Scheme of the Leidenfrost ratchet: an evaporating liquid drop is deposited on a heated metallic surface patterned with ratchet-like crenelations. The droplet will feel a force and will move in the indicated direction.
Different ratchets employed in the study. They all keep the same ratchet length, but varying its depth, they therefore differ basically in the aspect ratio: (a) H/λ = 0.23, (b) 0.11, (c) 0.07. All images have been taken by confocal microscopy.
(Top) Large water droplet of 3 mm diameter on a micro-ratchet at different instants after deposition: 500, 1000, 1500, and 2000 ms. Note that the roughness of the ratchets (λ ∼ 140 μm) is not visible at this scale. (Bottom) Small capillary water droplet of 1 mm diameter on a microratchet at different instants after deposition: 50, 100, 150, and 200 ms. Note that the capillary droplet (bottom) travels the same distance 10 times faster than the larger one (top).
Droplet velocity for different drop volumes. The velocities seem to stabilize around 80 mm/s for bigger droplets, but present larger and more fluctuating values for the smallest ones. (Inset) Experimental droplet velocity as function of time. Circles represent experimental measurements. Continuous line represents the model fit used to determine the terminal velocity.
Droplet acceleration for different ratchet temperatures and aspect ratios for drop volumes of 2 μl (∼1.6 mm diameter), measured as an average acceleration after a fixed traveling length of 4 cm. The dotted lines are only a guide to the eye.
Force measured for different drop volumes. Filled circles represent those measurements done with the velocity fit method and those with open squares represent those done with inclined plane method.
Representation of the viscous mechanism on a single microratchet: the vapor from the lower droplet surface penetrates into the ratchet, the ratchet geometry generates an horizontal pressure drop towards the right side, leading to a shear stress μ v ∂U/∂z at the droplets interface, which integrated over the contact area gives rise to a net thrust force towards the positive x-axis.
Propelling force measurements and predictions from Eqs. (8) and (10) for capillary drops and for heavy drops. Results from Lagubeau et al. (2011) are also plotted for comparison.
(a) Bouncing frequency for different volumes of capillary drops. (b) Terminal velocity of bouncing drops for different frequencies. Only data from droplets with bouncing amplitudes comparable to their size are plotted here. The corresponding data for propelling force have not been employed in the analysis.
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