Numerical Investigation of Evaporation Induced Self-Assembly of Sub-Micron Particles Suspended in Water

Self-assembly of sub-micron particles suspended in a water film is investigated numerically. The liquid medium is allowed to evaporate leaving only the sub-micron particles. A coupled CFD-DEM approach is used for the simulation of fluid-particle interaction. Momentum exchange and heat transfer between particles and fluid and among particles are considered. A history dependent contact model is used to compute the contact force among sub-micron particles. Simulation is done using the open source software package CFDEM which basically comprises of two other open source packages OpenFOAM and LIGGGHTS. OpenFOAM is a widely used solver for CFD related problems. LIGGGHTS, a modification of LAMMPS, is used for DEM simulation of granular materials. The final packing structure of the sub-micron particles is discussed in terms of distribution of coordination number and radial distribution function (RDF). The final packing structure shows that particles form clusters and exhibit a definite pattern as water evaporates away

Study of micro-sized particle deposition using DEM, CFD-DEM and SPH approach

Self-assembly and packing of colloids and micro or nano scale particles has become a subject of great interest due to widespread advancement of micro-scale technologies. In this thesis, several numerical analyses are performed to study the packing or self-assembly of micro or nano sized particles under dry or wet condition. Part one of the thesis is concerned with DEM simulation of micro-sized cohesive granular particles using two history dependent contact models. The simulation results are presented using porosity, coordination number, RDF and force distribution. It was observed that the particles with Gaussian distribution always have the lowest packing density while the mono-sized particles normally have the highest packing density. For cohesive particles, size distributions result in the same tendency of packing density but has much less variation with particle size. There is no significant effect of cohesion on coordination number but particle size and size distribution do influence the result. The differences in porosity, coordination number, RDF and magnitude of mean net force between the two models used are not substantial which show that any of the models can be used for simulation of particle packing. However, the Gran-Hooke-History model is found to be more efficient than the Modified Gran-Hertz-History model. In the second part self-assembly of micro-sized particles induced by evaporation is numerically investigated. The problem involves interaction between solid and fluid as well as interaction between fluids. The problem also involves phase change. A coupled CFD-DEM method is used to simulate the multiphase system. In the simulation liquid water film evaporates and leaves the particles at the container alone. Interesting patterns are seen to emerge as the liquid water film evaporates. The resulting packing structure analyzed in terms of the range of coordination number and radial distribution function also indicate the self-assembly of the particles. In the third part of the thesis low velocity SPH method developed by Seo et al., is used to simulate the cyclic packing of deformable two dimensional disks and study their packing behavior. The results obtained show that the average coordination number varies with packing fraction during jamming by conforming to the isostatic conjecture. Stress relaxation is seen to occur after several compression cycles which is marked by a decrease in coordination number and global pressure. Force distribution shows similar exponential behavior as the average force on the system is increased. The SPH method is also adapted to include the elasto-plastic behavior of materials. In future, the present work can be extended to include the contact friction of the particles. A method to achieve this is also shown by applying virtual work principal and penalty method

Numerical study of self-assembly of granular and colloidal particles

Self-assembly of granular materials and colloids are studied using several different computational methods such as Discrete Element Method (DEM), Smoothed Particle Hydrodynamics (SPH) method, finite volume Volume of Fluid and DEM (VOF-DEM) method and coupled VOF-Level Set and Dissipative Particle Dynamics (CVOFLS-DPD) method. A history dependent contact model is developed for the DEM and a cohesion model is introduced to study the packing of granular materials under cohesive forces. The study reveals granular size and size distribution has an important effect on the final packing structure. The study using SPH method reveals stress relaxation in a granular system subjected consecutive jamming cycles. However, above a certain initial packing fraction stress relaxation is found to be negligible. Further analysis reveals characteristics length and time scales for stress relaxation. Three-cycle basis is found to be the most preferred configuration of the particles as the granular system drives towards a more stable state. The study using VOF-DEM method reveals pattern formation by colloidal deposits as a thin film of fluid evaporates. Further analysis with CVOFLS-DPD method reveals interface forces on particles need to be carefully modeled to prevent escaping of particles during evaporation. The use of machine learning (ML) for computational study is also explored in this study. A machine-learned sub-grid scale (SGS) modeling technique is introduced for efficient and accurate prediction of reactants and products undergoing parallel competitive reactions in a bubble column. The machine-learned model replaces the iterative approach associated with the use of analytical profiles for previous sub-grid scale models for correcting concentration profiles in boundary layers.Includes bibliographical references