Recent studies have shown that bubbles inside a volatile liquid can spontaneously grow in the absence of bulk gas supersaturation through diffusion of air driven by differences in its partial pressure. Here, we demonstrate a distinct manifestation of this phenomenon in which perfluorocarbon oil-shell bubbles, formed following bubble detachment from a perfluorocarbon oil (PP1, F2 Chemicals)–water interface, become trapped and spontaneously expand at the water–air interface. Addition of the anionic fluorosurfactant Capstone FS-60 to the aqueous phase stabilizes the detached oil-shell bubbles at the water–air interface. The bubbles spontaneously grow from an initial diameter of approximately 1.5 mm to several millimeters within a few minutes. The initial growth rate reaches about 11 μm/s, substantially exceeding that observed for bubbles trapped at the PP1–water interface. Bubble-bursting measurements indicate progressive thinning and eventual depletion of the PP1 shell at bubble radii of approximately 2.5–3.0 mm. Bubble growth nevertheless continues after shell depletion as the system transitions into a surfactant-stabilized foam bubble containing PP1 vapor. An interfacial air-diffusion model attributes the enhanced growth to the short diffusion path across the thin liquid film separating the bubble from the atmosphere, driven by the difference in air partial pressure between the PP1-vapor-containing bubble interior and the surrounding atmosphere. Finally, multiple oil-shell bubbles form self-expanding foam clusters that continue to grow for hours, increasing their apparent foam volume by more than 2 orders of magnitude. These findings demonstrate how modifying the mass-transfer geometry of interfacial air diffusion can strongly accelerate bubble growth and enable the formation of self-inflating bubbles and foams under ambient conditions.

A small liquid droplet placed on an asymmetric ratchet surface heated above the Leidenfrost temperature is known to self-propel. Here, we examine an alternative configuration: a heated macroscopic alumina boat (14 cm long) decorated with asymmetric ratchets and floated in volatile perfluorocarbon liquid. Boats with 1 mm pitch ratchets, heated to 280 C, showed robust self-propulsion in the expected direction, reaching several centimeters per second and traveling more than one meter. An impulse–momentum balance scaling, derived from Leidenfrost droplet propulsion, predicts boat velocities of the same order but somewhat below experiments, suggesting enhanced efficiency of vapor flow in contact with bulk liquid. Propulsion efficiency depends strongly on ratchet geometry: for small pitches, bottom and sidewall ratchets act synergistically, while at larger pitches, sidewall ratchets reduce thrust by redirecting vapor. Herringbone-patterned ratchets further enhanced propulsion. Towing tests confirmed reduced drag when boats moved with their ratchet orientation, with ~10% lower drag coefficients compared to smooth boats.

Spontaneous bubble growths in liquids are usually triggered by rapid changes in pressure or temperature that can lead to liquid gas supersaturation. Here, we report alternative scenarios of the spontaneous growths of bubbles inside a high-saturation-vapor-pressure and high-air-solubility perfluorocarbon liquid (PP1) that were observed under ambient quiescent conditions. First, we investigate spontaneous bubble growth inside the single PP1 phase, which was left to evaporate freely. The bubble growth is explained by the difference in the PP1 vapor pressure inside the bubble and that above the freely evaporating PP1 interface. Next, we study the bubble growth inside the liquid PP1 covered with a layer of a second air-saturated immiscible liquid: low-air-solubility water or higher-air-solubility ethanol. In both cases, the bubble growth rates were accelerated, indicating mass transfer of air from the water or ethanol phases to the PP1 phase. The bubble growth rates significantly increase for bubbles trapped at the PP1–water or PP1–ethanol interfaces due to faster air diffusion through the thin PP1 liquid films separating the bubbles from the upper phases. Finally, we consider the case of bubbles inside millimeter-sized PP1 emulsion droplets in water or ethanol. The bubble growth inside the droplet leads to an increase in the PP1 droplet’s effective buoyancy and to the detachment of the droplets from the substrate. The observed bubble growth rate in the case of emulsion droplets was much faster for PP1 droplets in ethanol than for PP1 droplets in water (minutes vs hours). The underlying physical mechanism of the increase of bubble volumes is the spontaneous mass transfer of both air and PP1 vapor to the bubbles produced by a colloidal diffusion pump effect.

Hypothesis: The presence of hydrodynamic slip of water on smooth hydrophobic surfaces has been debated
intensely over the last decades. In recent experiments, the stronger bounce of free-rising bubbles from smooth
hydrophobic surfaces compared to smooth hydrophilic surfaces was interpreted as evidence for a significant
water slip on smooth hydrophobic surfaces.
Experiments: To examine the possible water-slip effect, we conduct well-controlled experiments comparing the
bouncing dynamics of millimeter-sized free-rising bubbles from smooth hydrophobic and hydrophilic surfaces.
The hydrophobic surfaces were graphite or Teflon, and the hydrophilic surfaces were glass or mica. To avoid
contamination, the experiments were conducted in pure water without any surface-active additives. Numerical
simulations were also used to compare the bounce of the bubble from a no-slip and free-slip walls.
Finding: Our experiments show that the free-rising bubbles in pure water bounce identically from the smooth
hydrophobic graphite or Teflon surfaces as from smooth hydrophilic mica or glass surfaces. The bubble bounce
from all four surfaces is in excellent agreement with the numerical simulation of a bubble bouncing from a flat,
no-slip wall. At the same time, numerical simulations for bubbles bouncing from a free-slip wall predict up to
two-fold stronger bouncing amplitudes. Our experiments and numerical simulations, including estimates of the
shear rates, confirm the no-slip boundary condition for water on smooth hydrophobic surfaces.

Superhydrophobic surfaces are expected to reduce drag on bluff bodies moving in water due to the introduction
of a thin air layer around the solid and the resulting partial slip boundary condition. The use of Leidenfrost vapor
layers, sustained on the surface of a heated metal body is a reliable method to estimate the maximum drag
reduction possible due to such air layers. In the past such an approach was used to estimate the drag reduction on
a free-falling heated sphere, in which case the form drag is the lead component of the drag force. Here, we extend
this approach to evaluate the effect of the thin gas layers on the hydrodynamic drag of free-falling streamlined
projectiles and towed model boats, where the form drag is minimal, and the skin friction drag is the lead
component of the drag force. By comparing the drag for streamlined bodies with and without sustained airlayers, we see only incremental drag reductions, for the sub-critical Reynolds number tested herein. The same
is true for towed model boats. These results hold both for superhydrophobic surface treatments and Leidenfrost
vapor layers. Thus, we concluded that for the investigated range of sub-critical Reynolds numbers, the skin
friction drag is less sensitive to the effect of the thin gas layers compared to the form drag. These novel findings
have important implications for the practical potential of energy savings using gas layers sustained on super
hydrophobic surfaces.