A Study On Heterogeneous Nucleation Of Soft Sphere Colloids By Optical Bragg Scattering

Nucleation and microstructural formation are one of the most challenging problems in condensed matter physics.They play an important role in many fields of materials science.The mechanical properties(such as toughness,hardness)from materials to their electromagnetic properties,for instance,electrical conductivity,magnetic hardness,are greatly dependent on the special solidified structure determined by the heterogeneous nucleation and its initial microstructures.Although important progress has been made in the experimental analysis and computer theoretical simulation for the later growth process of solidification,the early process of the nucleation kinetics on the microscale-the early process of that has not been explained satisfactorily.This refers not only to the homogeneous nucleation in the system,but also to the heterogeneous nucleation caused by the surface or seed.In view of the fact that the model of the classical nucleation theory is too simple to explain the details of some crystal nucleation,by the aid of optical Bragg scattering technologies,we take a binary mixture of charged soft sphere colloids as the sample system to study its heterogeneous nucleation and crystallization.The study is expected to be a model,which will enrich the nonclassical nucleation theory and will be helpful to design the defect free single crystal alloy.A series of the results are then achieved in the work of the thesis:(1).A computer aided soft control circuit has been designed,which can be used to adjust the concentration of the sample and the salt,to fast deionization on-line.It can also melt crystal by shearing and stop shear to study the colloidal nucleation and crystallization repeatedly.(2).Even at extremely low volume fraction,by means of Bragg microscopy,we realize an effective observation on the liquid-solid phase transition for the charged soft sphere colloids at different mixing ratios in case that the particle number density can be on-line adjusted accurately.(3).Supported by the self-designed optical Bragg microscopy,laser diffraction and microscopic techniques,we approach a reliable analysis of the heterogeneous nucleation mechanism and crystallization kinetics.(4).In a binary mixture of charged soft sphere colloids,we observed stable prenucleation clusters,which could play a role of nucleation seeds for heterogeneous nucleation.(5).Moreover,we observed two intermediate phases before finally forming crystal alloy.(6).A multistep model of heterogeneous nucleation is established,which further enriches and improves the nucleation theories nowadays.