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

This study reveals that the wettability, density, and size of floating microparticle rafts critically govern raindrop impact dynamics—specifically splash onset, jet formation, and microplastic aerosolization—by establishing distinct interfacial regimes that ultimately collapse under a unified geometric-inertial-capillary scaling law.

The Gist

Imagine the ocean surface not just as a smooth sheet of water, but as a crowded dance floor covered in tiny, floating marbles. These marbles are microplastics—tiny bits of plastic pollution that have gathered at the very top of the sea.

Now, picture a single raindrop falling from the sky and hitting this dance floor. What happens next? Does the rain just splash normally? Or does it kick the marbles into the air, sending them up into the atmosphere where they can travel thousands of miles?

This paper is a high-speed investigation into exactly that scenario. The researchers wanted to understand how raindrops interact with these floating "rafts" of microplastics and whether this interaction acts as a launchpad, shooting plastic pollution from the ocean into the sky.

Here is the story of their findings, broken down into simple concepts:

1. The Setup: A Raindrop vs. A Floating Raft

Think of the ocean surface covered in microplastics as a raft made of tiny, floating beads.

• The Raindrop: A standard raindrop (about the size of a small pea) falls onto this raft.
• The Variables: The researchers tested different types of "beads":
  • Size: Tiny beads (like sand) vs. large beads (like BBs).
  • Material: Light plastic vs. heavy glass.
  • Wettability (The "Coating"): Some beads are "hydrophilic" (they like water and sink a bit), while others are "superhydrophobic" (they hate water and sit high up, like a duck on a pond).

2. The Splash: When the Raft Gets Rough

When a raindrop hits clean water, it creates a crown-shaped splash. But when it hits a raft of plastic beads, the rules change.

• The "Roughness" Effect: Imagine running on a smooth track versus a track covered in pebbles. The pebbles make you stumble.
  • Small beads: If the beads are very small, they act like a smooth, dampening blanket. They actually stop the splash from getting too wild.
  • Large beads: If the beads are big, they act like a rough, bumpy surface. This roughness makes the splash much more violent, creating "fingers" of water that break apart into tiny droplets.
• The "Sticky" Factor: If the beads are "wet" (hydrophilic), they sink deeper into the water and hold on tight. They are hard to knock off. If they are "super-repellent" (superhydrophobic), they sit high up and barely touch the water. It's much easier to knock these high-sitting beads off their perch.

The Big Discovery: The researchers found a simple formula that predicts when a splash will happen. It depends on a mix of the bead's size, how heavy it is, and how much it hates water. If the beads are big, heavy, or very water-repellent, the splash happens much easier.

3. The "Worthington Jet": The Ocean's Trampoline

After the initial splash, something magical happens. The hole (cavity) created by the raindrop collapses. This collapse acts like a trampoline, shooting a vertical column of water straight up into the air. This is called a Worthington jet.

• The Problem: Usually, a raft of heavy beads acts like a heavy blanket on a trampoline. It absorbs the energy, making the jump (the jet) shorter and slower.
• The Surprise: However, if the beads are superhydrophobic (water-repellent), they don't act like a heavy blanket. Because they sit high up and don't sink, the water can still collapse efficiently underneath them.
• The Result: On these super-repellent rafts, the jet shoots up just as high as it would on clean water!

4. The "Liquid Marble" Effect: The Ultimate Escape

This is the most fascinating part. When the jet shoots up from a super-repellent raft, it doesn't just carry water; it carries the beads with it.

• The Armoring: As the water column shoots up, the tiny plastic beads rush to cover the surface of the water droplet, wrapping around it like a shell.
• The Liquid Marble: The result is a "liquid marble"—a droplet of water completely encased in a shell of plastic.
• Why it matters: These marbles are incredibly stable. They don't easily break apart or merge back with the ocean. They are like little armored vehicles that can survive the journey into the atmosphere, riding the wind for long distances.

5. The "Heavy Anchor" Problem

Not all rafts behave the same.

