Entanglement attenuates the entrained air film underneath polymeric droplets

Recent studies have revealed that droplets tend to rebound from the surface when the ambient air sufficiently decelerates the droplet, where an interstitial air layer prevents contact. While this air cushioning effect has been studied under Newtonian liquid droplets, the air entrainment mechanism underneath polymeric droplets is lacking in the literature. In this study, we demonstrate that for Weber numbers, We∽Ο(1-10), the spatiotemporal evolution of the air films is either enhanced or attenuated by the viscoelastic properties of the aqueous polymeric droplets. A submicron air layer is visualized during droplet impact of aqueous poly(ethylene oxide) and xanthan gum solutions from the dilute to the concentrated regimes with a high-speed total internal reflection microscopy (TIRM) technique. We observe that the slope of the air film is attenuated in the semi-dilute regime yet enhanced in the dilute and entangled regimes. Additionally, the air dimple inversion induced by the strong capillary wave upon impact is suppressed in the entangled regime for polymeric droplets with poly(ethylene oxide) and xanthan gum additives.