  Ejecta knot flickering, ablation tails, and fragmentation are expected
signatures associated with the gradual dissolution of high-velocity supernova
(SN) ejecta caused by their passage through an inhomogeneous circumstellar or
interstellar medium. Such phenomena mark the initial stages of the gradual
merger of SN ejecta with and the enrichment of the surrounding interstellar
medium. Here we report on an investigation of this process through changes in
the optical flux and morphology of several high-velocity ejecta knots located
in the outskirts of the young core-collapse SN remnant Cassiopeia A using {\sl
Hubble Space Telescope} images. Examination of WFPC2 F675W and combined ACS
F625W + F775W images taken between June 1999 and December 2004 of several dozen
debris fragments in the remnant's northeast ejecta stream and along the
remnant's eastern limb reveal substantial emission variations ('flickering')
over time scales as short as nine months. Such widespread and rapid variability
indicates knot scale lengths ~ 10^15 cm and a highly inhomogeneous surrounding
medium. We also identify a small percentage of ejecta knots located all around
the remnant's outer periphery which show trailing emissions typically 0.2" to
0.7" in length aligned along the knot's direction of motion suggestive of knot
ablation tails. We discuss the nature of these trailing emissions as they
pertain to ablation cooling, knot disruption and fragmentation, and draw
comparisons to the emission "strings" seen in eta Car. Finally, we identify
several tight clusters of small ejecta knots which resemble models of shock
induced fragmentation of larger SN ejecta knots caused by a high-velocity
interaction with a lower density ambient medium.