• Glass Beads: Even if you make glass beads super-repellent, they are so heavy (dense) that they act like anchors. When the jet tries to shoot up, the heavy glass beads weigh it down, preventing the jet from forming properly. No jet means no escape into the sky.
• Plastic Beads: Lighter plastic beads are easily lifted by the jet, making them the prime candidates for atmospheric transport.

The Bottom Line

This study reveals a hidden highway for pollution.

1. Raindrops hit the ocean.
2. If the surface is covered in light, water-repellent microplastics, the rain doesn't just splash; it creates a powerful jet.
3. This jet wraps the plastic in "liquid marbles" and shoots them high into the air.
4. These marbles can travel globally, meaning rain is actively helping to spread microplastics from the ocean to the atmosphere, and eventually, to the air we breathe.

In short: The ocean isn't just a sink for plastic; under the right conditions (rain + specific types of floating plastic), it becomes a launchpad, shooting pollution into the sky.

Technical Summary

1. Problem Statement

Microplastics are pervasive contaminants in both the hydrosphere and atmosphere. While ocean-to-atmosphere transfer is known to occur via breaking waves (bubble bursting), the role of raindrop impact on the ocean surface as a mechanism for microplastic aerosolization remains poorly understood.

• The Gap: Existing models focus on bubble-mediated transport or clean liquid surfaces. They do not account for the complex interfacial dynamics of a "particle raft" (a closely packed monolayer of microplastics floating at the air-sea interface, similar to the Sea Surface Microlayer).
• The Question: How do particle size, density, and wettability influence the splashing dynamics, cavity collapse, and Worthington jet formation when a raindrop strikes a floating microplastic raft? Specifically, under what conditions are microplastics ejected into the atmosphere?

2. Methodology

The authors employed a controlled experimental setup to simulate raindrop impact on particle rafts.

• Experimental Setup:
  • Droplet Generation: 2.55 mm water droplets were generated via a syringe pump and released onto a quiescent water pool.
  • Particle Rafts: The pool surface was coated with a closely packed, monodisperse monolayer of microparticles.
  • Materials: Three particle types were used with varying densities ($\rho_p$) and sizes ($D_p$):
    • Fluorescent Polyethylene (FPE): $\rho_p \approx 1.02$ g/cm³.
    • Polyethylene (PE): $\rho_p \approx 1.35$ g/cm³.
    • Glass: $\rho_p \approx 2.50$ g/cm³.
    • Sizes ranged from 115 µm to 780 µm.
  • Wettability Treatments: Particles were tested in two states:
    • Untreated: Native hydrophobic/hydrophilic properties.
    • Glaco-treated: Coated with a superhydrophobic nanoparticle layer (contact angles $\theta > 147^\circ$).
• Imaging: Orthogonal high-speed imaging (Phantom V710 and TMX5010 cameras) captured side-view and top-view dynamics at 7,500 fps to resolve:
  • Splash onset and morphology.
  • Cavity formation and depth.
  • Cavity collapse and Worthington jet formation.
  • Particle ejection and liquid marble formation.
• Characterization: Optical profilometry was used to measure the emerged height ($h_{above}$) and submerged depth ($h_{sub}$) to calculate the dimensionless immersion ratio ($h$) and apparent contact angle ($\theta$).

3. Key Contributions

1. Unified Scaling Law for Splashing: The authors propose a composite scaling parameter ($\Lambda$) that collapses splashing thresholds across all materials and treatments, combining geometric roughness, particle inertia, and wettability.
2. Identification of Two Jetting Regimes: The study distinguishes between an elastically responsive regime (smaller particles allowing wave-swell formation) and an inertially rigid regime (larger particles suppressing swell and acting as a load).
3. Discovery of "Marble-Forming" Jets: The authors identified a unique ejection pathway specific to superhydrophobic rafts where the Worthington jet becomes fully armoured by particles, fragmenting into stable "liquid marbles" that resist coalescence.
4. Mechanistic Link to Aerosolization: The work provides a predictive framework for quantifying how rainfall transfers microplastics from the ocean to the atmosphere, identifying specific interfacial conditions that maximize this risk.

