Interactions in droplet and particle system of near unity size ratio

- Near unity size ratio droplet-particle interaction studied on hydrophobic surface.
- Deposition to complete spreading observed with increasing Weber numbers.
- A time varying linear, power law and oscillatory regimes noted in lamella thickness.
- Non-isothermal interactions exhibited rebound and lamella breakup outcomes.
- Similar regimes observed with heat transfer except the third due to lamella breakup.

In this study, we report collision interactions between a single droplet (diameter: 2.48-2.61 mm) and a stationary hydrophobically coated thermally conductive particle (diameter: 3 mm) in the low impact Weber number range of ∼0.9-47.1 Experiments were performed under both cold and hot state with the aid of high speed imaging to capture the interaction dynamics. Two outcomes were observed in the cold state: deposition at low Weber numbers and complete wetting of particle surface through formation of a spreading lamella of thick peripheral rim at higher Weber numbers. The complete wetting behaviour of droplet exhibited three distinct regimes of temporal variations in liquid film thickness at the apex point of particle. In non-dimensional coordinates, these regimes included - a short interval regime of linear reduction in film thickness due to initial droplet deformation, a relatively larger interval of inertia dominated regime of non-linear reduction in the film thickness and a relatively time invariant large interval of gravity draining regime with an oscillatory transition state due to capillary effect. A novel non-invasive laser based particle heating system was deployed for the non-isothermal interaction cases which showed two outcomes - rebound at lower Weber numbers and complete wetting followed by disintegration of the lamella at higher Weber numbers. Variation in particle temperature was insignificant in the rebound regime however significant temperature reduction (∼10-70 °C) occurred due to increased wetted contact and nucleate boiling of secondary droplets at higher Weber numbers. Irrespective of the heat transfer effect at solid-liquid interface, the temporal variations in the film thickness followed the same trend in regime 1 and 2 as noted in the cold interaction cases. The effect of heat transfer was however uniquely characterised by the absence of regime 3 due to nucleate boiling at solid-liquid contact surface which led to rupture of the interface through eruption of vapour bubbles at the end of regime 2.

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