High-Speed Imagery Analysis of Droplet Impact on Van der Waals and Non-Van der Waals Soft-Textured Oil-Infused Surfaces

This study utilizes high-speed imaging to demonstrate that while both silicone oil and hexadecane can enhance droplet rebound on textured PDMS surfaces, silicone oil-infused substrates maintain robust, complete rebound across all impact velocities due to stable capillary retention, whereas hexadecane-infused surfaces exhibit velocity-dependent rebound and gradual performance degradation caused by lubricant loss.

The Gist

Imagine you are standing in a heavy rainstorm, but instead of getting wet, every single raindrop bounces off your jacket like a rubber ball. That is the dream of "super-repellent" surfaces. Scientists have been trying to create this for decades, but real-world rain is messy, fast, and powerful.

This paper is about a new, smarter way to build these "magic jackets" (surfaces) that can handle fast-moving water drops without failing. The researchers from IIT Goa used a mix of soft rubber, tiny Lego-like structures, and two different types of oil to figure out what works best.

Here is the story of their discovery, explained simply:

1. The Setup: Building a "Soft Lego" City

First, the scientists made a surface out of PDMS (a soft, rubbery material similar to what is used in kitchen spatulas or contact lenses). They didn't leave it flat; they stamped it with thousands of tiny, square pillars (like a microscopic city of Lego bricks).

• The City Layout: They made two versions of this city: one with the bricks very close together (5 microns apart) and one with them far apart (20 microns).
• The Coating: They treated the rubber with a chemical (OTS) to make it naturally hate water, like a duck's feathers.

2. The Secret Ingredient: The "Oil Bath"

To make the surface even better, they added oil. But they tried two different methods to get the oil there:

• Method A (The Paint Job): They dipped the surface in oil and pulled it out, leaving a thin layer of oil sitting on top of the Lego bricks.
• Method B (The Soak): They let the rubber soak up the oil like a sponge, so the oil was trapped inside the rubber and between the bricks.

They used two different "oils":

1. Silicone Oil (SO-5cSt): This oil is like a best friend to the rubber. They stick together tightly (Van der Waals forces).
2. Hexadecane: This oil is like a stranger to the rubber. They don't really like each other and don't stick well (Non-Van der Waals).

3. The Test: The High-Speed Water Drop

The scientists dropped water from different heights to see what happened. They used a high-speed camera (like a super-slow-motion movie) to watch the drops hit the surface. They measured the "Weber number," which is just a fancy way of saying "how hard is the drop hitting?"

What Happened with the "Paint Job" (Coated Surfaces)?

• The Problem: Because PDMS is porous (like a sponge), the oil sitting on top slowly got sucked into the rubber.
• The Result: The "Lego city" lost its oil layer. The water drop hit the rubber bricks directly.
  • If the drop hit gently, it bounced a little.
  • If the drop hit hard, it got stuck. The oil couldn't stay on top to act as a shield. It was like trying to run on a track where the rubber mats keep getting pulled away.

What Happened with the "Soak" (Absorbed Surfaces)?

This is where the magic happened. Because the oil was inside the rubber, it kept coming to the surface to refill the gaps.

• Scenario 1: The "Best Friend" Oil (Silicone Oil)

  • The Analogy: Imagine a water slide covered in a layer of slippery soap that never runs out because the slide itself is made of soap.
  • The Result: No matter how hard the water drop hit (even at high speeds), it always bounced off completely. The oil stayed in place, forming a perfect, invisible shield. The drop slid off without ever touching the rubber.
  • Why? The oil and the rubber loved each other so much that the oil formed a stable, continuous film that the water couldn't break.

• Scenario 2: The "Stranger" Oil (Hexadecane)

  • The Analogy: Imagine a water slide where the oil is only in the grooves, but the top of the slide is dry.
  • The Result: It worked okay for gentle drops. But when the drop hit hard, it smashed through the oil in the gaps and hit the dry rubber.
  • The Outcome: The drop stuck to the surface or only bounced partially. The oil was easily pushed away because it didn't "stick" to the rubber well enough to stay put under pressure.

4. The Durability Test: The "Endless Rain"

The scientists didn't just drop one water drop; they dropped the same spot over and over again to see how long the surface would last.

• The "Best Friend" Surface: It kept bouncing perfectly for about 17–20 hits before it started to wear out.
• The "Stranger" Surface: It gave up much faster, only lasting 4–8 hits.
• The Lesson: The oil that "loves" the rubber (Silicone) stays put and protects the surface much longer. The oil that doesn't (Hexadecane) gets washed away quickly.

The Big Takeaway

This paper teaches us that to make a surface that repels water perfectly, you can't just put oil on top. You have to:

1. Soak the material so the oil is stored inside like a reservoir.
2. Choose the right oil that chemically "loves" the material (like Silicone and PDMS).

If you do this, you create a surface that is like a self-repairing, slippery shield. Even if a water drop hits it like a bullet, the oil rushes to fill the gaps, and the drop bounces away clean. This could lead to better anti-icing planes, self-cleaning windows, or medical devices that don't get clogged with bacteria.

In short: To make water bounce, don't just paint the surface with oil; soak the sponge with the right kind of oil, and let the oil do the heavy lifting!

Technical Summary

1. Problem Statement

Droplet impact dynamics on solid surfaces are critical for applications ranging from agricultural spraying to anti-icing. While superhydrophobic surfaces (relying on trapped air) are well-studied, they suffer from instability under high pressure or impact, leading to wetting transitions. Slippery Liquid-Infused Porous Surfaces (SLIPS) offer a robust alternative by replacing air with a lubricating oil film.

