S1E9
Episode 9 (September 6, 2020)
Xiaohao Sun
University of Colorado at Boulder
Lisa Lee
Harvard University
Yuan Ma
Texas A&M University
Thermomechanics and Particle-Fusion Modeling of Vitrimers
Abstract:
Vitrimer possesses covalently crosslinked network that is capable of the topological rearrangement with network integrity preserved through thermally activated bond exchange reactions, thus showing a variety of unique properties such as stress relaxation, self-healing and recycling. Mechanics study on these behaviors can be useful to its practical applications and will be the focus of this talk. First, we study the bulk thermo-mechanical behaviors of vitrimers through finite element methods. The effects of the non-uniform and evolving temperature field is explored, with a focus on the complex coupling between mechanical deformation, heat conduction and bond exchange reactions. We show that in a thermoforming process, the non-uniform heating causes the material to creep in the high-temperature region, leading to redistribution of the deformation field and thus a final shape deviating from the prescribed shape. Such deviation can be controlled by adjusting the heating region, time and temperature to achieve desired final shapes of the vitrimer in a thermoforming process. Second, to gain insights into the powder-based recycling of vitrimers, we study the consolidation and fusion of vitrimer particles under heat press. A computational model is presented for this process, where multiple mechanisms are captured: random particle packing, large deformation, inter-particle contact, and thermally activated bulk stress relaxation and interface healing. Using the model, we evaluate the fusion extent by the evolution of porosity during consolidation and the effective modulus and strength of fused vitrimer, and study how the fusion process depends on the particle size and processing conditions.
The Transition Between Rotation and Counterrotation in Swirled Granular Media
Abstract:
Granular material in a swirled container exhibits a curious transition as the number of particles is increased: at low densities the particle cluster rotates in the same direction as the swirling motion of the container, while at high densities it rotates in the opposite direction. We investigate this phenomenon experimentally and numerically using a co-rotating reference frame in which the system reaches a statistical steady-state. In this steady-state the particles form a cluster whose translational degrees of freedom are stationary, while the individual particles constantly circulate around the cluster's center of mass, similar to a ball rolling along the wall within a rotating drum. We show that the transition to counterrotation is friction-dependent. At high particle densities, frictional effects result in geometric frustration which prevents particles from cooperatively rolling and spinning. Consequently, the particle cluster rolls like a rigid body with no-slip conditions on the container wall, which necessarily counterrotates around its own axis. Numerical simulations verify that both wall-disc friction and disc-disc friction are critical for inducing counterrotation.
Modeling and Experimental Investigation of the Finger-Screen Interaction
Abstract:
With the commercialization of haptic devices, understanding behavior under various environmental conditions is crucial for product optimization and cost reduction. Specifically, for surface haptic devices, the dependence of the friction force and the electroadhesion effect on the environmental relative humidity and the finger hydration level can directly impact their design and performance. This talk presents finger-surface friction force with and without electroadhesion with various relative humidity. The random multi-capillary model is introduced to describe the capillary formation in the interface. Mechanisms including changes to Young's modulus of skin, contact angle change and capillary force will be discussed separately with experimental and numerical methods.