  We show that the interaction of a particle with a directionally solidified
interface induces the onset of morphological instability provided that the
particle-interface distance falls below a critical value. This instability
occurs at pulling velocities that are below the threshold for the onset of the
Mullins-Sekerka instability. The expression for the critical distance reveals
that this instability is manifested only for certain combinations of the
physical and processing parameters. Its occurence is attributed to the reversal
of the thermal gradient in the melt ahead of the interface and behind the
particle.