However, a significant gap exists in understanding how soft, porous substrates (like PDMS) interact with lubricants under dynamic impact conditions. Specifically, the literature lacks a systematic comparison between:

• Coated vs. Absorbed states: How does the method of lubricant application (surface coating vs. bulk absorption) affect stability?
• Lubricant Chemistry: How do lubricants with different affinities for the substrate (Van der Waals vs. Non-Van der Waals interactions) influence droplet rebound and durability?
• Texture Geometry: How do micro-post spacings (5µm vs. 20µm) interact with these variables?

2. Methodology

Sample Fabrication:

• Substrate: Textured Polydimethylsiloxane (PDMS) surfaces were created via soft lithography using microtextured silicon masters.
• Geometry: Square micro-posts (10 × 10 × 10 µm) with two distinct inter-post spacings: 5 µm (high solid fraction) and 20 µm (low solid fraction).
• Functionalization: Surfaces were treated with Octadecyltrichlorosilane (OTS) to lower surface energy and create a non-polar interface.
• Lubricants: Two lubricants were selected based on their intermolecular affinity with the OTS-PDMS surface:
  1. Silicone Oil (SO-5cSt): High affinity (Van der Waals interaction), forming a stable thin film.
  2. Hexadecane: Low affinity (Non-Van der Waals interaction), prone to displacement.
• Application Methods:
  • Coated: Dip-coating to create a surface layer.
  • Absorbed: Immersion for 24 hours to allow bulk absorption into the porous PDMS matrix, followed by dip-coating to remove excess surface oil.

Experimental Setup:

• Impact Dynamics: Water droplets (2.8 mm diameter) were dropped from varying heights to achieve impact velocities between 0.88 and 3.70 m/s.
• Weber Numbers (We): Experiments covered We = 28, 63, 127, and 247.
• Imaging: High-speed photography (Phantom VEO 410, 5000 fps) captured spreading, retraction, and rebound behaviors.
• Analysis: Contact Angle Hysteresis (CAH) was measured to assess static wettability. Durability was tested via repeated impact cycles at the same location, followed by gravimetric analysis to measure lubricant loss.

3. Key Contributions

1. Differentiation of Lubricant Affinity: The study rigorously defines and experimentally validates the difference between Van der Waals (VdW) SLIPS (SO-5cSt) and Non-Van der Waals (nVdW) SLIPS (Hexadecane) on soft porous substrates.
2. Coating vs. Absorption Mechanism: It reveals that on porous PDMS, simple surface coating is transient due to oil absorption into the bulk. In contrast, pre-absorption creates a more stable, continuous lubricating layer that significantly alters impact dynamics.
3. Durability Quantification: The paper provides quantitative data on lubricant retention, demonstrating that VdW-SLIPS retain oil much longer than nVdW-SLIPS under repeated high-velocity impacts.
4. Regime Mapping: It establishes a comprehensive regime map correlating Weber number, post spacing, lubricant type, and application method to specific outcomes (Full Rebound, Partial Rebound, Deposition, Splashing).

4. Key Results

Static Wettability (CAH):

• SO-5cSt Absorbed: Exhibited extremely low CAH (~1.5°), indicating a continuous, stable oil film covering both the post tops and valleys.
• Hexadecane Absorbed: Showed higher CAH (~10–12°). The oil remained trapped in the valleys but failed to form a stable film on the post tops, leading to a hybrid wetting state where droplets contact both oil and solid.
• Coated Samples: Showed intermediate behavior but degraded over time as oil was absorbed into the PDMS, exposing the solid substrate.

Dynamic Impact Behavior:

• SO-5cSt Absorbed (VdW-SLIPS):
  • Result: Complete rebound occurred across all Weber numbers (28–247) for both 5µm and 20µm spacings.
  • Mechanism: The strong affinity creates a uniform, continuous lubricating layer that prevents water-solid contact. Even at high We (247), where prompt splashing occurred due to rim instability, the droplet retained enough momentum to fully rebound.
• Hexadecane Absorbed (nVdW-SLIPS):
  • Result: Highly dependent on We and spacing.
    • 5µm Spacing: Partial rebound at low We; complete deposition (no rebound) at high We.
    • 20µm Spacing: Partial rebound at low We; full rebound at intermediate We; deposition at high We.
  • Mechanism: Weak affinity allows the impacting droplet to displace the oil, penetrate the texture, and contact the solid PDMS-OTS substrate directly, causing energy dissipation and adhesion.

Durability and Oil Retention:

• SO-5cSt (VdW): Maintained full rebound for 17–20 impact cycles. Gravimetric analysis showed minimal mass loss, attributed to "cloaking" (a thin oil layer transferring to the droplet) rather than bulk displacement.
• Hexadecane (nVdW): Transitioned to partial rebound after only 4–8 cycles. Significant mass loss was observed due to the displacement and mixing of the lubricant with the impacting water droplet.

Scaling Laws:

• The maximum spreading factor ($\beta_{max}$) followed the power-law relationship $\beta_{max} \sim We^{0.30}$, consistent with the theoretical $We^{0.25}$ scaling for low-viscosity liquids on slippery surfaces.

5. Significance

This work provides critical design guidelines for engineering durable, anti-wetting surfaces:

• Material Selection: For applications requiring long-term durability (e.g., anti-icing, self-cleaning), selecting a lubricant with strong Van der Waals affinity to the substrate is more critical than the texture geometry itself.
• Fabrication Strategy: For porous soft materials like PDMS, bulk absorption of the lubricant is superior to simple surface coating, as it ensures a continuous film that resists displacement during high-energy impacts.
• Application Potential: The findings support the development of robust SLIPS for harsh environments where mechanical stress and repeated droplet impacts would otherwise degrade standard superhydrophobic or poorly infused surfaces. The ability to predict rebound vs. deposition based on lubricant chemistry allows for tailored surface designs in microfluidics, spray cooling, and biofouling prevention.