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Phase diagram morphology shapes droplet propulsion in chemical gradients

  • Stefan Köstler
  • Malcolm Steen
  • David Zwicker
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Abstract

Droplets in complex environments often encounter compositional gradients that drive their propulsion. For example, surfactant gradients induce surface-tension-driven propulsion via the Marangoni effect. Here, we show that such propulsion emerges generically, even in the absence of surfactants. Employing a thin-interface approximation, we derive a compact expression for the droplet velocity in terms of droplet size, viscosity, the sensitivity of surface tension to a regulating component, and the chemical potential gradient of that component. Our theory reveals that droplets move toward regions of lower stability, as encoded in the morphology of the phase diagram, particularly near critical points. Numerical simulations confirm these results, which establish a general route to predicting and designing droplet motility from the phase behavior of complex fluids.

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Cite

BibTeXRIS
BibTeX
@misc{kostler2026phase,
  title = {{Phase diagram morphology shapes droplet propulsion in chemical gradients}},
  author = {Stefan Köstler and Malcolm Steen and David Zwicker},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.24529},
  eprint = {2609.24529},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.24529},
}
RIS
TY  - UNPB
TI  - Phase diagram morphology shapes droplet propulsion in chemical gradients
AU  - Stefan Köstler
AU  - Malcolm Steen
AU  - David Zwicker
PY  - 2026
DA  - 2026/09/21
PB  - arXiv
DO  - 10.48550/arxiv.2609.24529
UR  - https://arxiv.org/abs/2609.24529
AB  - Droplets in complex environments often encounter compositional gradients that drive their propulsion. For example, surfactant gradients induce surface-tension-driven propulsion via the Marangoni effect. Here, we show that such propulsion emerges generically, even in the absence of surfactants. Employing a thin-interface approximation, we derive a compact expression for the droplet velocity in terms of droplet size, viscosity, the sensitivity of surface tension to a regulating component, and the chemical potential gradient of that component. Our theory reveals that droplets move toward regions of lower stability, as encoded in the morphology of the phase diagram, particularly near critical points. Numerical simulations confirm these results, which establish a general route to predicting and designing droplet motility from the phase behavior of complex fluids.
ER  -