Magneto-coalescence phenomena in colliding ferrofluid droplets
Abstract
We explore the collision hydrodynamics of a ferrofluid droplet falling freely onto a sessile droplet of the same liquid, in the presence of a horizontal magnetic field; a configuration that couples droplet-on-droplet coalescence with concomitant field-governed wetting and spreading. Using high-speed imaging, we track the events through crown formation, radial spreading, and rim detachment (under specific conditions), across three ferrofluid concentrations, two substrates of different wettability (glass and PET), and a range of impact velocities and magnetic field strengths. The maximum crown height is noted to scale as Hc,max/Dt∼ Fr0.5 (Fr: Froude number); well below the ballistic upper bound of Hc,max/Dt∼ Fr. At zero-field, the maximum spreading collapses onto the boundary-layer scaling β0,max∼ (We0/Oh)1/6 (We0: Weber number, Oh: Ohnesorge number) when expressed in terms of the merged impact velocity, and coalesced-drop size . With the field applied, a bulk-dissipation energy balance predicts βmax∼ Z1/5, where Z combines the magnetic-driving, and inertial-capillary-viscous terms, but the observations instead follow a markedly weaker ∼ Z1/11, a deficit traced to enhanced dissipation from the magnetoviscous effects, and manifested via an effective Ohnesorge number. Finally, rim detachment occurs beyond a field- and height-dependent threshold, described by a size-independent criterion Fr2Bo≈ 3550 (Bo: Bond number), above which the rim may fragment into daughter droplets. These scalings provide predictive tools for magnetically assisted printing, droplet-on-demand systems, and coating processes, where repeated droplet collisions occur on the residual liquid droplet or layer.
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BibTeX
@misc{bera2026magnetocoalescence,
title = {{Magneto-coalescence phenomena in colliding ferrofluid droplets}},
author = {Neeladri Sekhar Bera and Abhishek Kumar Jaiswal and Devranjan Samanta and Purbarun Dhar},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.30116},
eprint = {2609.30116},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.30116},
}
RIS
TY - UNPB
TI - Magneto-coalescence phenomena in colliding ferrofluid droplets
AU - Neeladri Sekhar Bera
AU - Abhishek Kumar Jaiswal
AU - Devranjan Samanta
AU - Purbarun Dhar
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2609.30116
UR - https://arxiv.org/abs/2609.30116
AB - We explore the collision hydrodynamics of a ferrofluid droplet falling freely onto a sessile droplet of the same liquid, in the presence of a horizontal magnetic field; a configuration that couples droplet-on-droplet coalescence with concomitant field-governed wetting and spreading. Using high-speed imaging, we track the events through crown formation, radial spreading, and rim detachment (under specific conditions), across three ferrofluid concentrations, two substrates of different wettability (glass and PET), and a range of impact velocities and magnetic field strengths. The maximum crown height is noted to scale as H_c,max/D_t∼ Fr^0.5 (Fr: Froude number); well below the ballistic upper bound of H_c,max/D_t∼ Fr. At zero-field, the maximum spreading collapses onto the boundary-layer scaling β_0,max∼ (We_0/Oh)^1/6 (We_0: Weber number, Oh: Ohnesorge number) when expressed in terms of the merged impact velocity, and coalesced-drop size . With the field applied, a bulk-dissipation energy balance predicts β_max∼ Z^1/5, where Z combines the magnetic-driving, and inertial-capillary-viscous terms, but the observations instead follow a markedly weaker ∼ Z^1/11, a deficit traced to enhanced dissipation from the magnetoviscous effects, and manifested via an effective Ohnesorge number. Finally, rim detachment occurs beyond a field- and height-dependent threshold, described by a size-independent criterion Fr^2Bo≈ 3550 (Bo: Bond number), above which the rim may fragment into daughter droplets. These scalings provide predictive tools for magnetically assisted printing, droplet-on-demand systems, and coating processes, where repeated droplet collisions occur on the residual liquid droplet or layer.
ER -