4. Key Results

A. Splashing Dynamics

• Threshold Control: Splashing onset is not governed by a single parameter but by a composite resistance.
  • Small/Deeply Immersed Particles (e.g., 115 µm untreated): Stabilize the spreading lamella, suppressing splashing (requires higher Weber numbers, $We \approx 251$).
  • Large/Weakly Immersed Particles (e.g., >550 µm or superhydrophobic): Act as a rough substrate, destabilizing the rim and promoting fingering. This lowers the splashing threshold significantly (down to $We \approx 65$ for superhydrophobic particles).
• Scaling Parameter ($\Lambda$): The transition from spreading to splashing collapses onto a single curve when plotted against:
  $$\Lambda = \left(\frac{D_p}{D}\right) \left(\frac{\rho_p}{\rho}\right) (1 - \cos\theta)^3$$
  Where $D_p$ is particle diameter, $D$ is droplet diameter, $\rho_p/\rho$ is density ratio, and $\theta$ is contact angle. High $\Lambda$ (large, dense, hydrophobic particles) leads to early splashing.

B. Particle Ejection Mechanisms

• Impact-Driven Ejection (Prompt Splash):
  • Untreated Rafts: Particle ejection is strongly suppressed due to strong capillary adhesion (deep immersion). Only a few particles detach even at high impact speeds.
  • Superhydrophobic Rafts: Weak capillary pinning allows immediate radial ejection of tens to hundreds of particles upon impact.
• Jet-Mediated Ejection (Worthington Jet):
  • Untreated Rafts: Particles are entrained within the rising jet. Large particles (550–780 µm) dampen the jet, creating slow, thick jets that pinch off single large droplets carrying particle clusters.
  • Superhydrophobic Rafts: The jet forms a continuous particle shell, creating liquid marbles. These are highly stable, resist coalescence, and can carry significant particle loads into the atmosphere.

C. Cavity Collapse and Jet Height

• Cavity Depth: Cavity depth decreases monotonically with increasing particle size for all treatments.
• Collapse Morphology:
  • Clean Water/Small Untreated: Sharp W-shape collapse (wave-swell mediated) $\rightarrow$ Slow, thick jet.
  • Superhydrophobic (Intermediate size): Soft W or Flat V shape. The energy is diverted into particle ejection, resulting in a faster, thinner jet that reaches heights comparable to or exceeding clean water.
  • Large/Inertially Rigid Rafts: Sharp V-shape collapse. The raft acts as a rigid load, suppressing wave swells and producing short, thick jets with poor vertical extension.
• Jet Height Scaling: The maximum jet height ($H_J$) follows a power law modulated by a resistance factor $\Pi$:
  $$H_J \propto \frac{Fr^\alpha}{1 + \Pi}, \quad \text{where } \Pi = \left(\frac{D_p}{D}\right) \left(\frac{\rho_p}{\rho}\right) (1 + \cos\theta)$$
  • Elastic Regime ($\alpha \approx 1.03$): Small particles; jet height scales nearly linearly with inertia.
  • Rigid Regime ($\alpha \approx 1.22$): Large particles; steeper scaling but significantly reduced jet height due to momentum dissipation.

5. Significance and Implications

• Environmental Risk Assessment: The study identifies that small (115–327 µm), weakly immersed, superhydrophobic microplastics present the highest risk for atmospheric transport. These particles facilitate the formation of "liquid marble" aerosols that are stable and persistent in the atmosphere.
• Mechanistic Insight: It challenges the view of the ocean surface as a simple liquid interface, showing that particulate monolayers fundamentally alter canonical droplet-impact physics (splashing thresholds, jet morphology, and breakup).
• Broader Applications: The findings extend beyond microplastics to other granular-fluid interactions, such as soil erosion by rain, impacts on sandy substrates, and the behavior of floating debris fields.
• Predictive Framework: The derived scaling laws ($\Lambda$ and $\Pi$) allow researchers to predict aerosolization rates under various rainfall conditions and ocean surface contamination levels, improving global models of microplastic distribution.

In conclusion, this work demonstrates that rainfall is a potent mechanism for microplastic transfer, but the efficiency is critically dependent on the physical properties of the floating particles. Superhydrophobicity and specific size ranges create a "perfect storm" for generating stable, particle-laden aerosols.