High-speed visualization and infrared thermography of pool boiling on surfaces having differing dynamic wettability

Abstract

Wettability is a key surface characteristic that affects boiling behavior. Recent studies have shown that surface wettability as described by a static contact angle offers an incomplete description of this behavior, whereas the dynamic wettability described by advancing and receding contact angles is required for accurate description of ebullition dynamics. The observed influence of dynamic surface wettability on boiling performance has been previously explained based on arguments of single-bubble growth and departure; however, the mechanisms by which these dynamic wettability regimes influence boiling performance at high heat fluxes are still unknown. In this study we experimentally reveal the mechanisms by which ambiphilic surfaces (having receding contact angle <90° and advancing angle >90°) offer better boiling performance compared to hygrophilic and hygrophobic surfaces (for which both receding and advancing contact angles are below or above 90°, respectively). These mechanisms are identified through analysis of surface temperature distributions acquired via high-speed infrared (IR) thermography, complemented with side-view visualizations. The low receding contact angle of ambiphilic and hygrophilic surfaces prevents dewetting during bubble growth, thereby preventing the premature critical heat flux (CHF) that occurs for hygrophobic surfaces. Furthermore, the high advancing contact angle of ambiphilic surfaces acts to significantly improve the nucleation rate per unit surface area, which is identified as the primary mechanism of nucleate boiling heat transfer coefficient enhancement over hygrophilic surfaces. These first-of-their-kind observations of the surface temperature distributions during boiling on surfaces spanning all regimes of dynamic wettability provide new understanding of the avenues available for performance enhancement by controlling the surface dynamic wettability.