Self-propulsion of inverse Leidenfrost drops on a cryogenic bath

Abstract

When deposited on a hot bath, volatile drops are observed to stay in levitation: the so-called Leidenfrost effect. Here, we discuss drop dynamics in an inverse Leidenfrost situation where room-temperature drops are deposited on a liquid nitrogen pool, and levitate on a vapor film generated by evaporation of the bath. In the seconds following deposition, we observe that the droplets start to glide on the bath along a straight path, only disrupted by elastic bouncing close to the edges of the container. Initially at rest, these self-propelled drops accelerate within a few seconds and reach velocities on the order of a few cm/s before slowing down on a longer time scale. They remain self-propelled as long as they are sitting on the bath, even after freezing and cooling down to liquid nitrogen temperature. We experimentally investigate the parameters that affect liquid motion, and propose a model, based on the experimentally and numerically observed (stable) symmetry breaking within the vapor film that supports the drop. When also the film thickness and the cooling dynamics of the drops are modeled, the variations of the drop velocities can be accurately reproduced.drops | self-propulsion | Leidenfrost | liquid nitrogen bath | gliding

Capillary orbits

“…A unique feature of this system is that, in the absence of any physical contact with the bath, friction forces remain extremely small 20–22 , and the rapid motion resembles usual Leidenfrost drops that are highly mobile when placed into external fields 23 . In addition, in our experiment, this small drag is almost perfectly compensated by a small propulsion force caused by a symmetry breaking within the film sustaining the drops 19 . The drops thus behave like nearly frictionless “cryogenic skaters”: they glide in perfectly straight trajectories and, once frozen, they keep a constant velocity v 0 ranging from 1 to 3 cm s −1 .…”

“…4, the drops are frozen to liquid nitrogen temperature, and h is fixed by the spontaneous evaporation of the bath, which maintains them in levitation. Following 19 , h is calculated: it is close to 10 μm for the two (almost identical) drops considered here. Integrating the equations of motion with this friction force provides a very good fit to the spiraling trajectory of Fig.…”

“…As illustrated in Fig. 1a, each drop is kept in an “inverse Leidenfrost” state above the liquid surface, a levitating state that is enabled by the continuous vapor flow produced by the cryogenic bath 16–19 . A unique feature of this system is that, in the absence of any physical contact with the bath, friction forces remain extremely small 20–22 , and the rapid motion resembles usual Leidenfrost drops that are highly mobile when placed into external fields 23 .…”

“…The liquid nitrogen bath is contained in a small beaker with characteristic size 5 cm. To avoid boiling, the small beaker is placed on a copper disk at the center of a sacrificial bath of liquid nitrogen 17,19 . The sacrificial bath has a diameter of 19 cm and is filled with 5 cm of liquid nitrogen.…”

Rectification of Mobile Leidenfrost Droplets by Planar Ratchets

“…As a result, droplets stay in a thermodynamically quasi‐stable state with elongated lifetime and enhanced mobility, providing extensive potentials in fluidic devices, energy conversion, and drag reduction . Distinct from ambient or condensation condition where the droplets are usually manipulated by wetting gradient and topographical asymmetry, droplets at Leidenfrost state exhibit a self‐activation and self‐rotation without the need of any asymmetric features …”