The influence of elasticity on Faraday instability*

In this work, we investigate the influence of parametric forcing on a viscoelastic fluid layer, considering both gravitationally stable and unstable configurations. In the absence of elasticity, when a viscous layer is vertically oscillated beyond a certain amplitude, large oscillations are observed at the interface due to resonant or Faraday instability. Adding a polymeric gel to this liquid imparts elasticity, affecting the damping rate of momentary disturbances, implying an altering of the threshold and temporal response of Faraday instability.

In gravitationally stable configurations, linear stability analysis shows that increased elasticity can either stabilize or destabilize the viscoelastic system. Specifically, in weakly elastic liquids, higher elasticity increases damping, raising the threshold for Faraday instability. Conversely, in strongly elastic liquids, elasticity reduces the damping rates, lowering the instability threshold.

While oscillatory instability occurs in Newtonian systems for all gravity levels, our findings reveal that parametric forcing can cause instability with monotonic response in viscoelastic systems at microgravity, below a critical frequency. Within the same frequency range, parametric forcing destabilizes viscoelastic liquid layers in gravitationally unstable configurations.  This contrasts with the case of Newtonian liquid layers, where parametric forcing always stabilizes an otherwise gravitationally unstable system.