Contact of soft solids on rigid substrates mediated by air

Before a soft solid can impact upon a rigid substrate, it must drain the air beneath it. If the impact is sufficiently rapid, the air will fail to drain, and instead compresses. Indeed, the air can compress sufficiently to significantly deform the soft impactor. Here we explore a critical transition in the dynamics between two regimes: first, where elastic and lubrication stresses balance, and second, where inertial and lubrication stresses balance. Using direct imaging of the interface with sub-micron resolution in our experiments, and varying the impactor's stiffness and geometry, we show how the progressing deformation front of the soft impactor can exceed the Rayleigh velocity of the impactor to fundamentally alter the contact dynamics. The transition between elastic and inertial stress dominance can be extremely sharp, highlighting an unexpected and rich dynamics during dynamic impact of compliant solids, with key implications for systems as diverse as biotribological interfaces, dense suspensions and the mundane rebound of a bouncy ball.