S3E8

Qiong Zhang

Hongyi Xiao

Siavash Monfared

Abstract of Talk 1

Empirical bedload sediment transport expressions commonly over- or underpredict sediment flux by more than a factor of two, even under controlled laboratory conditions. In this work, the Discrete Element Method and Lattice Boltzmann Method are coupled together to simulate 3D fluid-driven transport problems, in which the spherical sediment particles are fully resolved. After comparisons with flume experiments are made to test the numerical simulations, the grain-scale physics is studied, such as the flow field around individual particles and higher order descriptions of the granular motion. A more robust continuum model, unifying empirical models under various conditions and in different regimes, is further proposed based on the new grain-scale understanding of the mechanisms.

Biosketch of Speaker 1

Abstract of Talk 2

Understanding the interplay between plastic deformation and local structural change is important for disordered solids. In this study, quasi-static tensile experiments were performed using a monolayer of polydisperse granular spheres floating at an air-oil interface that induces capillary attractions between particles. Under tensile deformation, the strain in the monolayer localizes into an inclined shear band, upon which failure occurs, and the ductility of the monolayer can be tuned by controlling the capillary interactions via the particle size. Analysis of the local packing anisotropy indicates a strong structural signal in the shear band region in early stage deformation, which is a result of the interactions between local structure and local rearrangements. To capture such interactions, we used machine learning methods to develop a scalar field, softness, which indicates the likelihood of a certain structure to rearrange. Microscopic interactions between local rearrangements and their nearby softness field were extracted from the experimental results and were used to inform a structro-elasto-plastic model that can capture shear band formation and the brittle-to-ductile transition.

Biosketch of Speaker 2

Abstract of Talk 3

Utilizing a 3D mean-field lattice-gas model, we study capillary phase transition in granular aggregates and their inverse porous structures exhibiting various degrees of structural disorder. We find that the degree of confinement is related to the surface-surface correlation length with a connected path through the fluid domain. This entails insignificant fluid confinement in granular media as this correlation length approaches the bulk in contrast with more pronounced confinement in porous solids. The critical exponents estimated from finite-size scaling analysis map the liquid-gas transition inside these porous materials onto the three-dimensional random field Ising model universality class as hypothesized by F. Brochard and P.G. de Gennes. We attribute the underlying random fields to the local disorder in the intensity of fluid-solid interactions as a function of pore wall separation. Lastly, we demonstrate that the liquid-gas phase transition is of second order nature near capillary critical temperature Tcc and that Tcc represents a true critical temperature, independent of the degree of disorder and the nature of solid matrix, discrete or continuous.

Biosketch of Speaker 3