Dynamics Of Two-dimensional Active Magnetized Colloids On A Disordered Pinning Substrate

Colloid is a typical type of soft matter.With the continuous improvement of experimental conditions and techniques,active colloids have become one of the hot topics in the fields of material science,condensed matter physics,physical chemistry,biophysics and life science and so forth.It is a very active frontier field to study the self-organization behavior under non-equilibrium conditions by active colloids.In addition,the active colloids are of great significance for the directional delivery of drugs,cell separation or microsurgery.The first chapter of this thesis is an overview of soft matter and self-organization.In the second chapter,the colloids and the experimental preparations of twodimensional colloid systems are briefly introduced.In the third chapter,we first review the progress in the research of dynamics of charged and magnetized colloids,and then by the Langevin molecular dynamics simulations,we systematically investigate the dynamic characteristics of driven twodimensional active magnetized colloids on a disordered pinning substrate.The interactions between active magnetized colloidal particles are firstly modelled as the repulsive paralleled magnetic dipole and attractive Lennard-Jones potentials,and then modelled as the repulsive paralleled magnetic dipole and Mie-type attractive electric dipole potentials.By changing the factors such as pinning strength,repulsive strength,attraction strength and temperature,we systematically study the dynamic characteristics of these two kinds of interaction systems.Through studying the dynamic properties of active magnetized colloidal particles with repulsive paralleled dipoles and attractive Lennard-Jones interactions,we find for the weak depinning substrates,that living islands appear above the depinning.With an increase in the pinning strength,the critical pinning force increases and the living island disperse gradually.In addition,we find that,for the weak repulsion,the attraction between colloidal particles dominates,and there are living islands appearing above the depinning.The critical pinning force increases first and then decreases with the repulsion strength between colloidal particles(i.e.,there is a peak in the critical pinning force).When the repulsion strength is increased to a certain value,the living islands disappear.Further increasing the repulsion strength will lead to moving smectic and moving crystal structures above the depinning.In such a case,the repulsion between colloidal particles dominates.Furthermore,we find that,for the low attraction strength between colloidal particles,the critical pinning force remains unchanged basically and the plastic flows take place above the depinning.With an increasing attraction strength,the living island are gradually formed.When the attraction strength is increased to be large enough,the critical pinning force becomes unchanged again and the living islands and phase separation between different islands occur manifestly.Finally,we also find that,at low temperatures,the thermal fluctuations are small,the attractive potential between the colloidal particles and the pinning potential compete with each other,and there will be dispersed living islands above the depinning.As the temperature is increased further,the thermal fluctuations could compete with the pinning potential,conducive to the formation of clear living islands,but they smooth the pinning potential at high temperatures,leading to the melting of clusters and the homogeneous plastic flows arise above the depinning.Through investigating the dynamic properties of active magnetized colloidal particles with repulsive paralleled dipole and Mie-type attractive electric dipole interactions,we find that,with an increasing attraction exponent n of the Mie potential,the critical pinning force increases first and then decreases.The living islands which appear clearly above the depinning at low exponents disperse gradually with an increase in the exponent n,and they disappear once the n is larger than 6.Further increasing the exponent will lead to the smectic and even crystal flows above the depinning.In addition,with an increase in the repulsive strength between colloidal particles,the peak position of the critical pinning force will move to the lower repulsive strength.The peak disappears when n is increased over 6.Furthermore,we find that,regardless of whether the exponent n is less than 6 or greater than 6,the critical pinning force does not change basically with the attraction strength between colloidal particles.When the exponent n is equal to 6,the critical pinning force decreases with the attraction between colloidal particles.When the exponent n is greater than 6,the repulsion between colloidal particles dominates and the system is always in the plastic flows above the depinning.When the exponent n is less than 6,the attraction between colloidal particles dominates,and the system always shows ordered structures with living islands above the depinning.The results presented in this thesis provide an theoretical guidance for revealing the mechanism of biological self-organization and for controlling and adjusting the self-organization by the external fields.