Suppression of air bubble entrainment during drop impact on smooth surfaces*

When a drop impacts on a surface, it can entrain a central air bubble, undesirable in processes such as spray coating and inkjet printing. This air bubble is caused by a combination of the pre-contact drop-air interfacial deformation and the post-contact contact line dynamics. The volume of the entrained air bubble is maximal when the pressure within the interstitial air layer between the drop and the surface is less than the liquid inertial pressure and greater than the Laplace pressure due to the interfacial curvature. The approaches used in the past to suppress air bubble entrainment have been either by reducing the ambient pressure or by increasing the liquid inertia. Here, we present theory and experiments on drop impacting a smooth surface to achieve complete suppression of bubble entrainment at ambient pressure and low liquid inertia. We use the recently reported dimple mode of drop-surface contact to initiate contact at the center and the subsequent radial contact line propagation to suppress bubble entrainment. Several Newtonian and non-Newtonian liquids are used to study the effect of liquid rheology on the air bubble suppression mechanism, thereby providing insights on improving technologies such as inkjet printing, spray coating, and others.