Two dimensional colloidal crystals formed by particle self-assembly due to hydrodynamic interaction

Abstract

Aggregation and crystallization of colloidal particles deposited from suspensions on glass surfaces were studied. Trajectories of individual particles are tracked and recorded. Statistics over large amount of particles were made to measure the mean square displacement (MSD), <r 2 >, as functions of time, t. Isolated particles diffuse normally along the solid surface in a two dimensional random manner. A power law of <r 2 >~t α is obeyed both by short and long time scale MSDs of particles with neighbors. However, the exponent values are quite different. When the particle area fraction f≤ 80 %, the particles diffuse normally at the short time scale with a retarded diffusion coefficient. While at the long time scale, a superdiffusive behavior of the particles is detected due to collective motion of particles. When f>80 %, a spatial confinement effect shows in addition. The retarded particle dynamics and the collective particle movements both originate from the many-body hydrodynamic interaction enhanced by the quasitwo dimensional geometric conditions due to the existence of the substrate and the neighbor particles. If the substrate surface condition is favorable and the hydrodynamic interaction dominates, the long-range hydrodynamic interaction can lead particles in dense particle aggregations to self-assemble into long particle chains. This chaining behavior finally results in a phase transition taking place gradually over a large range of the particle area fraction.