Nan Xue

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Xue

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Nan

ORCID

0000-0002-4729-0741

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Publications 1 - 4 of 4

Droplets sliding on soft solids shed elastocapillary rails
Xue, Nan; Wilen, Lawrence A.; Style, Robert W.; et al. (2025)
Soft Matter
The surface tension of partially wetting droplets deforms soft substrates. These deformations are usually localized to a narrow region near the contact line, forming a so-called 'elastocapillary ridge.' When a droplet slides along a substrate, the movement of the elastocapillary ridge dissipates energy in the substrate and slows the droplet down. Previous studies have analyzed isotropically spreading droplets and found that the advancing contact line 'surfs' the elastocapillary ridge, with a velocity determined by a local balance of capillary forces and bulk rheology. Here, we experimentally explore the dynamics of a droplet sliding across soft substrates. At low velocities, the contact line is nearly circular, and dissipation increases logarithmically with speed. At higher droplet velocities, the contact line adopts a bullet-like shape, and the drag force levels off. At the same time, droplets shed a pair of 'elastocapillary rails' that fade away slowly behind them. These results suggest that sliding along the parallel edges of a bullet-shaped droplet dissipates less energy than surfing the wetting ridges at the front and back.
Elastomers Fail from the Edge
Xue, Nan; Long, Rong; Dufresne, Eric; et al. (2024)
Physical Review X
The performance of soft devices is limited by the fracture resistance of elastomers. Thus, understanding how fracture resistance changes with material and sample geometry is an important challenge. A key observation is that thicker elastomers can be significantly tougher than thinner ones. We show that this surprising toughness enhancement in thick samples emerges from the 3D geometry of the fracture process. In contrast to the classical picture of a 2D crack, failure is driven by the growth of two separate "edge"cracks that nucleate early on at a sample's sides. As loading is increased, these cracks propagate in towards the sample midplane. When they merge, samples reach their ultimate failure strength. In thicker samples, edge cracks need to propagate farther before meeting, resulting in increased sample toughness. We demonstrate that edge-crack growth is controlled by the elastomer's strain-stiffening properties. Our results have direct implications for how to effectively toughen elastomers by controlling their geometry and large-strain mechanical properties.
Three-Dimensional Self-Similarity of Coalescing Viscous Drops in the Thin-Film Regime
Kaneelil, Paul R.; Pahlavan, Amir A.; Xue, Nan; et al. (2022)
Physical Review Letters
Coalescence and breakup of drops are classic problems in fluid physics that often involve self-similarity and singularity formation. While the coalescence of suspended drops is axisymmetric, the coalescence of drops on a substrate is inherently three-dimensional. Yet, studies so far have only considered this problem in two dimensions. In this Letter, we use interferometry to reveal the three-dimensional shape of the interface as two drops coalescence on a substrate. We unify the known scaling laws in this problem within the thin-film approximation and find a three-dimensional self-similarity that enables us to describe the anisotropic shape of the dynamic interface with a universal curve.
Anomalous crystalline ordering of particles in a viscoelastic fluid under high shear
Sun, Sijie; Xue, Nan; Aime, Stefano; et al. (2023)
Proceedings of the National Academy of Sciences of the United States of America
Addition of particles to a viscoelastic suspension dramatically alters the properties of the mixture, particularly when it is sheared or otherwise processed. Shear-induced stretching of the polymers results in elastic stress that causes a substantial increase in measured viscosity with increasing shear, and an attractive interaction between particles, leading to their chaining. At even higher shear rates, the flow becomes unstable, even in the absence of particles. This instability makes it very difficult to determine the properties of a particle suspension. Here, we use a fully immersed parallel plate geometry to measure the high-shear-rate behavior of a suspension of particles in a viscoelastic fluid. We find an unexpected separation of the particles within the suspension resulting in the formation of a layer of particles in the center of the cell. Remarkably, monodisperse particles form a crystalline layer which dramatically alters the shear instability. By combining measurements of the velocity field and torque fluctuations, we show that this solid layer disrupts the flow instability and introduces a single-frequency component to the torque fluctuations that reflects a dominant velocity pattern in the flow. These results highlight the interplay between particles and a suspending viscoelastic fluid at very high shear rates.

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