  We have analyzed the expressed manifestation of the anisotropy of surface
energy density in the dynamics of ultrathin nanowires, which break up into
disjointed clusters when annealed below their melting temperature. The breakup
process is studied for different temperatures and orientations of the nanowire
axis relative to its internal crystal structure using the Monte Carlo kinetic
method. We have also presented an approximate analytical model of the
instability of nanowires. Generally, the interpretation of experimental results
refers to the theoretical model developed by Nichols and Mullins, which is
based on conceptions about the Rayleigh instability of liquid jets. In both
cases, the theories - which do not take into account the anisotropy of surface
energy density - predict the breakup of a nanowire/liquid jet with radius r
into fragments with an average length {\Lambda}=9r. However, the observed
value, {\Lambda}/r, often deviates from 9 either to lower values or to
substantially greater ones (up to 24-30). Our results explain various observed
features of the breakup and the significant variations in the values of its
parameter {\Lambda}/r depending on experimental conditions. In particular, the
ambiguous role of exchange by atoms of the surface of a nanowire with the
surrounding layer of free atoms formed as a result of their rather intensive
sublimation, which occurs in a number of cases, has been investigated. We have
shown that this exchange can lead both to a decrease, and to a significant
increase, in the parameter {\Lambda}/r. The obtained results could be
potentially useful in applications such as the development of optical
waveguides based on ordered nanoparticles chains.
