1. Introduction

Plasma surface treatment is widely used to enhance the wettability, adhesion, and surface properties of various materials, including plastics, semiconductors, automotive components, and medical devices. Traditionally, water contact angle measurement using a contact angle goniometer has been the primary method for evaluating the effectiveness of plasma activation. This approach assumes that the surface tension of water remains constant and that changes in contact angle directly correlate with modifications in the solid’s surface free energy.

However, recent findings indicate that plasma-treated surfaces can release soluble byproducts, which alter the surface tension of water itself. This can lead to misleading contact angle readings and create a pseudo-activation effect, where a lower contact angle is falsely interpreted as improved wettability.

This article explores the limitations of traditional plasma treatment evaluations, discusses the underlying mechanisms of the pseudo-activation phenomenon, and introduces a systematic characterization method combining the Wilhelmy plate method with water contact angle measurement to achieve accurate surface assessment.

2. Traditional Understanding of Plasma Surface Activation

2.1 Mechanisms of Plasma Surface Treatment

Plasma surface treatment modifies material surfaces through both physical and chemical processes. The key mechanisms include:

• Surface Functionalization: Plasma introduces polar functional groups (–OH, –COOH, –C=O), which increase surface free energy and enhance hydrophilicity.

• Surface Cleaning: Organic contaminants and hydrocarbons are removed, exposing a cleaner, more reactive surface.

• Microstructural Modification: Plasma etching roughens the surface at the micro/nanoscale, increasing its contact area with liquids and improving adhesion.

2.2 Reliance on Water Contact Angle Measurement

Traditionally, plasma activation has been evaluated using water contact angle measurements with a contact angle goniometer, based on the following assumptions:

• A decreased contact angle signifies increased surface free energy and improved wettability.

• An unchanged or increased contact angle indicates ineffective plasma activation.

However, this method assumes that water’s surface tension remains constant, which is not always the case. If plasma treatment alters the water’s surface tension, the observed contact angle changes may not accurately reflect the material’s true surface properties.

3. The Pseudo-Activation Phenomenon

3.1 What Is Pseudo-Activation?

Pseudo-activation refers to a scenario where water contact angle decreases after plasma treatment, not because the solid’s surface free energy has increased, but because soluble byproducts released from the treated surface lower the surface tension of the water itself.

3.2 Causes of Pseudo-Activation

Several factors contribute to this misleading effect:

• Deposition of Low-Molecular-Weight Byproducts: Plasma reactions generate oxidized, degraded, or fragmented molecules that dissolve in water and alter its surface tension.

• Surface Contamination Redistribution: Instead of complete removal, plasma may redistribute surface contaminants, modifying liquid-solid interactions.

• Chemical Reactions at the Solid-Liquid Interface: Some plasma-treated surfaces may catalyze changes in the test liquid itself, leading to variable surface tension effects.

3.3 Experimental Evidence of Pseudo-Activation

Case Study: Plasma-Treated Polyethylene (PE)

• Normally, PE has a water contact angle of ~90°. After plasma treatment, this typically reduces to 50°-60°.

• However, in some cases, the contact angle drops abnormally to 10°-20°, far below expected values.

• Wilhelmy plate measurements reveal that the water’s surface tension has dropped from 70 mN/m to 23 mN/m, indicating that soluble byproducts from the PE surface have altered the water’s properties.

This suggests that the observed contact angle reduction is not due to increased surface free energy but rather to altered water properties, making traditional evaluation methods unreliable.

4. Impact Across Different Industries

4.1 Semiconductor and Electronics Industry (Wafers, PCBs, LCDs)

O₂ or Ar plasma cleaning is used to remove organic residues and improve adhesion in semiconductor manufacturing. If the plasma treatment leaves behind fluorinated or oxidized residues, they can dissolve into water and alter its surface tension, leading to misleading contact angle measurements.

4.2 Automotive Industry (Plastic Components and Coatings)

Plasma is used to enhance adhesion for coatings and paints on plastics (e.g., PP, ABS). Some studies show that while contact angles decrease, adhesion strength does not improve correspondingly, indicating pseudo-activation.

4.3 Medical and Biotechnology Applications

Plasma modification is applied to biomaterials (e.g., PEEK, PTFE) to improve biocompatibility and hydrophilicity. If plasma-treated surfaces release small molecular fragments that affect the surface tension of biological fluids, it could lead to incorrect biocompatibility assessments.

5. The Limitations of Contact Angle Measurements

• Does Not Distinguish True Surface Free Energy Changes from Liquid Property Changes

• Assumes Constant Water Surface Tension, Which Is Not Always True

• Provides Conflicting Results When Testing with Different Liquids

6. Systematic Characterization Approach: Wilhelmy Plate Method + Contact Angle Measurement

KINO Scientific Instruments provides the TrueDrop®/RealDrop® contact angle goniometer that integrates optical contact angle measurement with the mechanical Wilhelmy plate method, allowing for more accurate characterization of plasma treatment effects.

Step 1: Wilhelmy Plate Method for Water Surface Tension Measurement Before evaluating plasma treatment using contact angles, the Wilhelmy plate method is used to confirm whether the treated surface alters water’s surface tension.

• If water’s surface tension remains unchanged, contact angle measurement can reliably indicate changes in solid surface properties.

• If water’s surface tension is altered, further investigation is required to determine the source of contamination or byproducts.

Step 2: Contact Angle Measurement on Plasma-Treated Surfaces Only after confirming that water’s surface tension remains unchanged should contact angle data be interpreted for surface free energy evaluation.

Step 3: Additional Surface Characterization Techniques (if necessary) To better understand surface modifications, other analytical techniques can be employed:

• X-ray Photoelectron Spectroscopy (XPS) – Identifies chemical groups introduced by plasma treatment.

• Fourier Transform Infrared Spectroscopy (FTIR) – Detects molecular changes on the surface.

• Atomic Force Microscopy (AFM) – Examines surface roughness at the nanoscale.

7. Conclusion

Plasma treatment evaluation has traditionally relied on water contact angle measurements using a contact angle goniometer, assuming that water’s surface tension remains constant. However, this assumption is flawed in cases where plasma-treated surfaces release soluble byproducts that alter water’s surface tension, leading to pseudo-activation effects.

To address this issue, a systematic characterization approach is recommended:

• Wilhelmy plate method to measure water surface tension.

• Water contact angle measurement only if water’s surface tension is unchanged.

• Complementary surface analysis for comprehensive material characterization.

By adopting this approach, researchers and engineers can avoid misinterpretations and ensure accurate evaluations of plasma-treated surfaces across various industries.