Researchers Answer Long-Standing Question on Stress in Thin Film Solid-Fluid Interactions

Recent experiments have managed to answer a long-standing question about the mechanical response of tin films and filaments at fluid interfaces, which are impacted by elastro-capillary stresses.  

According to the team of campus scientists that conducted the analysis, this is “one of those things you should be able to look up in a textbook, a very basic question. But when you do look into the literature, there’s confusion, and it has been going on for decades. Our work isn’t screaming ‘new discovery,’ but this basic question is what we have clarified.”  

A Long-Standing Basic Question  

The research was detailed in the journal Nature Materials and was led by experimental physicist Narayanan Meron, together with theoretical physicist Benny Davidovitch, polymer scientist Thomas Russell and a former UMass postdoctoral physics researcher, Deepak Kumar. He led the experiments and analysis for the final report and is now involved at the Indian Institute of Technology in Delhi.  

Menon said: “Deepak’s work addressed an assumption not tested carefully until now. Nobody got upset enough to devote a year to figuring it out.”  

According to co-author Davidovitch, the scientific question includes a mechanical reaction of tin films and filaments at fluid ports, which are affected by elasto-capillary stresses. Therefore, the core of the matter is the fine way by which surface energies of solid-liquid interfaces merge to create stresses in a solid body.  

To clarify, Menon explains: “Imagine something light and flexible, the thin film, floating on the surface of liquid – a leaf, the wing of a fly, a flower petal on a pond. It will stretch out and lay flat, pulled taut at the interface where it touches the water. You can try it out in your kitchen with a piece of plastic in a glass of water.”  

A thin film on the surface of liquid, which is pulled taut at the interface. As it is gently peeled away, marks of stress are revealed. [Image: University of Massachusetts Amherst]

The real question here is how much the thin film is being pressured, the researcher said. There are two answers that we find in the literature. One of them says that the tension relies on what the liquid is, while the other says it depends both on that and the way the solid and liquid engage,  

“Both assumptions cannot be correct, so we had to get to the bottom of it. Our experiments show with no doubt that only one of these answers is right,” Menon said.  

It Depends on the Liquid Type  

Kumar’s analyses unveiled that the solid-liquid interaction, where a crescent, or interface shaped as a lens forms at the liquid surface, only relies on what the liquid is, and not on the solid.   

Russell said: “When Deepak pulled a thin film from the surface of a liquid and showed us that the meniscii on the liquid and film-covered side were absolutely identical, we were amazed, but this is an unambiguous proof of one assumption.”  

According to Davidovitch, this matter had to be cleared up because a recent increase of interest in elasto-capillary events is in development in soft robotics, drug creation, and tissue engineering with responsive surfaces, besides other industries.  

The outcome doesn’t look outstanding, but it elucidates something that has been unclear for a while, Menon said: “The encouraging thing is that there are lots of simple things to figure out in science, which I find satisfying. You think that there is no space left for simple naive questions, but that is not true. This project was a nice reminder of that.” 

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