ERI: Incipient contact dynamics of viscoelastic drops

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). Before a droplet moving through a gas can contact a solid surface, the gas between the droplet and surface must be displaced. If the displacement is incomplete, some of the gas may be entrained as small bubbles inside the drop. This process has been studied extensively for common systems such as water droplets moving through air that impact various kinds of solid surfaces. This ERI award focuses on how the contact dynamics change when the liquid drop contains polymer additives. The impact of polymer-laden drops on solid surfaces is relevant to many applications including bioprinting, pesticide formulation, and the deposition of pathogen-laden respiratory droplets on surfaces. The influence of polymer additives on air displacement underneath droplets will be examined using a combination of experiments and theory. A state-of-the-art rheometer and a high-speed optical technique that can track the approach of droplets nanometers from contact will be used to assess the air entrainment in droplets containing commercially-relevant polymers such as polyethylene oxide. Results from the project will provide useful information to engineering practioners who must control air entrainment to ensure product quality in various applications. Graduate and undergraduate students will participate in the research, and research results will be incorporated into undergraduate and graduate courses. The project will partner with the Mayborn Musuem on the Baylor campus to enhance existing outreach efforts aimed at communicating science to students of all ages and the general public.Air entrainment is a common industrial problem in inkjet printing and various coating processes. Although the fundamental hydrodynamics underlying air entrainment has generated broad interest, the problem remains challenging because it spans orders of magnitude in spatiotemporal dynamics. Efforts to uncover the incipient contact of droplets have focused mainly on droplets composed of Newtonian liquids even though important applications involve polymeric drops. In order to fill this knowledge gap, this ERI project will analyze nanometer- to millimeter-scale dynamics using experiments and theory to uncover the contact routes for polymer-laden drops. The research comprises three tasks: (1) investigate the influence of polymer additives on air entrainment phenomena; (2) characterize the energy loss caused by the entrained air film during droplet spreading; and (3) analyze strategies to control the air entrainment mechanism. The influence of polymer concentration on air entrainment and the roles of shear thinning and elasticity will be investigated using a combination of high-speed total internal reflection microscopy and rheometric measurements. The project will develop new experimental tools for assessing the viscoelasticity of polymer-laden droplets, which will benefit researchers studying interfacial phenomena and multiphase fluid mechanics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.