  Tailored mixing inside individual droplets could be useful to ensure that
reactions within microscopic discrete fluid volumes, which are used as
microreactors in ``digital microfluidic'' applications, take place in a
controlled fashion. In this article we consider a translating spherical liquid
drop to which we impose a time periodic rigid-body rotation. Such a rotation
not only induces mixing via chaotic advection, which operates through the
stretching and folding of material lines, but also offers the possibility of
tuning the mixing by controlling the location and size of the mixing region.
Tuned mixing is achieved by judiciously adjusting the amplitude and frequency
of the rotation, which are determined by using a resonance condition and
following the evolution of adiabatic invariants. As the size of the mixing
region is increased, complete mixing within the drop is obtained.
