Impact of composite soft-rigid elastic projectiles: A numerical study.

We study by numerical simulation the impact of a one-dimensional composite projectile, composed of two superposed homogeneous parts, on an infinitely rigid and massive wall. The coefficient of restitution and the contact time are systematically measured as functions of the contrasts of mass and stiffness between the two parts. For purely elastic parts, these quantities show complex trends associated with different dynamics of the deformation waves propagating inside the projectile. A significant portion of the initial kinetic energy can be trapped in the deformation modes: the coefficient of restitution is lowest, about 0.2, when there is a strong stiffness contrast between the two parts and the stiff and soft parts are at the leading and trailing edges of the projectile respectively. In this case, we highlight the presence of multiple bounces, whose number increases as the proportion of the soft part increases. Finally, viscoelastic parts can be implemented in the same numerical framework to successfully recover the results obtained in real composite projectile impact experiments [D'Angelo et al., Phys. Rev. E 103, 053005 (2021)2470-004510.1103/PhysRevE.103.053005].

Atomistic Description of Interdroplet Ice-Bridge Formation during Condensation Frosting.

The propagation of frost in an assembly of supercooled dew droplets takes place by the formation of ice protrusions that bridge ice particles and still-liquid droplets. In this work, we develop a Kinetic Monte Carlo (KMC) model to study the formation kinetics of the ice protrusions. The KMC simulations reproduce well the experimental results reported in the literature. The elongation speed of the ice protrusions does not depend on the droplet size but increases when the interdroplet distance decreases, the temperature increases, or the substrate wettability increases. While 2D diffusion of the water molecules on the substrate surface is sufficient to explain the process kinetics, high 3D (vapor) water-molecule concentration can lead to the development of 3D lateral branches on the ice protrusions. A 1D analytical model based on the water-molecule concentration gradient between a droplet and a nearby ice particle reproduces well the simulation results and highlights the relation between the protrusion elongation kinetics and parameters like the interdroplet distance, the water diffusivity, and the concentration gradient. The bridge-formation time has a quadratic dependence on the droplet-ice distance. Comparing the simulations, the analytical model, and the experimental results of the literature, we conclude that the propagation of frost on a flat substrate in an assembly of supercooled dew droplets with interdroplet spacing larger than about 1 µm is limited by water-molecule diffusivity.

Use of compliant actuators for throwing rigid projectiles.

Muscles and tendons, actuators in robotics, and various sports implements are examples that exploit elasticity to accelerate objects. Tuning the mechanical properties of elastic elements connecting objects can greatly enhance the transfer of mechanical energy between the objects. Here, we study experimentally the throw of rigid projectiles by an actuator, which has a soft elastic element added to the distal end. We vary the thickness of the elastic layer and suggest a simple mass-spring chain model to find the properties of the elastic layer, which will maximize the energy transfer from the actuator to the projectile. The insertion of a soft layer, impedance matched to the ejection frequency of the projectile mass, can increase the throwing efficiency by over 400%. Finally, we identify that very thick and very soft compliant layers could potentially lead to high efficiency and flexibility simultaneously.

Dynamics of Frost Propagation on Breath Figures.

We investigate the process of condensation frosting on flat surfaces using thermal imaging microscopy. This method is particularly well-suited to characterize the frosting of polydisperse assemblies of dew droplets, also called breath figures, that transform into ice droplets by the propagation of frost fronts. The front propagation speed is found to be a nonmonotonous function of the characteristic droplet size of the breath figure. In our experimental conditions, the propagation speed is maximum around 70 µm s-1 for a characteristic droplet radius of around 300 µm. We mainly show that the frost propagation speed is governed by the competition between two characteristic time scales. The first one is the freezing time of individual droplets, and the other one is the formation time of interdroplet ice bridges that grow from frozen to liquid droplets. In addition, the experiments reveal that the mean ice bridge speed is constant regardless of the characteristic radius of the liquid droplets in the breath figure. A theoretical mean-field analysis without any adjustable parameters recovers all of the features of the front propagation observed in experiments.

