  We propose a theory for the multi-ring pattern of the deposits that are
formed when droplets of suspension are dried on a substrate. Assuming a
standard model for the stick-slip motion of the contact line, we show that as
droplets evaporate, many concentric rings of deposits are formed, but are taken
over by a solid-circle pattern in the final stage of drying. An analytical
expression is given to indicate when ring-pattern changes to solid-circle
pattern during an evaporation process. The results are in qualitative agreement
with existing experiments, and the other predictions on how the evaporation
rate, droplet radius and receding contact angle affect the pattern are all
subject to experimental test.
