Mechanism comparisons of transport-deposition-reentrainment between microplastics and natural mineral particles in porous media: A theoretical and experimental study.

The migration and distribution of microplastic particles (MPs) in the natural environment has attracted global attention in recent years. However, little is known about the transport-deposition-reentrainment differences between MPs and natural mineral particles in porous media. In this study, polystyrene (PS) and silica (SiO2) particles, representing model MPs and natural mineral particles, respectively, were selected to study the responses of different particle types to changes in specific particle size and flow velocity. Three typical particle sizes and various flow velocities were chosen to compare and delineate the transport-deposition-reentrainment characteristics of PS and SiO2 in a packed-bed laboratory column. Collector efficiency was calculated using Tufenkji and Elimelech (TE) equation. The particle fractions released from the collector surfaces were predicted using DLVO theory and force analysis. Two types of particles were attached in the secondary minimum, which were either retained on the collector surface or reentrained to the fluid. The staged elution experiment wherein the flow velocity was increased experienced a period of flow shock, thus breaking the force balance of the particle. An increase in the flow velocity resulted in various degrees of particle elution. The breakthrough experiment at a specific flow velocity showed that the corresponding velocity alteration in staged elution experiment contributed to reentrainment to varying extents. When the effect of gravity on particle deposition was negligible, the particle size was larger, and the lower the velocity for releasing the particles. However, the opposite tendency was observed when considering the effect of gravity on particle deposition. Moreover, the deposition, mainly due to gravity, easily causes particle reentrainment as the flow velocity increases. This study further predicts and reveals the nature of transport and deposition differences between MPs and natural mineral particles, which helps to further assess the risk and potential of groundwater contamination with MPs of different sizes.

Critical instability at moving keyhole tip generates porosity in laser melting.

Laser powder bed fusion is a dominant metal 3D printing technology. However, porosity defects remain a challenge for fatigue-sensitive applications. Some porosity is associated with deep and narrow vapor depressions called keyholes, which occur under high-power, low-scan speed laser melting conditions. High-speed x-ray imaging enables operando observation of the detailed formation process of pores in Ti-6Al-4V caused by a critical instability at the keyhole tip. We found that the boundary of the keyhole porosity regime in power-velocity space is sharp and smooth, varying only slightly between the bare plate and powder bed. The critical keyhole instability generates acoustic waves in the melt pool that provide additional yet vital driving force for the pores near the keyhole tip to move away from the keyhole and become trapped as defects.