between the wire and droplets. Theoretical analyses were conducted to develop a general model reflecting the underlying physics of the transition between the three distinct modes. The diameter and the impact velocity of the drops are crucial for determining the threshold temperature of the jumping mode, whereas liquid properties such as the nucleate boiling temperature and We numbers of the drops are more decisive factors for the initiation of splash mode. The outcomes of this work will contribute toward advancing the fundamental understanding and application using the drop impacts on high-curvature structures at high temperatures. (C) 2018 Elsevier Ltd. All rights reserved.