  The significance of the magnitude of Prandtl number of a fluid in the
propagation direction of induced convection rolls is elucidated. Specifically,
we report on the physical mechanism to account for the formation and
orientation of previously unexplored supra-wavelength periodic surface
structures in dielectrics, following melting and subsequent capillary effects
induced upon irradiation with ultrashort laser pulses. Counterintuitively, it
is found that such structures exhibit periodicities, which are markedly, even
multiple times, higher than the laser excitation wavelength. It turns out that
the extent to which the hydrothermal waves relax depends upon the laser beam
energy, produced electron densities upon excitation with femtosecond pulsed
lasers, magnitude of the induced initial local roll disturbances and the
magnitude of the Prandtl number with direct consequences on the orientation and
size of the induced structures. It is envisaged that this elucidation may be
useful for the interpretation of similar, albeit large-scale periodic or
quasi-periodic structures formed in other natural systems due to thermal
gradients, while it can also be of great importance for potential applications
in biomimetics.
