| Summary: | The hydrodynamic behavior of droplet impingement has been a topic of interest for many years due to its potential applications. Due to the advances in nano-fabrication the fabrication micro and nano-structured surfaces has become more economical. Which has motivated the study of droplet impingement on patterned surfaces in the recent years. However, the study of droplet train impingement at superheat temperatures on such pattern surfaces are lacking. Thus, this study aims to investigate the transition of splash angle and wetted area diameter of various droplet train on a TiO2 nanotube surface across a temperature range of T = 150°C to T = 430°C. Four different droplet train are used in the experiment conducted. Namely Case A (f = 40 kHz, We = 285), Case B (f = 40 kHz, We = 333), Case C (f = 40 kHz, We = 376) and Case D (f = 40 kHz, We = 285). At low temperatures of T = 150°C water is observed to accumulate around the impingement site with occasional nucleation bubbles. As the surface temperature increases nucleation bubbles erupt more vigorously and occurs at higher frequency which results in the wetted area expanding at a faster rate. As surface temperature continues to increase the behavior transits from erupting of vapor bubbles to random splashing. This point is referred to the peak wetted area diameter temperature (Tw, peak). Any further increase in surface temperature after Tw, peak will result in a decreasing rate of wetted area expansion. In Case A, Tw, peak = 190°C while In Case C, Tw, peak is between 190°C - 210A°C. Thus a higher Weber number results in a lower Tw, peak. No significant differences are seen when comparing Case B and Case D which meant that frequency is not a factor in affecting Tw, peak. In all four cases at high surface temperatures of T = 310°C to T = 430°C, a distinct splash angle is observed within a short window of t = 1.43ms to t = 8.57ms. In all cases the splash angle starts as ϕ = 40° to ϕ = 50°. The phenomenon typically has a longer duration at higher temperatures of T = 390°C to T = 430°C. Future studies can include different surfaces with micro-level structures, nano-level structures or even hierarchical surfaces. Detailed heat flux and surface temperature measurements can also be conducted to allow a more in-depth analysis and understanding of the impingement behavior.
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