Dynamics Of Colloidal Crystallization

Crystalline materials are broadly used in our daily life.Crystallization is the process by which crystalline materials are purified and produced.To understand the mechanism of crystallization,scientists have studied it in theory and experiment more than one century.Nevertheless,our understanding of crystallization is still far from being complete.The main challenge arises from the fact that molecules and atoms are too small to observe and follow directly.In contrast,colloidal particles are large enough to be observed directly by light microscopes and move much more slowly to be followed.Most importantly,colloidal particles suspended in solutions behavior like ‘atoms’ in exhibiting phase transitions between solid,liquid and gas states.In the last decades,the studies of phase transitions in colloidal systems have produced a lot of insights to our understanding of crystallization.However,unlike atoms,colloidal particles are polydisperse in size and as the size polydispersity is high enough,crystallization may be suppressed dynamically.To understand the effect of polydispersity,crystallization in a two-dimensional binary colloidal system controlled by an alternating electric field is studied.It is found that no crystalline structures can be formed as the system is fast quenched to a supersaturated state.Nevertheless,as the system is quenched gradually,size fractionation occurs and gives rise to local crystalline domains consisting of identical particles.It follows that crystallization is still possible to occur even in a polydisperse system if the size fractionation is allowed dynamically.It has been known that crystal nuclei may grow by absorbing adatoms or via Ostwald ripening.In recent studies,it is found that nuclei coalescence via oriented attachment also plays an important role in the growth of nanocrystals.In the oriented attachment,two approaching crystal nuclei unify their orientations first through rotation and then coalesce into one single crystal.In our study,the rotation and the coalescence of crystal nuclei are studied in a two-dimensional colloidal model system.Facilitated with the techniques of image processing and particle tracking,the rotation-mediated nuclei coalescence is investigated at the single-particle level.A relation between the nuclei size and the rotation speed is investigated.