🔬 condensed matter

Ambient-environment dependence of dynamic contact angles: Droplet tilting vs. captive bubble methods

This study experimentally demonstrates that while dynamic contact angles measured via the captive bubble method in water generally correspond to those obtained in air for smooth and sandpaper-polished surfaces, significant discrepancies arise for microstructured hydrophobic surfaces, which exhibit hydrophilic behavior with minimal hysteresis in degassed aqueous environments.

Original authors: Koki Iwasaki, Hiroyuki Ebata, Hiroaki Katsuragi

Published 2026-03-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Koki Iwasaki, Hiroyuki Ebata, Hiroaki Katsuragi

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to figure out how "sticky" or "slippery" a surface is. In the world of science, we call this wettability. The easiest way to test this is to drop a water droplet on a table and see how it sits. Does it bead up like a mercury ball (hydrophobic/repelling water), or does it spread out flat like a puddle (hydrophilic/loving water)?

For a long time, scientists have only tested this in air. But what if you want to know how a surface behaves underwater? Maybe you are designing a submarine hull, a medical implant, or a ship's bottom to prevent barnacles.

This paper asks a simple but tricky question: If we measure how a surface interacts with water in the air, does that tell us the same thing as measuring it underwater?

To answer this, the researchers used two different "games" to test the surface.

The Two Games: Droplets vs. Bubbles

The Droplet Game (In Air): This is the classic test. You put a drop of water on a surface and tilt the table until the drop slides off. You measure the angle of the drop just before it moves.
The Bubble Game (Underwater): This is the "Captive Bubble" method. Instead of a water drop, you trap an air bubble against the surface while it's submerged in water. You push the bubble against the surface and then pull it away, measuring the angle just like the droplet game.

The big question was: Do these two games give the same score?

The Findings: When the Rules Match

The researchers tested three types of surfaces, and here is what they found using some fun analogies:

1. The Smooth Surfaces (The Polished Floor)

Imagine a perfectly smooth glass table.

Result: Whether you put a water drop on it in the air or an air bubble against it underwater, the results were identical.
The Takeaway: If the surface is smooth, it doesn't matter if you test it in the air or underwater; the "stickiness" is the same. The bubble method works perfectly here.

2. The Rough Surfaces (The Sandpaper)

Imagine a surface that has been sanded down with sandpaper. It has tiny hills and valleys.

Result: Even with the roughness, the air and underwater tests matched up very well.
The Analogy: Think of a sponge. In the air, water soaks into the tiny holes (this is called the Wenzel state). Underwater, an air bubble gets stuck in those same holes, but it's the "reverse" of the sponge (the Reversed Wenzel state).
The Takeaway: Even though the physics is slightly different (water filling holes vs. air filling holes), the final "angle" measurement comes out the same. The bubble method is reliable for rough surfaces too.

3. The Micro-Structured Surfaces (The Lotus Leaf Effect)

This is where things got interesting. The researchers used a surface with tiny, microscopic pillars (like a forest of tiny trees), similar to a lotus leaf.

In Air: The water drop sits on top of the "trees," trapping air underneath. It beads up and rolls off easily. It looks super hydrophobic (water-repelling).
Underwater (The Surprise): When they put this same surface underwater and degassed it (removed trapped air bubbles from the "trees"), the behavior flipped completely! The surface acted super hydrophilic (water-loving). The air bubble couldn't get stuck in the "trees" anymore, so it slid off easily with almost no resistance.
The Analogy: Imagine a Velcro strip. In the air, the water drop is like a fuzzy ball that gets stuck in the hooks (hydrophobic). But underwater, if you remove the air that was holding the hooks open, the water just washes right over the smooth plastic underneath, and the "stickiness" disappears.

Why Does This Matter?

This study is like finding a new rule for a video game.

For Smooth and Rough Surfaces: You can safely use the "Bubble Game" (underwater) to predict how a surface will behave in the "Droplet Game" (air). It's a time-saver!
For Micro-Structured Surfaces: You cannot assume the underwater behavior is the same as in the air. If you design a surface to repel water in the air (like a self-cleaning window), it might actually attract water and let things stick to it when it's underwater, unless you account for those tiny trapped air bubbles.

The Bottom Line

The researchers built a special machine to trap air bubbles against surfaces underwater and compared it to the standard water-drop test.

They discovered that for most surfaces, air and water tell the same story. However, for surfaces with tiny, engineered textures (like those found in nature or high-tech materials), the story changes completely underwater. If you want to know how a surface behaves on a submarine or a medical device, you can't just test it in a lab in the air; you have to test it where it will actually live: underwater.

1. Problem Statement

While measuring the contact angle of a water droplet on a solid surface in air is a standard method for evaluating surface wettability, many applications (e.g., marine science, medicine) require understanding wettability in aqueous environments. The captive bubble method (attaching an air bubble to a solid surface underwater) is the standard technique for this. However, it has not been experimentally verified whether dynamic contact angles measured via the captive bubble method in water correspond to those measured via the droplet tilting method in air.

