1. Introduction

Plasma surface treatment is widely used to improve the wettability, adhesion, and surface properties of various materials, involving industries such as plastics, semiconductors, automotive components, and medical equipment. Traditionally,Measurement of water contact angleIt is the main method for evaluating the effectiveness of plasma activation. This method assumesThe surface tension of water remains constantAnd infer the change in surface free energy of solids based on the change in contact angle.

However, recent studies have shown that,The surface after plasma treatment may release soluble by-productsThusChanging the surface tension of water itselfThis may lead to misleading contact angle measurement results, resulting inPseudo activation phenomenonThe decrease in contact angle does not necessarily mean an increase in the surface free energy of the solid, but rather a change in the surface tension of water.

This article explores the limitations of traditional plasma surface treatment evaluation methods, analyzes the mechanism of pseudo activation phenomenon, and proposes a combination ofWilhelmy board methodandMeasurement of water contact angleofSystematic characterization methodTo achieve more accurate surface assessment.

2. Traditional plasma surface activation principle

2.1 Mechanism of plasma surface treatment

Plasma surface treatmentPhysical and chemical interactionsThe main mechanisms for changing the surface of materials include:

1. Surface functionalization

• Introduction of plasma on material surfacePolar functional groups（–OH、–COOH、–C=O）， Increase surface free energy and enhance hydrophilicity.

2. surface cleaning

• Plasma removes organic pollutants and hydrocarbons, making the surface cleaner and more active.

3. Microstructure modification

• By plasma etching, micro nano scale rough structures are formed on the surface to improve wettability and adhesion.

2.2 Limitations of Traditional Water Contact Angle Measurement

Traditionally, the evaluation of plasma activation effectiveness relies onMeasurement of water contact angleAnd based on the following assumptions:

• Decreased contact angle→ It is believed that the surface free energy increases and the wettability improves.

• The contact angle remains unchanged or increases→ It is believed that plasma treatment is ineffective or has limited effectiveness.

However, this methodImplicitly assuming that the surface tension of water remains constantBut if after plasma treatmentThe surface tension of water changesThe change in contact angle may not truly reflect the characteristics of the solid surface.

3. Pseudo activation phenomenon

3.1 Definition of pseudo activation

The pseudo activation phenomenon refers to:The contact angle of water decreases after plasma treatment, but the reason is not due to an increase in solid surface free energy, but rather due to the release of soluble byproducts on the surface, which reduces the surface tension of water.

3.2 Possible causes of pseudo activation

The main factors causing pseudo activation include:

1. Low molecular weight byproduct deposition

• Plasma reaction generationOxidation, degradation, or fragmentation of small molecule compoundsDissolve in water and change its surface tension.

2. Redistribution of surface pollutants

• Plasma may not haveeffectiveRemove pollutants, but redistribute them to change the behavior of the liquid-solid interface.

3. Solid-liquid interface chemical reaction

• Some plasma modified surfaces may catalyze changes in the physical and chemical properties of water, thereby affecting surface tension.

3.3 Experimental evidence of pseudo activation

Case study: Plasma treated polyethylene (PE)

• Before processing, PE'sThe contact angle of water is about 90 °After plasma treatment, it usually decreases to50°-60°.

• However, in some cases, the contact angleAbnormal decrease to 10 ° -20 °Far below the normal range.

• Using the Wilhelmy plate method to measure the surface tension of water, it was found thatThe surface tension of water decreases from 70 mN/m to 23 mN/mThis indicates that the soluble by-products released from the surface of PE affect the properties of water.

• This indicates that,The decrease in contact angle is not due to an increase in surface free energy, but rather a change in surface tension of waterThis leads to distortion in traditional evaluation methods.

4. The impact of pseudo activation phenomenon on different industries

4.1 Semiconductor and Electronics Industry (Wafer, PCB, LCD)

• In semiconductor manufacturing,O ₂ or Ar plasma cleaningUsed to remove organic residues and improve adhesion.

• If there is residue after plasma treatmentFluoride or oxidation by-productsDissolving in water may alter the surface tension of the water, leading to incorrect contact angle assessment.

4.2 Automotive Industry (Plastic Parts and Coatings)

• PP, ABS and other automotive plastics are often treated with plasma to enhance coating adhesion.

• Research has found that althoughThe contact angle of water decreases, but the adhesion of the coating does not significantly improveThis indicates the possibility of a pseudo activation effect.

4.3 Medical and Biotechnology Applications

• Biomaterials such as PEEK and PTFE are modified by plasma to improve their performanceBiocompatibility and hydrophilicity.

• If plasma treatment causes the release of small molecule substances on the surface, it mayChanging the surface tension of biological fluidsThus affectingCell interaction and biocompatibility assessment.

5. Limitations of traditional contact angle measurement

1. Unable to distinguish between surface free energy changes and liquid surface tension changes

2. Assuming the surface tension of water is constant, but it may actually vary

3. Contradictory results may arise when measuring with different liquids

6. Systematic characterization method: Wilhelmy plate method+contact angle measurement:

RealDrop ®/ TrueDrop ® The contact angle measuring instrument integrates optical contact angle and mechanical platinum plate method to achieve integrated testing.

Step 1: Measure the surface tension of water using the Wilhelmy plate method

• Before conducting contact angle measurement, first confirm using the Wilhelmy plate methodDoes plasma treatment change the surface tension of water.

• If the surface tension of water remains constant, contact angle measurement can reflect changes in surface free energy.

• If the surface tension of water has changed, further analysis of its source is required.

Step 2: Contact angle measurement

• Contact angle data can only be used to evaluate solid surface properties when it is confirmed that the surface tension of water has not changed.

Step 3: Additional surface analysis techniques, if necessary

Combining other characterization methods to conduct in-depth analysis of surface modification:

• XPS (X-ray photoelectron spectroscopy)Determine the changes in surface chemical groups after plasma treatment.

• FTIR (Fourier Transform Infrared Spectroscopy)- Detect changes in surface molecular structure.

• AFM (Atomic Force Microscopy)Analyze surface roughness and nanoscale morphology changes.

7. Conclusion

Traditional evaluation methods for plasma treatmentDependent on water contact angle measurementBut itImplied assumption that the surface tension of water remains constantHowever, when plasma treatment causes the release of soluble byproducts on the surface and changes the surface tension of water, it can triggerPseudo activation phenomenonThis leads to misleading contact angle measurements.

To avoid misinterpretation, it is recommended to useSystematic characterization method：

1. Measurement of surface tension of water using the Wilhelmy plate method.

2. Confirm that the surface tension of water remains unchanged before measuring the contact angle.

3. Comprehensive characterization using surface analysis techniques.