Loïc Tadrist

Wind and gravity mechanical effects on leaf inclination angles

In a tree, the distribution of leaf inclination angles plays an important role in photosynthesis and water interception. We investigate here the effect of mechanical deformations of leaves due to wind or their own weight on this distribution. First, the specific role of the geometry of the tree is identified and shown to be weak, using models of idealized tree and tools of statistical mechanics. Then the deformation of individual leaves under gravity or wind is quantified experimentally. New dimensionless parameters are proposed, and used in simple models of these deformations. By combining models of tree geometry and models of leaf deformation, we explore the role of all mechanical parameters on the Leaf Inclination Angle Distributions. These are found to have a significant influence, which is exemplified finally in computations of direct light interception by idealized trees.

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.

Faraday instability and subthreshold Faraday waves: surface waves emitted by walkers

A walker is a fluid entity comprising a bouncing droplet coupled to the waves that it generates at the surface of a vibrated bath. Thanks to this coupling, walkers exhibit a series of wave-particle features formerly thought to be exclusive to the quantum realm. In this paper, we derive a model of the Faraday surface waves generated by an impact upon a vertically vibrated liquid surface. We then particularise this theoretical framework to the case of forcing slightly below the Faraday instability threshold. Among others, this theory yields a rationale for the dependence of the wave amplitude to the phase of impact, as well as the characteristic timescale and length scale of viscous damping. The theory is validated with experiments of bead impact on a vibrated bath. We finally discuss implications of these results for the analogy between walkers and quantum particles.

Interaction of two walkers: Perturbed vertical dynamics as a source of chaos

Walkers are dual objects comprising a bouncing droplet dynamically coupled to an underlying Faraday wave at the surface of a vibrated bath. In this paper, we study the wave-mediated interaction of two walkers launched at one another, both experimentally and theoretically. Different outcomes are observed in which either the walkers scatter or they bind to each other in orbits or promenade-like motions. The outcome is highly sensitive to initial conditions, which is a signature of chaos, though the time during which perturbations are amplified is finite. The vertical bouncing dynamics, periodic for a single walker, is also strongly perturbed during the interaction, owing to the superposition of the wave contributions of each droplet. Thanks to a model based on inelastic balls coupled to the Faraday waves, we show that this perturbed vertical dynamics is the source of horizontal chaos in such a system.

Bilayer curling and winding in a viscous fluid

Induced spontaneous curvature is a new mechanism of microcapsules bursting by nucleation and growth of a hole surrounded by a curling rim. Here we study the dynamics of curling on a macroscopic scale induced by the elastic curvature of a bilayer of tracing paper and tape after soaking by water. The ribbon which is fully stretched at time t = 0 is immersed in a viscous oil. We observe the winding from the free end and find two regimes: rolling at constant velocity at short times, and a slowdown at long times. We interpret these two regimes by a balance of the elastic driving force and the friction force on the roll.

Foliage motion under wind, from leaf flutter to branch buffeting

The wind-induced motion of the foliage in a tree is an important phenomenon both for biological issues (photosynthesis, pathogens development or herbivory) and for more subtle effects such as on wi-fi transmission or animal communication. Such foliage motion results from a combination of the motion of the branches that support the leaves, and of the motion of the leaves relative to the branches. Individual leaf dynamics relative to the branch, and branch dynamics have usually been studied separately. Here, in an experimental study on a whole tree in a large-scale wind tunnel, we present the first empirical evidence that foliage motion is actually dominated by individual leaf flutter at low wind velocities, and by branch turbulence buffeting responses at higher velocities. The transition between the two regimes is related to a weak dependence of leaf flutter on wind velocity, while branch turbulent buffeting is strongly dependent on it. Quantitative comparisons with existing engineering-based models of leaf and branch motion confirm the prevalence of these two mechanisms. Simultaneous measurements of the wind-induced drag on the tree and of the light interception by the foliage show the role of an additional mechanism, reconfiguration, whereby leaves bend and overlap, limiting individual leaf flutter. We then discuss the consequences of these findings on the role of wind-mediated phenomena.