Specifically, there is a knowledge gap regarding:

Whether dynamic contact angles are consistent between air and water for smooth and rough surfaces.
How microstructured surfaces (which exhibit hydrophobicity in air via the Cassie–Baxter state) behave dynamically in water, particularly regarding the potential trapping of air bubbles in surface grooves.

2. Methodology

The authors constructed two experimental systems to measure dynamic contact angles (advancing and receding) and compared results across different surface types.

Experimental Systems:

Captive Bubble Method (Water): An air bubble (~43 µL) was generated at a syringe tip and pressed against a submerged sample using a universal testing machine. The bubble was compressed (receding angle) and then detached (advancing angle) by moving the syringe vertically. This compression method prevented lateral bubble movement, ensuring stability.
Droplet Tilting Method (Air): A water droplet (~33 µL) was placed on a sample mounted on a rotating stage. The stage was tilted until the droplet slid, allowing measurement of advancing (lower side) and receding (upper side) angles.

Sample Materials:

Smooth Surfaces: PET, PC, PTFE, and PMMA plates.
Rough Surfaces (Wenzel State): PET and PMMA plates polished with sandpaper (grit 280, 400, 600). These were designed to be in the Wenzel state in air (liquid penetrates roughness) and the reversed gas–liquid Wenzel state in water.
Microstructured Surfaces (Cassie–Baxter State): PMMA plates with laser-processed pillars (pitch 50 µm, depth ~63 µm, top 15 µm × 15 µm). These exhibited hydrophobicity in air.
- Critical Pre-treatment: To ensure accurate underwater measurements, microstructured samples were subjected to ultrasonic degassing in water to remove trapped air bubbles from the grooves before testing.

3. Key Contributions

Systematic Comparison: The study provides the first experimental verification comparing dynamic contact angles of smooth and rough surfaces measured in air (droplets) versus water (bubbles) using consistent methodologies.
Degassing Protocol: The authors demonstrated the critical importance of removing trapped air from microstructured surfaces before underwater testing to avoid artifacts caused by the "lotus-leaf effect" (trapped air pockets).
Theoretical Validation: The study successfully applied the Cassie–Baxter equation to the inverted scenario (air bubble on a solid in water), validating the model for predicting apparent contact angles in aqueous environments.

4. Results

A. Smooth Surfaces:

Dynamic contact angles measured in air and water were nearly identical for all smooth polymer samples (PET, PC, PTFE, PMMA).
Standard deviations were low (max 2.2° for bubbles, 3.7° for droplets), indicating high reproducibility and confirming that the captive bubble method yields results comparable to the droplet method for smooth surfaces.

B. Rough Surfaces (Sandpaper-polished):

Surfaces in the Wenzel state (air) and reversed Wenzel state (water) showed similar dynamic contact angles in both environments.
Observation: Roughness generally reduced contact angles compared to smooth surfaces. The receding angle decreased more significantly than the advancing angle.
Mechanism: This is attributed to enhanced wetting (Wenzel relation) combined with increased pinning effects from surface asperities. While minor discrepancies existed (likely due to manual polishing variability), the overall trends were consistent between air and water.

C. Microstructured Surfaces (Hydrophobic PMMA):

Drastic Difference: Surfaces that were hydrophobic in air (high contact angle) exhibited superhydrophilic behavior (very low contact angle) in water under degassed conditions.
Hysteresis: The hydrophilic behavior in water was accompanied by very small hysteresis, contrasting with the larger hysteresis often seen in air.
Mechanism (Inverted Cassie–Baxter):
- In air, the droplet sits on top of pillars (Cassie–Baxter state).
- In water (degassed), the air bubble does not penetrate the grooves but contacts the pillar tops and the surrounding water (Reversed Gas–Liquid Cassie–Baxter state).
- Using the Cassie–Baxter equation with the calculated area fraction of the solid ( $f_1 \approx 0.09$ ) and the liquid phase ( $f_2 \approx 0.91$ ), the theoretical contact angles ( $\theta_a \approx 26.3^\circ$ , $\theta_r \approx 17.6^\circ$ ) matched the experimental data closely.
- This confirms that without trapped air, the microstructure effectively makes the surface hydrophilic to the bubble.

5. Significance

Validation of Methods: The study confirms that the captive bubble method is a reliable proxy for droplet measurements for smooth and Wenzel-state rough surfaces, validating its use for underwater wettability studies.
Environmental Dependence: It highlights that surface wettability is not an intrinsic property of the material alone but is heavily dependent on the ambient environment (air vs. water) and the presence of trapped gas.
Applications: These findings are crucial for fields where underwater surface interactions are critical, including:
- Marine Science: Understanding biofouling and organism attachment on ship hulls or sensors.
- Medicine: Designing biomaterials where interfacial properties dictate biological adhesion.
- Fluid Mechanics: Improving models for droplet/bubble dynamics on textured surfaces.

In conclusion, the paper establishes that while smooth and Wenzel-state rough surfaces behave similarly in air and water, microstructured hydrophobic surfaces undergo a fundamental wetting transition in degassed water, behaving hydrophilically due to the inverted Cassie–Baxter state.