A new theoretical model that accounts for the excess surface energy, viscous dissipation, surface adhesion, the droplet wetting state, the droplet and fiber radii, and the fiber surface property is developed to predict the coalescence-induced droplet jumping velocity on hydrophobic fibers. The effects of the droplet and fiber radii on the energy terms, the jumping velocity, and the jumping critical contact angle of symmetric and/or asymmetric coalescence are investigated. The results indicate that for the symmetric coalescence, the jumping velocity falls after first increasing along with the increase of the radii of the droplets, despite the larger excess surface energy. However, for the asymmetric coalescence, the highest jumping velocity is achieved when the droplets have the same size. With the increase of the radii difference between the two coalesced droplets, the jumping velocity gradually decreases, even if a greater excess surface energy is released upon coalescence.
Although droplet
self-jumping on hydrophobic fibers is a well-known
phenomenon, the influence of viscous bulk fluids on this process is
still not fully understood. In this work, two water droplets’
coalescence on a single stainless-steel fiber in oil was investigated
experimentally. Results showed that lowering the bulk fluid viscosity
and increasing the oil–water interfacial tension promoted droplet
deformation, reducing the coalescence time of each stage. While the
total coalescence time was more influenced by the viscosity and under-oil
contact angle than the bulk fluid density. For water droplets coalescing
on hydrophobic fibers in oils, the expansion of the liquid bridge
can be affected by the bulk fluid, but the expansion dynamics exhibited
similar behavior. The drops begin their coalescence in an inertially
limited viscous regime and transition to an inertia regime. Larger
droplets did accelerate the expansion of the liquid bridge but had
no obvious influence on the number of coalescence stages and coalescence
time. This study can provide a more profound understanding of the
mechanisms underlying the behavior of water droplet coalescence on
hydrophobic surfaces in oil.
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