  Bimetallic nanoparticles (BNPs) exhibit diverse morphologies such as
core-shell, Janus, onion-like, quasi-Janus, and homogeneous structures.
Although extensive effort has been directed towards understanding the
equilibrium configurations of BNPs, kinetic mechanisms involved in their
development have not been explored systematically. Since these systems often
contain a miscibility gap, experimental studies have alluded to spinodal
decomposition (SD) as a likely mechanism for the formation of such structures.
We present a novel phase-field model for confined (embedded)systems to study
SD-induced morphological evolution within a BNP. It initiates with the
formation of compositionally modulated rings as a result of surface-directed SD
and eventually develops into core-shell or Janus structures due to
coarsening/breakdown of the rings. The final configuration depends crucially on
contact angle and particle size -Janus is favored at smaller sizes and higher
contact angles. Our simulations also illustrate the formation of metastable,
kinetically trapped structures as a result of competition between capillarity
and diffusion.
