Study On Elastic Properties Of Colloidal Crystals By Electrokinetic Behaviour Of Colloidal Particles

As a typical soft matter system,colloidal suspension is widely present in many fields such as pharmaceutics,environment,material and chemical industry.Colloidal crystals,which are self-assembled by colloidal particles,have great prospect in the application of photonic communication and computer.Moreover,colloidal crystals have been consid-ered as model systems of atomic or molecular crystals and have gotten considerable atten-tion in the area of condensed matter physics,owing to their huge difference in time and spatial scales from the atomic or molecular systems,so that there are more measurement methods which can be used to study the microscopic structures of the matters.The thesis is aimed at the mechanical properties of colloidal crystals,which is closely related to the surface charge properties of colloidal particles.Therefore,the effective sur-face charges and electrokinetic behaviour of colloidal particles are firstly studied both theoretically and experimentally.Based on this investigation,the elastic properties(in-cluding extension,compression and shear)of colloidal crystals and the effect of porosity are further studied.The main results of the thesis are as follows:1.The effective charges on the surface of particles obtained from four kinds of ways are systematically compared with each other,in order to figure out the causes of differences among them.These effective charges include mean field renormalized charge,effective titrimetric charge,effective mobility charge and effective elasticity charge.The physical mechanism for the discrepancies among these effective charges are investigated,and the applicability of each effective charge is presented.2.Based on the electrokinetic models for the electrophoresis of colloidal particles in the suspension with electrolytes,a new electrokinetic model is developed for the salt-free colloidal suspension by fundamental theories within fluid mechanics and electrochem-istry.On the effective charge of colloidal particles,the electrophoresis mobility can be predicted by the new model which is coincident with experimental results.Moreover,the influences of the volume fraction and effective surface charge of particles on the elec-trophoresis mobility are discussed.The effects of counterions dissociated from colloidal particles,hydrogen and hydroxide ions from water dissociation are all considered in the new model,so that the previous model in the literature for suspensions with electrolytes can be considered as a special case of the new model.Therefore,the model developed in the thesis can be more extensively applied.3.Based on the above mentioned electrokinetic model for particles inside the salt-free suspension,an analytic expression for the electrophoretic mobility of colloidal parti-cles is derived by employing the order-of-magnitude analysis.The mobility of particles can be calculated more easily by the analytic expression.Meanwhile,the hydrodynamic mechanism of the electrophoresis of particles get deeply understood.4.According to the hydrodynamics of electrophoresis of particles in salt-free sus-pensions,a novel method measuring Young's moduli of colloidal crystals is proposed.In this method,the field strength in the suspension is determined to get the values of stress,and the nearest inter-particle distances are measured to get the values of strain in differ-ent parts of the colloidal crystals.Then the stress-strain curve of extension/compression can be obtained to analyse the mechanical properties of colloidal crystals including the value of Young's moduli as well.The present method is a very practical way to study the mechanical properties of colloidal crystals because of its higher efficiency and wider applicable range compared to the gravity settling method in the literature.5.The effect of crystalline porosity is introduced to the shear modulus-pair poten-tial model,so that the shear moduli of colloidal crystals can be precisely estimated from the effective charge of particles.Therefore,the new model solved the problem that the theoretical values of the shear moduli of colloidal crystals deviate from the experimental results.Moreover,the mechanisms for the mechanical properties of colloidal crystals can be well understood by the new model.As mentioned above,in the thesis theoretical models have been developed and ex-perimental methods have been established based on the combination of the elasticity of colloidal crystals and the electrokinetic behaviour of colloidal particles.And the micro-mechanism of the elasticity of colloidal crystals is also explored.The thesis is of great value for the utilization of electrokinetic properties of colloidal particles,and further un-derstanding and controlling the mechanical properties of colloidal crystals in the future.