EAGER: Unravelling the Spatiotemporal Dynamics of Three-Phase Contact Line on Soft Surfaces by Transmission X-Ray Microscopy

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Understanding and controlling the complex dynamics of liquid droplets that impact and spread over solid surfaces has intrigued scientists for more than a century because of the potential applications these phenomena have in broad areas of technology, including fuel combustion, spray coating, pesticide deposition, and inkjet printing. However, most research has considered impact and spreading over rigid synthetic surfaces with uniform textures, which is not the case, for example, in most biological systems. As a result, it remains challenging to predict the behaviors and outcomes of droplet impact on soft surfaces, mainly due to the complexity of the dynamic processes that occur at the droplet rim and the three-phase (solid-liquid-air) contact zone. The goal of this EAGER project is to uncover the fundamental mechanisms that govern contact line dynamics on soft substrates. To examine the details of contact line motion, the researchers will use ultrafast transmission X-ray microscopy (TXM). As a powerful and non-destructive tool, TXM will provide unprecedented spatial and temporal resolutions by imaging multiphase topography and transport in otherwise opaque media. The outcome of this project will have a broad impact on a variety of fields. New research opportunities from this project will be integrated with educational endeavours involving underrepresented college students and high school students in western Virginia. Three-phase contact line dynamics on soft surfaces is a ubiquitous process, and its underlying mechanism is of significant scientific and technological importance. TXM offers unprecedented temporal and spatial resolutions, phase-contrast with the edge-enhancement capability, and ultrahigh and nondestructive penetrability to examine three-phase contact line dynamics on soft materials. Soft substrates with well-characterized flexibility and elasticity will be fabricated. Liquid droplet impact dynamics will be examined with TXM. By visualizing contact line topology and tracking its dynamic evolution, droplet interfacial dynamics including elasto-viscous effects, contact line friction and dissipation on soft surfaces will be revealed for complex, mobile, multiphase and soft liquid-solid interfaces. By imaging the air nanobubbles that may be trapped under an advancing contact line, the mechanism of contact line slip may be revealed. The proposed efforts could not only advance our understanding of the underlying physics governing multiphase interfacial transport on soft surfaces, but also open a new avenue to developing novel bio-inspired interfacial materials and provide guidelines on modifying elasto-viscous properties for controlling droplet impact behaviors.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.