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中国物理学会期刊

周期性爆沸诱导的液滴自驱动

CSTR: 32037.14.aps.70.20211386

Self-propulsion droplet induced via periodic explosive boiling

CSTR: 32037.14.aps.70.20211386
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  • 将液滴沉积在高于Leidenfrost温度的表面上, 液滴将悬浮在自身的蒸汽垫上, 这使液滴具有惊人的移动性, 通常通过构造不对称的微纳结构表面对液滴下方的蒸汽流进行校正, 实现液滴自驱动. 但液滴运动方向和液滴输运速度(10—40 cm/s)具有局限性. 本实验构造Leidenfrost传热面和撞击面, Leidenfrost传热面用于悬浮液滴并为其提供足够的能量, 当Leidenfrsot液滴(燃料)与撞击面(点火器)接触时, 粗糙环的大量微/纳米腔不仅会向液滴产生额外的辐射热量, 而且还会提供成核点以在约10 ms内触发爆炸沸腾. 利用周期性爆炸沸腾产生的脉冲推力Fth实现液滴自驱动. 液滴运动初始阶段由于惯性力Fi占主导, 液滴撞击多为镜面反射, 液滴运动轨迹较为混乱, 随着液滴直径减小, 脉冲推力Fth占主导, 液滴轨迹通过Leidenfrost传热面中心. 同时液滴输运速度高达68.91 cm/s, 这是在以往微纳结构表面液滴自驱动从未实现的. 这一发现为高温下操纵液滴提供了新的思路, 有望在微流控和传热等两相系统中得到重要的应用.

     

    When a droplet is placed on a surface with a temperature higher than Leidenfrost temperature, it will be levitated on its own vapor cushion, which makes the droplet have amazing mobility. Generally, the vapor flow under the droplet is corrected by constructing asymmetry micro/nano textured surfaces to realize the self-propulsion of the droplet. However, the control of droplet dynamics becomes uncertain due to the complicated interaction among liquid-vapor-solid phases, and the direction of droplet motion and droplet transport velocity (10-40 cm/s) have limitations. Leidenfrost heat transfer surface and impact surface are constructed in this experiment. When the surface of Leidenfrost heat transfer for droplet levitation contacts the droplet which the sufficient energy flows towards, the rough ring surface acts as an igniter. When a warm Leidenfrsot droplet (fuel) contacts skirt ring (igniter), abundant micro/nano cavities of rough skirt ring not only generate additional radiation heat flux towards droplet but also provide nucleation sites to trigger explosive boiling on a ~10 ms time scale. The thrust force Fth generated by periodic explosive boiling realizes the self-propulsion of droplets. In the initial stage of droplet motion, the inertial force Fi is dominant, and the droplet impact is mostly specular reflection, and the droplet trajectory is chaotic. With the decrease of droplet diameter, the pulsed thrust Fth is dominant, and the droplet trajectory passes through the center of the Leidenfrost heat transfer surface. Our experimental results show that the droplet passes through the center of the Leidenfrost heat transfer surface in a wide diameter range (D = 0.671-1.576 mm). For the last (150th) collision with the rough ring, the curved trajectory exists instead of line trajectory with D \sim 0.105 mm. For a very small droplet, the drag force Fd is dominant, which prevents the droplet from continuing to move. The droplet is followed by sessile droplet evaporation until it totally disappears. At the same time, the droplet transport velocity is as high as 68.91 cm/s, which has not been realized before. The present work provides a new method to manipulate droplet motion at a high temperature. Our Leidenfrost system is simple, cost-effective and lasts long-term operation because it does not depend on complicated micro/nano fabrications, which is expected to be used in microfluidic and heat transfer two-phase systems.

     

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