  A densely packed colloidal suspension with plasticity, called paste, is known
to remember directions of vibration and flow. These memories in paste can be
visualized by the morphology of desiccation crack patterns. Here, we find that
paste made of charged colloidal particles cannot remember flow direction. If we
add sodium chloride into such paste to screen the Coulombic repulsive
interaction between particles, the paste comes to remember flow direction. That
is, one drop of salt water changes memory effect in the paste and thereby we
can tune the morphology of desiccation crack patterns more precisely.
