Journal
SOFT MATTER
Volume 17, Issue 27, Pages 6646-6660Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/d1sm00387a
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Funding
- Ecole Doctorale Physique en Ile de France
- Kyoto University Foundation (Public Interest Incorporated Foundation)
- JSPS KAKENHI [20J15804]
- Grants-in-Aid for Scientific Research [20J15804] Funding Source: KAKEN
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The behavior of self-propelled water-in-oil droplets in capillaries of different cross-sections was experimentally investigated. It was found that the velocity of the droplet decreases with increasing confinement in straight capillaries, but even very long droplets can still swim at very high confinement. In stretched circular capillaries, the non-uniform thickness of the lubrication layer around the droplet leads to neck formation and eventual spontaneous splitting events for large enough confinement, highlighting the critical role of the droplet's interface activity under confinement.
We investigate experimentally the behavior of self-propelled water-in-oil droplets, confined in capillaries of different square and circular cross-sections. The droplet's activity comes from the formation of swollen micelles at its interface. In straight capillaries the velocity of the droplet decreases with increasing confinement. However, at very high confinement, the velocity converges toward a non-zero value, so that even very long droplets swim. Stretched circular capillaries are used to explore even higher confinement. The lubrication layer around the droplet then takes a non-uniform thickness which constitutes a significant difference to usual flow-driven passive droplets. A neck forms at the rear of the droplet, deepens with increasing confinement, and eventually undergoes successive spontaneous splitting events for large enough confinement. Such observations stress the critical role of the activity of the droplet interface in the droplet's behavior under confinement. We then propose an analytical formulation by integrating the interface activity and the swollen micelle transport problem into the classical Bretherton approach. The model accounts for the convergence of the droplet's velocity to a finite value for large confinement, and for the non-classical shape of the lubrication layer. We further discuss on the saturation of the micelle concentration along the interface, which would explain the divergence of the lubrication layer thickness for long enough droplets, eventually leading to spontaneous droplet division.
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