Role of interfacial friction for flow instabilities in a thin polar ordered active fluid layer

We construct a generic coarse-grained dynamics of a thin inflexible planar layer of polar-ordered suspension of active particles, that is frictionally coupled to an embedding isotropic passive fluid medium with a friction coefficient $\Gamma$. Being controlled by $\Gamma$, our model provides a unified framework to describe the long wavelength behaviour of a variety of thin polar-ordered systems, ranging from {\em wet} to {\em dry} active matters and free standing active films. Investigations of the linear instabilities around a chosen orientationally ordered uniform reference state reveal generic moving and static instabilities in the system, that can depend sensitively on $\Gamma$. Based on our results, we discuss estimation of bounds on $\Gamma$ in experimentally accessible systems.