Impact dynamics of composite elastorigid projectiles onto solid surfaces.

We investigate the impact of composite objects. They consist of a soft layer on top of a rigid part with a hemispherical impacting end. The coefficient of restitution (e) of such objects is studied systematically as a function of the mass ratio and of the nature of the materials. For rather elastic materials, the coefficient of restitution is a nonmonotonic function of the mass ratio and exhibits important variations. The dynamics of the impact can be characterized by several bounces depending on the ratios between the four timescales at play. These include the duration of contact of the rigid part with the substrate and the time for the elastic waves to travel back and forth in the soft layer. In that sense, describing these projectiles requires one to take into account both the Hertzian theory of contact and the elastic waves described by Saint-Venant's approach.

Cavitation Mean Expectation Time in a Stretched Lennard-Jones Fluid under Confinement.

We investigate the nucleation of cavitation bubbles in a confined Lennard-Jones fluid subjected to negative pressures in a cubic enclosure. We perform molecular dynamics (MD) simulations with tunable interatomic potentials that enable us to control the wettability of solid walls by the liquid, that is, its contact angle. For a given temperature and pressure, as the solid is taken more hydrophobic, we put in evidence, an increase in nucleation probability. A Voronoi tessellation method is used to accurately detect the bubble appearance and its nucleation rate as a function of the contact angle. We adapt classical nucleation theory (CNT) proposed for the heterogeneous case on a flat surface to our situation where bubbles may appear on flat walls, edges, or corners of the confined box. We finally calculate a theoretical mean expectation time in these three cases. The ratio of these calculated values over the homogeneous case is computed and compared successfully against MD simulations. Beyond the infinite liquid case, this work explores the heterogeneous nucleation of cavitation bubbles, not only in the flat surface case but for more complex confining geometries.

Dynamic Wetting Properties of Mesh Substrates with Tunable Water Adhesion.

In nature, wetting phenomena are present nearly everywhere and are a source of inspiration for liquid transportation. A good understanding of the underlying dynamic phenomena that governs wettability is therefore extremely important for researchers involved in bio-inspired surfaces. Herein, we study the adhesive behavior with water of mesh substrates modified with structured copolymers in order to tune the surfaces from parahydrophobic states (high water adhesion) to superhydrophobic states (low water adhesion). Using the ejection test method (ETM), a new technique that consists of the ejection of water droplets deposited onto a substrate with the aid of a catapult system, we experimentally demonstrate that the elasticity of the mesh substrate can be exploited for efficient vertical actuation of droplets.

Ferrofluid Leidenfrost droplets.

We experimentally investigate the behavior of ferrofluid Leidenfrost droplets subject to a static magnetic field gradient. The droplets are deposited on a hot substrate and trapped over the vertical axis of a permanent magnet placed at a distance d above the substrate. Several effects are observed. Firstly, the droplet evaporation rate is strongly influenced by the distance d. Secondly, the droplet takes off from the substrate when its radius decreases to a critical value. The introduction of an effective gravity, which accounts for the magnetic force, allows a successful description of these effects. Finally, we observe an instability for which the droplet starts bouncing with irregular amplitudes. This behavior is qualitatively interpreted by introducing the synchronization of the free fall time between successive bounces with the period of the fundamental vibration mode of the droplet.

Experimental Characterization of Droplet Adhesion: The Ejection Test Method (ETM) Applied to Surfaces with Various Hydrophobicity.

We study the wetting and the adhesive behavior of substrates made by electropolymerization of copolymers of pyrene substituted with fluoroalkyl and adamantyl groups. The hydrophobicity and water adhesion properties can be tuned by the molar percentage (mol %) of each pyrene monomer so that the substrate properties can vary from superhydrophobic to parahydrophobic, with respectively low and high water adhesion. The ejection test method (ETM) is proposed as an original tool to discriminate and characterize such substrates. Using a catapult-like apparatus, a droplet initially at rest on the surface is subject to a large acceleration and is subsequently ejected. Depending on the surface properties and initial catapult acceleration, the ejection is more or less efficient and occurs with or without fragmentation of the droplet. The ETM is shown to be a complementary test to the lateral adhesion and hysteresis classical measurements. This work is of importance for the understanding of adhesion phenomena on various surfaces and for a better quantitative characterization of their adhesive properties.

