  We study an instability of thin liquid-vapor layers bounded by rigid parallel
walls from both below and above. In this system, the interfacial instability is
induced by lateral vapor pressure fluctuation, which is in turn attributed to
the effect of phase change: evaporation occurs at a hotter portion of the
interface and condensation at a colder one. The high vapor pressure pushes the
interface downward and the low one pulls it upward. A set of equations
describing the temporal evolution of the interface of the liquid-vapor layers
is derived. This model neglects the effect of mass loss or gain at the
interface and guarantees the mass conservation of the liquid layer. The result
of linear stability analysis of the model shows that the presence of the
pressure dependence of the local saturation temperature mitigates the growth of
long-wave disturbances. The thinner vapor layer enhances the vapor pressure
effect. We find the stability criterion, which suggests that only slight
temperature gradients are sufficient to overcome the gravitational effect for a
water/vapor system. The same holds for the Rayleigh-Taylor unstable case, with
a possibility that the vapor pressure effect may be weakened if the
accommodation coefficient is below a certain critical value.
