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Published on: April 17, 2015
Droplet impact on a particle-laden liquid interface: Worthington jet dynamics and microplastic ejection from floating
Muhammad Hamza Iqbal1, Alfonso Arturo Castrejón-Pita2, José Rafael Castrejón-Pita1
1Department of Mechanical Engineering, University College London, London, WC1E 7JE, United Kingdom.
Hypothesis:
Particle-laden liquid interfaces can modify droplet-impact dynamics by altering lamella destabilisation, cavity collapse, and Worthington jet formation. We hypothesise that the size, density, and wettability of floating microparticles govern whether raindrop impact produces liquid splashing, particle ejection, or jet-mediated transport from the interface.
Experiments:
We investigate millimetric water-droplet impact on quiescent pools covered by closely packed microparticle monolayers, referred to as particle rafts. Thde rafts were formed using particles with diameters of 115-780 μm, densities of 1.02-2.50 g/cm3, and apparent contact angles of 37-157 degrees. Untreated hydrophilic or hydrophobic particles were compared with Glaco-treated superhydrophobic particles to isolate the role of wettability and immersion depth. Droplet impacts were performed under raindrop-relevant conditions, spanning 5 ≲ We ≲ 422 and 6 ≲ Fr ≲ 479. Orthogonal high-speed imaging and image analysis were used to quantify splashing thresholds, cavity depths, jet heights, and particle-transport pathways.
Findings:
Particle rafts strongly modify canonical droplet-impact dynamics through geometric roughness, particle inertia, and capillary adhesion. Deeply immersed particles stabilise the spreading lamella and suppress particle ejection, whereas weakly immersed and superhydrophobic particles promote lamella fingering, splashing, and rapid detachment. The raft also controls cavity collapse, separating the dynamics into elastically responsive and inertially rigid regimes. Small and intermediate superhydrophobic rafts generate particle-armoured Worthington jets that fragment into liquid marbles, while larger or denser rafts suppress wave-swell formation, reduce jet height, and limit ejection. Splashing thresholds and jet heights are captured using geometric-inertial-capillary scaling, identifying when rainfall can transfer microplastics from water to air.
