Dynamics of wrinkling in ultrathin elastic sheets

The wrinkling of thin elastic objects provides a means of generating regular patterning at small scales in applications ranging from photovoltaics to microfluidic devices. Static wrinkle patterns are known to be governed by an energetic balance between the object's bending stiffness and an effective substrate stiffness, which may originate from a true substrate stiffness or from tension and curvature along the wrinkles. Here we investigate dynamic wrinkling, induced by the impact of a solid sphere onto an ultra-thin polymer sheet floating on water. The vertical deflection of the sheet's centre induced by impact draws material radially inwards, resulting in an azimuthal compression that is relieved by the wrinkling of the entire sheet. We show that this wrinkling is truly dynamic, exhibiting features that are qualitatively different to those seen in quasi-static wrinkling experiments. Moreover, we show that the wrinkles coarsen dynamically because of the inhibiting effect of the fluid inertia. This dynamic coarsening can be understood heuristically as the result of a dynamic stiffness, which dominates the static stiffnesses reported thus far, and allows new controls of wrinkle wavelength.Comment: 8 pages, 4 figures. Please see published version for supplementary movies and SI Appendi

Turbulence-particle interactions on surfaces

Thesis: S.M., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 105-110).The physics of adhesion and detachment of particles in ventilation ducts is important to understand and control contaminant and pathogen dispersal indoors. This thesis presents an experimental characterization of parameters which affect the resuspension of settled micro-particles and spores in a turbulent airflow channel. We examine, quantify, and analyze the role of relative humidity (RH), air temperature, particle size, and surface properties on particle detachment rate and mode. This is done using a combination of high-speed imaging in a turbulent channel where spores and particles are deposited initially followed by image-processing and particle-tracking. First, we show that ambient moisture hinders particle detachment, however, we also find that this is only true for a relative humidity higher than 60% RH. At lower air saturation, we show that, instead, another effect dominates, leading to a different mode of detachment. Instead of individual particle detachment, it is a collision dynamics leading to cluster formation that dominates the pattern of detachment of particles from surfaces. We find that collisions lead to aggregations of particles on the surface in the form of clusters of self-similar sizes. We find that the larger the cluster (above 5 particles) the more anisotropic its shape, similarly to what was observed in prior literature examining clusters of air-suspended particles in channel flows. We examined and quantified the role of initial particle surface concentration, mean air velocity, and particle surface properties on these results. Our study have implications in the control of pathogen and contaminant dispersal in confined geometries, relevant for a wide range of applications.by Maxime Inizan.S.M