  In this work we study the evolution of Boltzmann's entropy in the context of
free expansion of a one dimensional interacting gas inside a box. Boltzmann's
entropy is defined for single microstates and is given by the phase-space
volume occupied by microstates with the same value of macrovariables which are
coarse-grained physical observables. We demonstrate the idea of typicality in
the growth of the Boltzmann's entropy for two choices of macro-variables -- the
single particle phase space distribution and the hydrodynamic fields. Due to
the presence of interaction, the growth curves for both these entropies are
observed to converge to a monotonically increasing limiting curve, on taking
the appropriate order of limits, of large system size and small coarse graining
scale. Moreover, we observe that the limiting growth curves for the two choices
of entropies are identical as implied by local thermal equilibrium. We also
discuss issues related to finite size and finite coarse gaining scale which
lead interesting features such as oscillations in the entropy growth curve. We
also discuss shocks observed in the hydrodynamic fields.
