  How far a particle moves along the optical axis in a holographic optical trap
is not simply dictated by the programmed motion of the trap, but rather depends
on an interplay of the trap's changing shape and the particle's material
properties. For the particular case of colloidal spheres in optical tweezers,
holographic video microscopy reveals that trapped particles tend to move
farther along the axial direction than the traps that are moving them and that
different kinds of particles move by different amounts. These surprising and
sizeable variations in axial placement can be explained by a dipole-order
theory for optical forces. Their discovery highlights the need for real-time
feedback to achieve precise control of colloidal assemblies in three dimensions
and demonstrates that holographic microscopy can meet that need.
