Alignment and self-assembly of elongated micronsize rods in several flow fields

Microfluidic alignment has attracted attention as a possible tool for the orientation of micronsize rods suspended in a solvent and the deposition of these rods in ordered arrays on solid surfaces. In the present work the following situations are realized experimentally: (i) a flow through an abruptly converging (or diverging) joint between wide and narrow straight channels which entrains and aligns (or misaligns) microrods and (ii) a tiny droplet of the order of a hundred microns that contains microrods is displaced due to Marangoni convection forces in a channel subjected to a temperature gradient. The flow near the advancing contact line orients the random suspension of rods and guides them towards the channel bottom where they are deposited in an ordered array. In parallel with this experimental study, the processes of microrod reorientation and alignment in the above-mentioned flows and in a similar sinklike flow into an issuing jet were studied using solutions of the Fokker-Planck equation for the orientation probability density function. (c) 2006 American Institute of Physics.