Droplet Dynamics, Coalescence, and Mobility in Space

Is the sessile droplet mobility parameter a material parameter that can be used to predict drop spreading and coalescence? Here we examine this question using experiments and data from the ISS.
Droplet Dynamics, Coalescence, and Mobility in Space

Why study coalescing drops?

Droplet coalescence is ubiquitous in industrial processes and technical applications: self-cleaning surfaces, anti-frost coating mechanisms, condensation heat transfer processes, 3D printing, to name a few. Engineered surfaces can induce or retard a coalescence event, optimizing the above applications. As a result, two cooperative needs have arisen in regard to informing surface design for application: experimental characterization of drop dynamics on engineered surfaces and accompanying predictive modeling. As experimental results become available, simulations can be developed and verified to provide predictive capabilities for drop behaviors on engineered surfaces. Once valid models exist, simulations can be used to provide information not easily obtainable by the experimentalist (e.g. drop velocity fields or energy evolutions). Further, in cases where experiments may be difficult to run, simulations can be utilized instead of a lab-based study. Ultimately, by understanding the influence of surface wettability on drop dynamics, surfaces can be better designed for critical technical applications. See our article here at this DOI: 10.1038/s41526-022-00190-y

 What is this work?

In this work we experimentally analyze droplet coalescent dynamics between two sessile droplets on several substrates and compare to simulations. Two different simulation approaches are implemented: a widely known heuristic model and a less popular analytic model, which offers a more physical understanding. Until recently, one difficulty with applying the analytic model is measuring a necessary input, the mobility parameter. The mobility parameter is a non-negative number that characterizes a drop's tendency to move or resist movement. Historically it has been used as a fitting parameter for a given solid-liquid-gas system. However, if droplet mobility is a material parameter, it can be measured in one context and successfully applied in another, opening up exciting possibilities for anticipating rapid wetting and dewetting behaviors. In this work we find the analytic model better predicts droplet coalescence than the heuristic model, motivating the claim contact-line mobility is a material parameter that is the impetus of our current studies to prove it. However such studies are tricky to conduct on Earth because of the element of gravity in the physical scaling necessary to design experiments.

 What's next?

On Earth, inertial-capillary contact line motions are fast and small. On the International Space Station (ISS), owing to reduced gravity, inertial-capillary spreading takes place at time scales nearly two orders of magnitude slower and length scales extended one order of magnitude larger. This allows droplet dynamics during coalescence and vibration to be resolvable through imaging on temporal and spatial scales unmatched on Earth. By way of hydrodynamic similitude, observations from the ISS informs our limited understanding of rapidly moving contact lines on Earth. To this end, vibrational and coalescence experiments were launched to the ISS in December 2020, and our group is currently analyzing the results.

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