Superpropulsion of Droplets and Soft Elastic Solids.

We investigate the behavior of droplets and soft elastic objects propelled with a catapult. Experiments show that the ejection velocity depends on both the projectile deformation and the catapult acceleration dynamics. With a subtle matching given by a peculiar value of the projectile/catapult frequency ratio, a 250% kinetic energy gain is obtained as compared to the propulsion of a rigid projectile with the same engine. This superpropulsion has strong potentialities: actuation of droplets, sorting of objects according to their elastic properties, and energy saving for propulsion engines.

Stability of the hydrophilic and superhydrophobic properties of oxygen plasma-treated poly(tetrafluoroethylene) surfaces.

Poly(tetrafluoroethylene) (PTFE) materials were exposed to low and high-energy oxygen plasma, and the stability of the materials' surface was evaluated using contact angle, surface roughness, and surface chemistry characterizations. Lower-energy oxygen plasma treatments exhibited hydrophilic behavior with contact angles as low as 87°, and the higher-energy oxygen plasma treatments exhibited superhydrophobic behavior with contact angles as high as 151°. The wettability of all the treated samples as stored in air and in water was found to be stable in time as evidenced by the statistically insignificant differences in the advancing, receding, and hysteresis contact angles. Low contact angle hysteresis (θH<5°) and low sliding angle (α≈4°) were exhibited by the superhydrophobic surface. The surface morphology was found to be responsible for the changes in the wettability of the PTFE samples since (1) there was an increase in the surface rms roughness as the plasma discharge energy was increased, and (2) there were no significant changes in the observed group frequencies of the FT-IR spectra of the treated PTFE from the untreated PTFE.

Room temperature water Leidenfrost droplets.

We experimentally investigate the Leidenfrost effect at pressures ranging from 1 to 0.05 atmospheric pressure. As a direct consequence of the Clausius­Clapeyron phase diagram of water, the droplet temperature can be at ambient temperature in a non-sophisticated lab environment. Furthermore, the lifetime of the Leidenfrost droplet is significantly increased in this low pressure environment. The temperature and pressure dependence of the evaporation rate is successfully tested against a recently proposed model. These results may pave the way for reaching efficient Leidenfrost micro-fluidic and milli-fluidic applications.

Jet impact on a soap film.

We experimentally investigate the impact of a liquid jet on a soap film. We observe that the jet never breaks the film and that two qualitatively different steady regimes may occur. The first one is a refractionlike behavior obtained at small incidence angles when the jet crosses the film and is deflected by the film-jet interaction. For larger incidence angles, the jet is absorbed by the film, giving rise to a new class of flows in which the jet undulates along the film with a characteristic wavelength. Besides its fundamental interest, this paper presents a different way to guide a micrometric flow of liquid in the inertial regime and to probe foam stability submitted to violent perturbations at the soap film scale.

Take off of small Leidenfrost droplets.

We put in evidence the unexpected behavior of Leidenfrost droplets at the later stage of their evaporation. We predict and observe that, below a critical size Rl, the droplets spontaneously take off due to the breakdown of the lubrication regime. We establish the theoretical relation between the droplet radius and its elevation. We predict that the vapor layer thickness increases when the droplets become smaller. A satisfactory agreement is found between the model and the experimental results performed on droplets of water and of ethanol.

Ratchetlike motion of a shaken drop.

We study the motion of a drop lying on a plate simultaneously submitted to horizontal and vertical harmonic vibrations. The two driving vibrations are adjusted to the same frequency, and, according to their relative amplitude and phase difference DeltaPhi, the drop experiences a controlled directed motion with a tunable velocity. We present a simple model putting in evidence the underlying mechanism leading to this ratchetlike motion of the drop. Our model includes the particular case DeltaPhi=pi corresponding to the climbing of a drop on a vertically vibrated inclined substrate, as recently observed by Brunet et al. [Phys. Rev. Lett. 99, 144501 (2007)10.1103/PhysRevLett.99.144501]. This study gives insights in the fundamental issue of wetting dynamics and offers new possibilities of controlled motion in droplet microfluidics applications.

