What is it about?
When droplets of complex fluids such as polymer solutions, gels, or everyday products like shampoo merge, they behave in ways that simple Newtonian fluids like water never do. Classical coalescence theory, built on centuries of studying water and oil, does not prepare you for what happens when elasticity enters the picture. We discovered a new elasto-viscous regime — a previously hidden dynamical state in which elastic and viscous forces compete on equal terms during droplet merging, producing scaling behavior that existing frameworks cannot predict. These hidden dynamics are not merely of academic interest; they are critical to understanding and designing pharmaceuticals, inkjet printing, surface coatings, and food formulations where complex fluid coalescence is central.
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Why is it important?
We provide the first experimental proof, supported by quantitative numerical simulations, that droplet coalescence in complex fluids such as concentrated polymer solutions is governed by a previously uncharacterized elasto-viscous regime - a dynamical state in which elastic and viscous forces compete on equal terms. This overturns long-standing textbook assumptions about how droplets merge, assumptions that hold for simple Newtonian fluids like water but fail entirely for viscoelastic materials. The findings reframe our understanding of a phenomenon as ubiquitous as it is fundamental, with direct implications for drug delivery, inkjet and additive manufacturing, food formulation, and biomedical systems where complex fluid coalescence plays a controlling role.
Perspectives
Droplets merging - what could be simpler? And yet, nature hid an entire regime of physics inside this everyday phenomenon, waiting for someone to look carefully enough. That is what excites us most about this work. Newtonian fluids like water and air have been studied for centuries; their coalescence dynamics are largely settled science. But complex fluids - polymers, gels, biological fluids, emulsions - represent a vast, comparatively uncharted territory where the rules are different and surprises remain. We found one such surprise. We suspect there are many more. The simplest phenomena, looked at through the right fluid, can reveal profound and unexpected complexity.
Aloke Kumar
Read the Original
This page is a summary of: Elasto-viscous regime in coalescence of viscoelastic droplets, Proceedings of the National Academy of Sciences, April 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2528750123.
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