The role of inter-particle friction on rheology and texture of wet granular flows

The combined effect of cohesion and finite size on the collapse of wet granular columns

The collapse of low-saturation liquid-containing granular materials is prevalent in nature and industrial processes, and understanding the associated transient dynamics is extremely important for exploring such complex flow processes. In this paper, the collapse of a finite-size wet granular column is systematically studied and the determinants affecting its dynamics are analyzed based on the discrete element model for wet particles and the corresponding small-scale experiments. With the aid of parametric analysis, the dimensionless cohesion parameter containing the system size and grain-scale bond number is proposed, and its relevance in characterizing column stability and collapse dynamics of wet granular materials is further confirmed. For the collapse of wet granular columns with a fixed aspect ratio, the initial height contained in the cohesion parameter is verified to be a manifestation of the finite size effect, which is present in a wet granular collapse and is coupled with the cohesive effect. Such a coupling effect is taken into account in our proposed scaling laws that can be applied to uniformly describe the deposit morphology of wet granular columns after collapse.

Effects of Particle Size Distribution with Efficient Packing on Powder Flowability and Selective Laser Melting Process

The powder bed-based additive manufacturing (AM) process contains uncertainties in the powder spreading process and powder bed quality, leading to problems in repeatability and quality of the additively manufactured parts. This work focuses on identifying the uncertainty induced by particle size distribution (PSD) on powder flowability and the laser melting process, using Ti6Al4V as a model material. The flowability test results show that the effect of PSDs on flowability is not linear, rather the PSDs near dense packing ratios cause significant reductions in flowability (indicated by the increase in the avalanche angle and break energy of the powders measured by a revolution powder analyzer). The effects of PSDs on the selective laser melting (SLM) process are identified by using in-situ high-speed X-ray imaging to observe the melt pool dynamics during the melting process. The results show that the powder beds made of powders with dense packing ratios exhibit larger build height during laser melting. The effects of PSD with efficient packing on powder flowability and selective laser melting process revealed in this work are important for understanding process uncertainties induced by feedstock powders and for designing mitigation approaches.

Scaling behavior of the tensile strength of viscocohesive granular aggregates

We numerically analyze the tensile strength of a single wet agglomerate modeled as a viscocohesive aggregate impacting a flat surface by using the discrete-element simulations. The viscocohesive agglomerate composed of primary spherical particles with the inclusion of the interstitial liquid in the form of the capillary bridges characterized by the cohesive and viscous forces between particles is extracted from a cuboidal sample of granular materials by applying a spherical probe. The tensile strength is measured from the impact test of a wet agglomerate by systematically varying different values of the surface tension of the interstitial liquid, the liquid viscosity, and the impact speed. We show that the tensile stress increases immediately when the collision occurs between the agglomerate and the flat surface. The peak of the tensile stress obtained after the collision, then decreases smoothly with increasing the particle movement. The maximum tensile stress is defined to be the tensile strength of such agglomerate. It is remarkable that the normalized tensile strength of such agglomerate can be well described as a function of a dimensionless impact number that incorporates the capillary number and Stokes number (calculated from the surface tension and the viscosity of the liquid and the impact rate of the agglomerate), thus providing the confirmation for the unified representation of the liquid properties and the impact rate of wet granular media.