Recent experiments and theories have dismissed the role of increased extensional viscosity during impact of viscoelastic droplets. Here we show that for relatively low Weber numbers, $\mathrm{We}=\rho U_0^2{D}_0/\gamma =O\left({10}^0–{10}^1\right)$, where $\rho$ is the density, $U_0^{}$ the impact velocity, $D_0$ the droplet diameter, and $\gamma$ the surface tension, droplets tend to bounce on an air film with a thickness, $h$, which sets off capillary waves that eventually focus into a single wave. This focusing causes rapid deformation of the droplet thus producing high strain rates, which we verified using a particle-tracking method. Without the addition of polymers, the capillary wave focusing generates droplet contact with the substrate; however, with the addition of polymers, contact is inhibited even for relatively small polymer concentrations (10 ppm). We attribute the inhibition of contact to the large increase in the extensional viscosity near the center of the droplet, which dissipates the kinetic energy of the droplet during impact and deformation.

Recent studies have revealed the air-cushioning effect of droplet impact upon various surfaces and although pure water droplets have extensively been studied, the air entrainment dynamics for aqueous polymeric droplets was the focus of this study. Herein, droplets of low to moderate Weber numbers, We ∼ O(1−10), displayed air film thickness gradients which was strongly influenced by the viscoelastic properties of the aqueous polymeric droplets in the dilute to the semidilute unentangled regimes. Aqueous polyethylene oxide droplets impacting a smooth thin oil film surface formed a submicrometer air layer, moments prior to impact, which was tracked by a high-speed total internal reflection microscopy technique. The radial changes in the air film thickness were related to the polymer concentration, thus providing an alternative tool for comparing the rheometer-derived overlap concentrations with a contactless optical technique.

Marangoni flow is the motion induced by a surface tension gradient along a fluid-fluid interface. In this study, we report a Marangoni flow generated when a bath of surfactant contacts a pre-wetted film of deionized water on a vertical substrate. The thickness profile of the pre-wetted film is set by gravitational drainage and so varies with the drainage time. The surface tension is lower in the bath due to the surfactant, and thus a liquid film climbs upwards along the vertical substrate due to the surface tension difference. Particle tracking velocimetry is performed to measure the dynamics in the film, where the mean fluid velocity reverses direction as the draining film encounters the front of the climbing film. The effect of the surfactant concentration and the pre-wetted film thickness on the film climbing is then studied. High-speed interferometry is used to measure the front position of the climbing film and the film thickness profile. As a result, higher surfactant concentration induces a faster and thicker climbing film. Also, for high surfactant concentrations, where Marangoni driving dominates, increasing the film thickness increases the rise speed of the climbing front, since viscous resistance is less important. In contrast, for low surfactant concentrations, where Marangoni driving balances gravitational drainage, increasing the film thickness decreases the rise speed of the climbing front while enhancing gravitational drainage. We rationalize these observations by utilizing a dimensionless parameter that compares the magnitudes of the Marangoni stress and gravitational drainage. A model is established to analyse the climbing front, either in the Marangoni-driving-dominated region or in the Marangoni-balanced drainage region. Our work highlights the effects of the gravitational drainage on the Marangoni flow, both by setting the thickness of a pre-wetted film and by resisting the film climbing.