LEIDENFROST LIQUID DROPS ON MINIATURIZED RATCHETS: THE INFLUENCE OF DROP IMPACT SPEED Jeong Tae Ok 1* , Daniel Sang–Won. Park 2 and Sunggook Park 2 1 Midwestern State University, USA and 2 Louisiana State University, USA ABSTRACT The experimental setup equipped with a high speed camera and an injection system with a needle and a syringe pump was used to examine the influence of the impact and motion of water drops on micro scale asymmetric ratchets under various conditions. The results show good repeatability. The highest acceleration is always observed at the beginning of drop motion. The impact speed of water drop does not significantly affect the drop motion. KEYWORDS: Leidenfrost effect, asymmetric ratchets, liquid drop impact INTRODUCTION The Leidenfrost phenomenon occurs when liquid drops on a hot surface produce an insulating vapor layer, keeping them from boiling rapidly. Directional motion of Leidenfrost drops was achieved by asymmetric ratchets due to the local vapor pressure gradient [1]. The floating liquid drop moves toward the gently inclined side of the ratchet teeth and immediately reaches a terminal velocity of the order of 10 cm/s. The drop mobility strongly depends on the surface temperature and size of the ratchets [2]. Most studies related to drop impact on hot surfaces have been carried out for cooling applications such as fuel combustors, spray quenching and fire protection systems. In terms of heat transfer, as the impact speed increases the net heat transfer enhances in Leidenfrost regime. The maximum heat transfer has been observed when drop spreads most and start pushing against the wall. Thus, higher temperature is required to isolate liquid drop from the wall with an impact, which is called as dynamic Leidenfrost temperature. For the dynamics of Leidenfrost drops, the kinetic energy effectively transfers to surface energy and then vice versa due to its elasticity. This temperature-independent elasticity of Leidenfrost drop strongly decreases with Weber number (We), which represents the indicative of the competition between kinetic energy and surface energy [3]. In this study, we investigate the influence of initial liquid drop impact on the motion along the microratchets at two typical temperature regimes (L regime: near the Leidenfrost temperature, H regime: far above Leidenfrost temperature) by alternating the needle height where drops are released. EXPERIMENTAL Large area nickel electroplated microscale ratchets as shown in Figure 1 was used to examine the influence of water drop impact speed on the mobility. The overall area and thickness of the sample were 10 × 5 cm 2 and 2 mm. A high speed camera (Kodak Ektapro 1000HRC) was used to obtain the trajectory and velocity characteristics of drop motion over the entire ratchets surface. Figure 1: Electroplated nickel micro-ratchets (l: 150 μm, h: 30 μm) (a) schematics, (b) photograph, (c) Scanning electron micrographs. 978-0-9798064-7-6/μTAS 2014/$20©14CBMS-0001 1226 18th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 26-30, 2014, San Antonio, Texas, USA