Droplet Impact on Microparticle Raft: Wettability, density and size govern splashing and microplastic ejection from rafts under raindrop impact

Abstract

Raindrop impact on the ocean has been proposed as a mechanism for microplastic transfer from seawater to the atmosphere, yet the interfacial dynamics governing particle ejection from floating microplastics remain largely unexplored. We investigate droplet impact onto microparticle monolayers (rafts) spanning a wide range of sizes, contrasting densities, and wettabilities, under raindrop-relevant impact conditions. Particle rafts strongly influence splash dynamics, cavity collapse, and Worthington jet formation. Splash onset is controlled by particle-induced roughness and capillary adhesion: deeply immersed particles stabilise the spreading lamella, producing only microdroplets, whereas weakly immersed particles destabilise the rim, promoting fingering and splashing. Following impact, raft characteristics govern wave-swell dynamics, separating into elastic and rigid regimes. Superhydrophobic rafts enable particle ejection upon impact and form particle-armoured Worthington jets that fragment into liquid marbles, providing an efficient aerosolisation pathway. In contrast, less hydrophobic rafts show limited detachment upon impact but still support Worthington jet-mediated transport. Despite these differences, splash thresholds and Worthington jet heights collapse under a simple geometric-inertial-capillary scaling. These results show how particulate monolayers modify canonical droplet-impact and identify the interfacial conditions under which rainfall transfers microplastics from ocean to atmosphere, and inform related droplet-granular processes such as soil erosion, and impacts on sandy substrates.

Figures (8)

• Figure 1: Experimental configuration and characterisation of floating microparticle rafts.a Sketch of the droplet–raft impact apparatus. Millimetric water droplets are generated from a syringe pump and needle and released onto a quiescent pool whose surface is coated with a closely packed particle raft. A high-speed side-view imaging system records the droplet impact, crater formation and Worthington jet dynamics. b Representative top-view (top) and side-view (bottom) images from the high-speed cameras showing the hexagonally packed monolayer of $780~\mathrm{\mu m}$ FPE microparticles and the corresponding interfacial profile at the particle scale. c Optical profilometer scans for hydrophilic $625~\mathrm{\mu m}$ glass spheres (top) and Glaco-treated $550~\mathrm{\mu m}$ FPE spheres (bottom), used to determine the emerged height ($h_\text{above}$) of the particle above the undisturbed interface. d Schematic immersion geometries for hydrophilic, hydrophobic and superhydrophobic microparticle spheres. The submerged depth ($h_\text{sub}$), emerged height ($h_\text{above}$), and the apparent contact angle ($\theta$) are indicated (see Table \ref{['table1']}), corresponding to the geometric relation used to compute wettability at negligible Bond number. Not to scale.
• Figure 2: Splashing dynamics on particle rafts. (a,b) Impact outcomes mapped in the ($We$, $D_p$) plane for (a) clean water + untreated and (b) Glaco-treated (superhydrophobic) particle rafts, showing the transition from spreading to splashing across particle diameters and Weber numbers. The dashed lines show the empirical trend of the splashing threshold highlighting the progressive decrease in the critical Weber number as particle diameter increases. The Glaco treatment (superhydrophobic) produces a shallow particle immersion and large apparent contact angles that shift the splashing threshold to low Weber numbers. (c,d) Side-view snapshots of droplet impact at $t=0.05~\mathrm{ms}$, after impact at $We=346\pm3$, comparing splashing morphologies on untreated (c) and Glaco-treated (d) particle rafts. For both, the splashing morphology evolves from classical crown splashing on water to prompt splashing for small particles ($115 - 327~\mathrm{\mu m}$), before reverting to crown-like splashing for larger particles ($550$ and $780~\mathrm{\mu m}$). (e) Our scaling, $\Lambda$, collapse of all experiments in the ($We,~\Lambda$) plane. The green dashed line indicates the splashing threshold on clean water for reference.
• Figure 3: Impact-driven particle ejection during droplet splashing on FPE/PE rafts at $We\approx415$.(a,b) Side-view snapshots at $t = 2.8~\mathrm{ms}$ after impact comparing particle splash for untreated (a) and Glaco-treated (b), illustrating the strong contrast between suppressed splash for hydrophobic particles and prolific radial ejection for superhydrophobic particles. For untreated rafts, only limited particle detachment is observed, whereas treated rafts exhibit widespread scattering due to minimal interfacial pinning.
• Figure 4: Influence of the particle raft size on the Worthington jet height. Solid symbols represent untreated particles, while hollow symbols and represent Glaco-treated particles; dashed and dotted lines added to guide the eye.(a,b) Maximum dimensionless jet height, $H_J/D$, as a function of the Froude number for clean water and particle rafts of varying diameter; (a) shows results for untreated particles and (b) shows Glaco-treated particles. For untreated rafts, the onset of jetting shifts systematically to higher $Fr$ with increasing particle size with the maximum jet height decreasing monotonically. In contrast, Glaco-treated rafts exhibit earlier jet formation and a non-monotonic dependence of jet height on particle diameter, with the tallest jets observed for intermediate sizes ($231$ and $327~\mathrm{\mu m}$). In panel (b), dashed lines for untreated rafts are shown for reference. (c,d) Side-view snapshots of the Worthington jet at the instant of maximum height at $Fr=417\pm3$, comparing untreated (c) and Glaco-treated (d) particle rafts with clean water.
• Figure 5: Cavity depth and collapse morphology underlying jet formation on particle rafts. Solid symbols and dashed lines represent untreated particles, while hollow symbols and dotted lines represent Glaco-treated particles.(a,b) Maximum dimensionless cavity depth, $H_C/D$, as a function of the Froude number $Fr$ for clean water and particle rafts composed of FPE and PE particles of varying diameter. Untreated particles seen in (a) and Glaco-treated seen in (b). For both conditions, cavity depth decreases monotonically with increasing particle size. In panel (b), dashed lines for untreated rafts are shown for reference. Shaded backgrounds denote the dominant cavity collapse shape identified from side-view imaging (legend). (c,d) Representative side-view images of cavity collapse for untreated (c) and Glaco-treated (d) particle rafts at $Fr\approx471$, illustrating the transition between sharp W, soft W, flat V and sharp V morphologies with increasing particle size and surface treatment. Shaded boundaries denote the dominant cavity collapse shape.