Title: Particle Dynamics Simulation of Microstructure Evolution towards Functionally Graded Material Manufacturing
Description: Functionally graded materials (FGMs) have attracted significant attention in academia and industry, because of its unique material properties, such as thermal, electrical, or mechanical properties, which are characterized by the continuous spatial variation in macroscale due to the graded distribution of material phases in the microstructure. In our recent invention, a hybrid solar panel has been fabricated to integrate a FGM layer with water tubes cast inside between the photovoltaic (PV) cell and the substrate, so that the thermal harvest efficiency can be substantially increased, as well as the structural integrity and material efficiency. To scale up the technology from the laboratory to mass production, sedimentation-based methods (static sedimentation and vibration-sedimentation) have been explored as an efficient and economic manufacture method under different solid load, in which aluminum (Al) and high-density polyethylene (HDPE) powders are mixed in a liquid and a graded mixture can be obtained at the end of the sedimentation process.

To optimize the manufacturing process, a better understanding of the physics lying behind the experimental observations is required. However, the conventional continuum methods may not be applicable due to the complex fluid effect and particle-particle interactions in this huge many-particle system. My Ph.D. studies aim to develop particle based approaches for fundamental understanding of particle interactions and optimization of the manufacturing process of FGMs. Due to its natural advantage to capture physics at a fine scale and capability to address complex fluid effect and boundary problems, particle based methods are particularly suitable to simulate the fabrication process with a large number of particles. Dissipative particle dynamics (DPD) was used to study the interactions between liquid particles. In this work, we proposed a reduced rough sphere model to use the DPD to understand the interactions between liquid and solid particles and thus simulate the hydrodynamic behavior of solid particle moving in the DPD liquid.

And discrete element method (DEM) is used to describe the interaction between solid particles. The DPD/DEM hybrid model has successfully simulated the static sedimentation process and revealed the underlying physicist behind the sedimentation approach for the solid load less than 10 vol%. For a higher solid load, particle jamming is commonly observed if the solid-liquid particle size ratio is not large enough, which however makes the computation cost formidably expensive. Therefore, a modified DEM model is developed, in which liquid particles are disregarded but a drag force as a function of porosity is introduced to simulate the particle-fluid interaction. Therefore, the vibration-sedimentation of a high solid load suspension can be successfully simulated. The particle-based methods are general and can be applied in particle mixing as well, which also involves the microstructure evolution and has a wide application in the pharmaceutical industry, food processing, energetic materials, as well as many other industries.

To accelerate the mixing rate, normally the liquid binder is used to wet the granular composite, in which the liquid content is close to zero. The cohesion effect is reported to play an important role in the mixing process. As the volume fraction of the liquid binder increases, the cohesion accelerates the mixing process and enhances the homogeneity of the mixture, however, if it continuously increases, extremely large cohesion force may prevent the mixture from being mixed and segregation occurs. To investigate the high shear mixing process, DEM model is developed to simulate the interaction between solid particles, while the liquid bridge model, which implements the capillary force in the particulate method, is used to describe the cohesion effect. Quantitative and comprehensive studies are performed to investigate the effect of the liquid binder's volume fraction on the mixing rate and homogeneity of the final mixture. The results have shown that there exists a critical volume fraction of the liquid binder to achieve a good homogeneity in the mixture when the filling height is comparable to the blades height.

When the filling level is high, cohesion slows down the mixing process and decreases the mixing quality.