  Freezing colloidal suspensions widely exists in nature and industry.
Interface instability has attracted much attention for the understandings of
the pattern formation in freezing colloidal suspensions. However, the interface
instability modes, the origin of the ice banding or ice lamellae, are still
unclear. In-situ experimental observation of the onset of interface instability
is still absent up to now. Here, by directly imaging the initial transient
stage of planar interface instability in directional freezing colloidal
suspensions, we proposed three interface instability modes, Mullins-Sekerka
instability, global split instability and local split instability. All the
three instability modes come from the competition of the solute boundary layer
and the particle boundary layer, which only can be revealed from the initial
transient stage of planar instability in directional freezing.
