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

Abstract

Aggregation and crystallization of colloidal parti- cles deposited from suspensions on glass surfaces were stud- ied. Trajectories of individual particles are tracked and record- ed. Statistics over large amount of particles were made to measure the mean square displacement (MSD), , as func- tions of time, t. Isolated particles diffuse normally along the solid surface in a two dimensional random manner. A power law of ~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%,theparticlesdiffusenormallyattheshorttimescalewith a retarded diffusion coefficient. While at the long time scale, a superdiffusive behavior of the particles is detected due to col- lective motion of particles. When f>80 %, a spatial confine- ment effect shows in addition. The retarded particle dynamics and the collective particle movements both originate from the many-body hydrodynamic interaction enhanced by the quasi- two dimensional geometric conditions due to the existence of thesubstrateandtheneighborparticles.Ifthesubstratesurface condition is favorable and the hydrodynamic interaction dom- inates, the long-range hydrodynamic interaction can lead par- ticles 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.