Thermal Antibubbles: When Thermalization of Encapsulated Leidenfrost Drops Matters.

Antibubbles are ephemeral objects composed of a liquid drop encapsulated by a thin gas shell immersed in a liquid medium. When the drop is made of a volatile liquid and the medium is superheated, the gas shell inflates at a rate governed by the evaporation flux from the drop. This thermal process represents an alternate strategy for delaying the antibubble collapse. We model the dynamics of such "thermal" antibubbles by incorporating to the film drainage equation the heat-transfer-limited evaporation of the drop, which nourishes the gas shell with vapor, as for Leidenfrost drops. We demonstrate that the inflation of the gas shell is drastically inhibited by the thermalization of the initially colder drop. Because of this thermalization effect, smaller drops evaporate much faster than larger ones.

Impact of a pressurized membrane: Coefficient of restitution.

Pressurized membranes are usually used for low cost structures (e.g., inflatable beds), impact protections (e.g., airbags), or sport balls. The last two examples deal with impacts on the human body. Underinflated protective membranes are not effective whereas overinflated objects can cause injury at impact. The coefficient of restitution represents the ability of a membrane to dissipate energy during an impact. Its dependence on membrane properties and inflation pressure is investigated on a model experiment using a spherical membrane. The coefficient of restitution increases with inflation pressure but decreases with impact speed. For a spherical membrane, it is shown that kinetic energy is lost by transfer to vibration modes. A physical modeling of a spherical membrane impact is built considering a quasistatic impact with small indentation. Finally, the dependency of the coefficient of restitution with mechanical parameters, pressurization, and impact characteristics is given.

Penetrating a granular medium by successive impacts.

We consider the penetration dynamics of a vertical cylinder into a dry granular medium subjected to successive impacts. The depth of the impactor below the free surface z_{N} first evolves linearly with the impact number N and then follows a power-law evolution z_{N}∝N^{1/3}. The depth reached by the cylinder after a given number of impacts is observed to increase with the impact energy, but to decrease with its diameter and the density of the granular medium. We develop a model that accounts for the quasistatic and inertial granular forces applying on the cylinder to rationalize our observations. This approach reveals the existence of two intrusion regimes for large and small impact numbers, allowing all data to be rescaled on a master curve. Then, we extend the study to the effect of sidewalls on the dynamics of the impactor. We show that lateral confinement changes the dependence of the impactor depth on the impact number z_{N}(N). This effect is accounted for by considering the increase of the granular drag with the lateral confinement.

Designing a Contact Fingertip Sensor Made Using a Soft 3D Printing Technique.

The development of highly compliant materials and actuators has enabled the design of soft robots that can be applied in rescue operations, in secure human-robot interactions, to manipulate fragile devices or objects, and for robot locomotion within complex environments. To develop reliable solutions for soft robotics applications, devices with the ability to deform and change shape are required, which must be equipped with appropriate sensors capable of withstanding large deformations at suitable speeds and respond repeatedly. This work presents a methodology to build strain sensors made of sensitive, thin, and conductive channels printed inside a soft matrix, using three-dimensional printing. As proof of concept, rectangular beams and semispherical caps embedded with sensitive circuits are developed that are designed to deform under applied forces and detect the gradual contact with objects. The rectangular beam with conductive lines separated from the neutral plane exhibits a quasi-linear electrical response as a function of the applied shear strain. Mechanical diodes, which trigger an activated response once a given deformation onset is exceeded, are implemented using circumferential conductive channels that are centered with the spherical body sensor. Sinusoidally shaped conductive channels located at a given distance from the spherical surface produce a monotonic electrical response, which detects deformations over a broad range. Linear sensors, with enhanced sensitivity to compression, are created if the sensitive conductive channels are oriented along the compression direction. Numerical calculations, used to guide the design of the sensor, show the capability of these sensors to measure simultaneous normal and tangential forces, making them suitable for applications involving fragile object manipulation and robot locomotion. An example of application of these sensors in the control of the forces applied by soft gripper lifting an object is given.

Optimal propulsion of an undulating slender body with anisotropic friction.

