A Molecular Dynamics Study On Phase Transitions And Structures Of Patchy Particles

The anisotropic particle system is the system of particles between which the in-teraction depends not only on their spatial coordinates but also on the orientations,due to the influence of various factors such as the structure,shape,electronic properties of the particles and/or the environment they stay in.The introduction of orientational coordinates makes the phase space of the system have much more degrees of freedom and more complex symmetries,so the study of its general properties is of fundamental importance in physics.On the other hand,anisotropic interactions are common in var-ious quantum and classical systems of different scales in nature,among which are spin systems,magnetic systems,polar molecules,liquid crystals,coarse-grained biomacro-molecules,etc.For this reason,the research on anisotropic particle systems has broad application potential and prospects.In the latest decade patchy colloidal particles have become one of the research hotspots in the field of soft matter physics.It is a typical mesoscopic substance with anisotropic interaction.In the experiment,the researchers can design and prepare patchy colloidal particle systems with various properties and structures and complex interaction forms.The different ways of self-assemblies of the patchy colloidal particles provide the basis for better understanding and designing molecular machines.At the mesoscopic scale,the manipulation techniques and observation methods are more abundant than the microscopic scale.Because of these features,people study the patchy colloidal particle systems to deepen their understanding of anisotropic interactions.In this thesis,the Doye model is employed to explore the phase structure of the two-dimensional patchy colloidal systems.The diffusion and melting behavior of these systems are studied.A solid-solid phase transition,in which the domains of patch ori-entations are formed spontaneously with the increase of the temperature,is discovered in very low temperature environments.The phenomenon,mechanism and nucleation process of this phase transition are also studied.The author explains and summarizes the obtained results and physical mechanisms from the general perspective of anisotropic interactions,pointing out that because the order of orientations can be easily destroyed at high temperatures,the effects related to any particular orientations disappear due to time averaging,and the systems behave like isotropic ones.The long-time diffusion behavior of the liquid patchy particle system is normal diffusion,and the melting process of the two-dimensional system is a two-step phase transition.These results are closely consistent with the isotropic of the system.The short-time diffusion behavior of the liquid system is ballistic diffusion,and the spontaneous patch orientation domain formation in the two-dimensional system at low temperature are the direct consequences of the anisotropic interaction.