Self-propelled Leidenfrost droplets on a heated glycerol pool

Abstract

The development of contactless sample manipulation for microfluidic purposes has attracted significant attention within the physicochemical fields. Most existing studies focus on the interactions of unheated liquid substrates and on heated/unheated solid substrates. Therefore, the dynamics of droplets on heated liquid pools have yet to be explored. Here, we present an experimental investigation on the levitated and self-propelled droplets on a heated pool. We aim to identify the effect of the pool temperature and the thermophysical properties of droplets on the dynamics of a self-propelled Leidenfrost droplet on a heated pool. The motion of droplets after levitation on the heated pool is visualized. To elucidate the self-propulsion of Leidenfrost droplets, we quantify the thickness of the vapour film between the approaching droplet and the pool surface. Our experimental results show a quantitative agreement with the simple model prediction for self-propelled Leidenfrost droplets. Our results provide deeper physical insights into the dynamics of Leidenfrost droplets on a heated pool for contactless and contamination-free sample manipulation.

Interaction and motion of two neighboring Leidenfrost droplets on oil surface

“…悬浮于平板表面上的液滴，蒸发过程会出现移动 [8] 、弹跳 [9] 和振荡 [10] 的随 机现象，这给液滴控制造成难度。在平板表面上制造规则锯齿结构，可以有效地 引导液滴底部的蒸汽流。使其在液滴两侧非对称排放，从而打破液滴底部粘性力 的平衡，驱动液滴定向运动 [11] 。此外，不均匀的壁温分布影响了液滴局部汽化 速度以及液滴内部 Marangoni 流，在液滴底部形成非对称的蒸汽膜，引导液滴向 冷的方向移动 [12] 。相比于刚性壁面，液滴在可变形表面的运动更为复杂。这是 由于液滴和液池之间蒸汽层形态不同于平坦刚板上， 气膜厚度呈现不同的标度律 [13] 。高温液池上悬浮的液滴，颈部蒸汽膜出口位置局部振荡极易引起气膜整体 不稳定，从而破坏蒸汽膜的对称性，使液滴在液池表面自发移动 [13,14] 。在移动 过程中， 液滴前缘出口气膜厚度总是大于尾缘， 液滴遵循蒸汽主流的方向 [14,15] 。 鉴于此，Gauthier 等 [16] 分析液滴与弯月面碰撞动力学，以探测界面形状并鉴别 液滴尺寸。由于液池表面光滑，液滴可在较低过热度下进行 Leidenfrost 蒸发。 因此，与液滴在固体壁面上的蒸发相比，液滴在液池表面更易形成 Leidenfrost 状态 [1,17] 。 最近 Pacheco 等 [18] 通过观察两个液滴在 Leidenfrost 状态下的碰撞-合并行为， 发现沉积在热的凹板上不同液体的液滴碰撞时， 接触区域较高饱和温度液滴加热 较低温度液滴而发生的 Leidenfrost 效应推迟了液滴的合并。综合液滴固-液界面 发生的两处 Leidenfrost 蒸发，他们将这一行为称为三重 Leidenfrost 效应 (Triple Leidenfrost Effect)。 相比于处在 Leidenfrost 状态不同液体的液滴间碰撞-合并行为， 了解相同液体液滴的蒸发动力学同样重要，其有助于解析喷雾冷却的相变机制， 然而却鲜有报道。对于单个液滴，蒸发速率受到蒸气扩散速率和液滴大小等因素 的影响；然而，对于简单排列的几个液滴或者有复杂尺寸分布的液滴，蒸发过程 液滴间协同作用显著 [19,20] 。这种相互影响导致液滴内出现非对称流场，从而影 响液滴蒸发 [21] 。因此，对于多个 Leidenfrost 液滴，除了非对称气膜产生的驱动 机制外，还需要考虑液滴间相互作用对运动的影响，其中涉及到的动力学较为复 杂。 与以往对常温或震动液池表面多滴作用机理研究 [22][23][24][25] 和高温固体表面不同 液体液滴的 Leidenrost 蒸发机制研究 [18] 界线 [26,27] 。由此我们可以获得液滴表面温度的分布。 图 2 (a) 油池在水平方向和深层方向的温度分布; (b) 硅油和 FC-72 的表面温度测量的校准;…”

Leidenfrost drop dynamics: a forgotten past and modern day rediscoveries

“…It has now been found in a variety of settings. These include: (i) liquid drops carefully placed onto liquid substrates [79][80][81], (ii) liquid drops or solid platelets placed onto a solid substrate whose surface has been uniformly textured using a series of asymmetric grooves to form a ratchet-the so-called Leidenfrost ratchet [29,[82][83][84][85][86], or (iii) uneven solid platelets placed onto a smooth solid surface [87]. In all cases an asymmetry seems to be the primary cause of the motion.…”

Micro Sponge Balls: Preparation and Characterization of Sponge-like Cryogel Particles of Poly(2-hydroxyethyl methacrylate) via the Inverse Leidenfrost Effect

“…Although this phenomenon has shown good application potential in many fields, such as cryogenics, aerospace, green chemistry, and surface science, its main mechanism remains largely unexplored. This is primarily attributed to the boiling-film’s presence on the droplet surface, which apart from restricting their contact with the reaction substrate, also inhibits their dynamic behaviors, like self-propulsion, gliding, spinning, bobbing, and bouncing. − The use of the inverse Leidenfrost effect for the preparation of polymer particles could enable us to achieve a surfactant-free strategy, to alleviate these complications.…”