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15 juin 2017: 1 événement

  • Séminaires Invités

    Jeudi 15 juin 16:00-17:30 - Douglas P. HOLMES - Mechanical Engineering, Boston University

    Morphing of Sheets and Shells

    Résumé : Swelling-induced deformations of slender structures occur in many biological and industrial environments, and the shapes and patterns that emerge can vary across many length scales. The dynamics of fluid movement within elastic networks, and the interplay between a structure’s geometry and its boundary conditions, play a crucial role in the morphology of growing tissues, the shrinkage of mud and moss, and the curling of cartilage, leaves, and pine cones. We aim to utilize swelling-induced deformations of soft mechanical structures to dynamically shape materials. Adaptive structures that can bend and fold in an origami-like manner provide advanced engineering opportunities for deployable structures, soft robotic arms, mechanical sensors, and rapid-prototyping of 3D elastomers.
    Swelling is a robust approach to structural change as it occurs naturally in humid environments and can easily be adapted into industrial design. Small volumes of fluid that interact favorably with a material can induce large, dramatic, and geometrically nonlinear deformations. This talk will examine the geometric nonlinearities that occur as slender structures are swollen – surfaces will crease, beams will bend and snap, circular plates will warp and twist, and fibers will coalesce and detach. I will describe the intricate connection between materials and geometry, and present a straightforward means to permanently morph 2D sheets into 3D shapes.

    contact : Catherine Quilliet

    Lieu : LIPhy, conference room - 140 Avenue de la Physique 38402 Saint Martin d’Hères

    En savoir plus : Séminaires Invités

15 juin 2017: 1 événement

  • Soutenances de Thèse/HDR

    Jeudi 15 juin 10:00-12:30 - Adel Djellouli

    [PhD defence] Swimming through shell buckling

    Résumé : "Microswimmers, and among them aspirant microrobots, are generally bound to cope with flows where viscous forces are dominant, characterized by a low Reynolds number (Re). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities.
    Here, we propose a swimming mechanism which uses the buckling instability triggered by pressure waves to propel a spherical hollow shell. The particularity of this mechanism is that it fulfills naturally
    the necessary condition of swimming at low Re. In addition, the swiftness of the instability might produce inertial effects even at the microscopic scale.
    With a macroscopic experimental model we show that a net displacement is produced at all Re regimes. We put in evidence the role of geometrical parameters, shell material properties and rheology of the surrounding
    fluid on the swimming efficiency. An optimal displacement is reached at intermediate Re. Using time-resolved PIV measurements, we explain that non-trivial history effects take place during the instability and enhance net displacement. Using a simple model, validated by the study of shell dynamics, we show that due to the fast activation induced by the instability, this regime would be reachable by microscopic shells. The rapid dynamics
    would also allow high frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1 cm/s for a remote controlled microrobot, a suitable value
    for biological applications such as drug delivery."

    En savoir plus : Soutenances de Thèse/HDR