Computer Simulation On Self-assembly Behavior Of Two Dimensional Anisotropic Colloid Particle Systems

The self-assembly behavior(including phases and dynamics)in two-dimensional(2D)systems is afundamentalsubjectin condensed matter physics and statistical physics.2D colloidal systems show rich phase behavior including novel phase structures.The dynamics of 2D colloid systems is also easily to be tracked and visualized,thus providingreference systems for understandingsimilar features in more complex 3D systems.However,it remains challenging to predict the self-assembly behavior of any certain 2D colloid system even for particles as simple as disks or spheres.A full understanding of the relationship of the self-assembly behavior and particle properties particularly the shape of particles is needed.To this end,in this thesis,we employ Monte Carlo(MC)and molecular dynamical(MD)simulation methods to study the self-assembly behaviors of different 2D hard anisotropic particles,including the phase behavior of rounded hexagon and kite shaped particle systems and dynamics of rounded square and annular sector particle(ASP)systems.In 2D rounded hexagon systems,the experimental results show a phase sequence of isotropic(I)-hexagonal rotator crystal(RX)-hexagonal crystal(HX)-frustrated hexagonal crystal(FHX),which is certainly different from perfect hexagons due to the effect of corner roundness.To get a better understanding of the effect of corner roundness on the phase behavior of rounded hexagons,we systematically studied the phase behaviors of hexagons with different corner-roundness by MC simulations.The results show that the corner roundness of hexagon weakens the coupling between the translationalandthe orientational order of particles(i.e.between the translationalandthe rotational motion of particles),and the confinementof translational motion always takes place before rotational motion becomes confined as the system is gradually condensed(i.e.,the phase transition appears at a lower surface fraction for translational order than that for orientational order).The more rounded the hexagon is,the more dramatic the decoupling is.Hard kite-shaped particles is an important class of shapes in 2D system in condensed matter physics.We systematically investigated the melting phase transition of a series of kite-shaped particles with different apex angle α(54°≤α≤144°)of 72°-α-(144°-α/2)-(144°-α/2)kite using MC simulation.We show that tetratic-ordered phase emerge in these systems which particles are rotationally asymmetric and have opposite 72° and α≈90° internal angles.A new tetragonal rectangular crystal(TRX)phase possessing(quasi-)long-range 4-fold molecular-orientational order and translational order are observed.Finally,we introduce local polymorphic configurations of neighboring particle pairs(LPC-NPPs)method to understand the observed phase behavior,and apply it to other tested systems successfully.Besides structures,dynamics of rounded square systems and ASP systems are also investigated by MD simulations.In rounded square systems,we observed three types of translational collective motion pattern in different corner-rounded square systems,including hopping,gliding and a mixture of gliding and hopping.To explain the observed phenomena,a simple geometrical model is also proposed.In the ASP systems,by MD simulations,we find the dynamics of ASPs are closely related to the dimerization reaction of particles in system.In all range window of density we tested,from low density in isotropic state to high density in near jamming state,the dynamics of ASPs are all heterogeneous:in low density region(ΦA<0.30),based on the different diffusion patterns between dimer and monomer state particles,system shows a weak heterogeneity on dynamics;in high density region(ΦA>0.30),particles with weak motion ability(slow particles)always form the core of heterogeneous motion,and particles with strong motion ability(fast particles)flows surround those cores,which serve for the multi-distribution of characteristic time scale in system.Particles near the contact lines of slow particles and fast particles always provided more opportunity to participate reactions.In summary,the investigations of the self-assembly behaviors of 2D anisotropic colloid systemsare reported in this thesis.By performing MC simulations,we demonstrate that it is possible to control the self-assambledstructuresof anisotropic particles by appropriately designing the anisotropic particle shape.In addition,by performing MD simulations,we reveal the close relationship between the dynamics and structures.These results provide a new insight on synthesis of materials with specific functions.