Vendredi 26 septembre 2025 à 11h ; salle 250 - IUSTI
Abstract: Elastic helical rods offer a simple yet powerful model for exploring complex nonlinear phenomena observed in everyday physical systems. When these rods are unwound, they form a topological defect known as a perversion, which connects helices of opposite chirality—transitioning from a right-handed to a left-handed helix—similar to the tendrils of climbing plants such as cucumbers and vines. Upon rotation, the perversion can travel along the rod, enabling a complete reversal of the helix’s chirality.
In the first part of the talk, we will show that this phenomenon can be captured by a biphasic model and can be understood through an analogy with phase transitions: the system transitions from a purely right-handed (positive chirality) to a purely left-handed (negative chirality) state via a mixed state. This mixed state, where both chiralities coexist, is characteristic of the mature shape of plant tendrils.
In the second part of the talk, we integrate the Kirchhoff theory of rod writhing with the Lockhart model of plant growth to develop a simplified morphoelastic framework that describes tendril evolution under axial constraints. Our model reproduces key experimental observations, including the sudden onset of coiling associated with the writhing instability.