This study investigates theoretically and numerically the propulsive sliding of a slender body. The body sustains a transverse and propagative wave along its main axis, and undergoes anisotropic friction caused by its surface texture sliding on the floor. A model accounting for the anisotropy of frictional forces acting on the body is implemented. This describes the propulsive force and gives the optimal undulating parameters for efficient forward propulsion. The optimal wave characteristics are effectively compared to the undulating motion of a slithering snakes, as well as with the motion of sandfish lizards swimming through the sand. Furthermore, numerical simulations have indicated the existence of certain specialized segments along the body that are highly efficient for propulsion, explaining why snakes lift parts of their body while slithering. Finally, the inefficiency of slithering as a form of locomotion to ascend a slope is discussed.

Helical Locomotion in a Granular Medium.

The physical mechanisms that bring about the propulsion of a rotating helix in a granular medium are considered. A propulsive motion along the axis of the rotating helix is induced by both symmetry breaking due to the helical shape, and the anisotropic frictional forces undergone by all segments of the helix in the medium. Helix dynamics is studied as a function of helix rotation speed and its geometrical parameters. The effect of the granular pressure and the applied external load were also investigated. A theoretical model is developed based on the anisotropic frictional force experienced by a slender body moving in a granular material, to account for the translation speed of the helix. A good agreement with experimental data is obtained, which allows for predicting the helix design to propel optimally within granular media. These results pave the way for the development of an efficient sand robot operating according to this mode of locomotion.

Low-resistive vibratory penetration in granular media.

Non-cohesive materials such as sand, dry snow or cereals are encountered in various common circumstances, from everyday situations to industry. The process of digging into these materials remains a challenge to most animals and machines. Within the animal kingdom, different strategies are employed to overcome this issue, including excavation methods used by ants, the two-anchor strategy employed by soft burrowers such as razor-clams, and undulatory motions exhibited by sandfish lizards. Despite the development of technology to mimic these techniques in diggers and robots, the limitations of animals and machines may differ, and mimicry of natural processes is not necessarily the most efficient technological strategy. This study presents evidence that the resisting force for the penetration of an intruder into a dry granular media can be reduced by one order of magnitude with small amplitude (A ≃ 10 µm) and low frequency (f = 50 - 200 Hz) mechanical vibrations. This observed result is attributed to the local fluidization of the granular bed which induces the rupture of force chains. The drop in resistive force on entering dry granular materials may be relevant in technological development in order to increase the efficiency of diggers and robots.

On the shape of giant soap bubbles.

We study the effect of gravity on giant soap bubbles and show that it becomes dominant above the critical size [Formula: see text], where [Formula: see text] is the mean thickness of the soap film and [Formula: see text] is the capillary length ([Formula: see text] stands for vapor-liquid surface tension, and [Formula: see text] stands for the liquid density). We first show experimentally that large soap bubbles do not retain a spherical shape but flatten when increasing their size. A theoretical model is then developed to account for this effect, predicting the shape based on mechanical equilibrium. In stark contrast to liquid drops, we show that there is no mechanical limit of the height of giant bubble shapes. In practice, the physicochemical constraints imposed by surfactant molecules limit the access to this large asymptotic domain. However, by an exact analogy, it is shown how the giant bubble shapes can be realized by large inflatable structures.

Are leaves optimally designed for self-support? An investigation on giant monocots.

Leaves are the organs that intercept light and create photosynthesis. Efficient light interception is provided by leaves oriented orthogonal to most of the sun rays. Except in the polar regions, this means orthogonal to the direction of acceleration due to gravity, or simply horizontal. The leaves of almost all terrestrial plants grow in a gravity field that tends to bend them downward and therefore may counteract light interception. Plants thus allocate biomass for self-support in order to maintain their leaves horizontal. To compete with other species (inter-species competition), as well as other individuals within the same species (intra-species competition), self-support must be achieved with the least biomass produced. This study examines to what extent leaves are designed to self-support. We show here that a basic mechanical model provides the optimal dimensions of a leaf for light interception and self-support. These results are compared to measurements made on leaves of various giant monocot species, especially palm trees and banana trees. The comparison between experiments and model predictions shows that the longer palms are optimally designed for self-support whereas shorter leaves are shaped predominantly by other parameters of selection.