Cosine law at the atomic scale: toward realistic simulations of Knudsen diffusion.

We propose to reconsider the diffusion of atoms in the Knudsen regime in terms of a complex dynamical reflection process. By means of molecular dynamics simulations, we emphasize the asymptotic nature of the cosine law of reflection at the atomic scale, and carefully analyze the resulting strong correlations in the reflection events. A dynamical interpretation of the accommodation coefficient associated with the slip at the wall interface is also proposed. Finally, we show that the first two moments of the stochastic process of reflection depend nonuniformly on the incident angle.

Vibration of submillimeter-size supported droplets.

We study the fundamental vibration mode of supported submillimeter-size droplets. Using an analogy with a simple oscillator we derive a semianalytical expression for the eigenfrequency and the scaling law of the energy dissipation within the droplet. The experimental results obtained for mercury drops deposited on glass are compared with the model. The agreement is satisfactory for the eigenfrequencies on the whole range of size we considered (from 0.04 to 0.9 mm). The scaling law for the dissipation is recovered for radii larger than 0.1 mm but fails for smaller droplets. We finally discuss possible applications related to the use of vibrations to effectively reduce the hysteresis of the wetting angle and therefore increase the mobility of the supported droplets.

Stretching an adsorbed polymer globule.

Using molecular dynamic simulation, we study the stretching of an adsorbed homopolymer in a poor solvent with one end held at a distance ze from the substrate. We measure the vertical force f on the end of the chain as a function of the extension ze and the substrate interaction energy w. The force reaches a plateau value at large extensions. In the strong adsorption limit, we show that the plateau value increases linearly in w in good agreement with a theoretical model. In the weak adsorption limit, a polymer globule with a layered structure is formed and elastically deformed when stretched. In both cases a simple theoretical model permits us to predict the relation between the necessary force to fully detach the polymer and its critical extension.

Phase-field approach for faceted solidification.

We extend the phase-field approach to model the solidification of faceted materials. Our approach consists of using an approximate gamma plot with rounded cusps that can approach arbitrarily closely the true gamma plot with sharp cusps that correspond to faceted orientations. The phase-field equations are solved in the thin-interface limit with local equilibrium at the solid-liquid interface [A. Karma and W.-J. Rappel, Phys. Rev. E 53, R3017 (1996)]. The convergence of our approach is first demonstrated for equilibrium shapes. The growth of faceted needle crystals in an undercooled melt is then studied as a function of undercooling and the cusp amplitude delta for a gamma plot of the form gamma=gamma0[1+delta(/sin theta/+/cos theta/)]. The phase-field results are consistent with the scaling law Lambda approximately V(-1/2) observed experimentally, where Lambda is the facet length and V is the growth rate. In addition, the variation of V and Lambda with delta is found to be reasonably well predicted by an approximate sharp-interface analytical theory that includes capillary effects and assumes circular and parabolic forms for the front and trailing rough parts of the needle crystal, respectively.

Measuring kinetic coefficients by molecular dynamics simulation of zone melting.

Molecular dynamics simulations are performed to measure the kinetic coefficient at the solid-liquid interface in pure gold. Results are obtained for the (111), (100), and (110) orientations. Both Au(100) and Au(110) are in reasonable agreement with the law proposed for collision-limited growth. For Au(111), stacking fault domains form, as first reported by Burke, Broughton, and Gilmer [J. Chem. Phys. 89, 1030 (1988)]. The consequence on the kinetics of this interface is dramatic: the measured kinetic coefficient is three times smaller than that predicted by collision-limited growth. Finally, crystallization and melting are found to be always asymmetrical and here again the effect is much more pronounced for the (111) orientation.