  We describe results of measurements of the orientational motion of glass
microrods in a microchannel flow, following the orientational motion of
particles with different shapes. We determine how the orientational dynamics
depends on the shape of the particle and on its initial orientation. We find
that the dynamics depends so sensitively on the degree to which particle
axisymmetry is broken that it is difficult to find particles that are
sufficiently axisymmetric so that they exhibit periodic tumbling ("Jeffery
orbits"). The results of our measurements confirm earlier theoretical analysis
predicting sensitive dependence on particle shape and its initial orientation.
Our results illustrate the different types of orientational dynamics for
asymmetric particles predicted by theory.
