1. Introduction
In fields such as materials science, semiconductor manufacturing, and coating technology, solid surface wettability (contact angle) is a key indicator for evaluating surface properties. However, up to 99% of users and instrument manufacturers confuse solid surface chemical diversity (surface energy differences) with surface cleanliness (residual contaminants), leading to inaccurate contact angle measurement results and, consequently, affecting product quality and R&D efficiency. This article systematically analyzes this cognitive misconception, starting from theoretical mechanisms, testing methods, and industry case studies, and proposes standardized solutions.

2. Core Concept Clarification: Chemical Diversity vs. Cleanliness
Solid Surface Chemical Diversity (Chemical Diversity): Refers to the differences in surface free energy caused by the inherent properties of the material, such as material composition, crystal face orientation, and functional group distribution. For example, the surface energy of the (111) plane and the (100) plane of a silicon wafer are different.
Impact: It leads to different contact angles for the same liquid on the surface, which is an intrinsic characteristic of the material.

Solid Surface Cleanliness (Cleanliness): Refers to whether the solid surface contains organic contaminants (such as fingerprints, oils), surfactants, chemical reagent residues (such as release agents, cutting fluids), etc.
Impact: Contaminants can alter surface tension, leading to systematic errors in contact angle measurements (usually causing smaller values), which obscure the material’s intrinsic characteristics.

Key Difference: Chemical diversity is an inherent property of the material, while cleanliness is a variable caused by external contamination. Confusing these two factors leads to misjudging test results, such as attributing the impact of contaminants to material defects.

3. Mechanism of Cleanliness Impact on Contact Angle Measurements
Surface Tension Reduction Effect
Contaminants (such as surfactants) adsorb onto the solid surface, forming a low-surface-energy layer, causing a decrease in the surface tension of the test liquid (e.g., from 72 mN/m to 65 mN/m). According to Young's equation:

$\cos \theta = \frac{\gamma_{sv} - \gamma_{sl}}{\gamma_{lv}}$

When the surface tension of the test liquid (water) $\gamma_{lv}$ decreases, the contact angle $\theta$ will systematically become smaller, unrelated to the actual surface energy.

Contact Angle Measurement Error Amplification
The uneven distribution of contaminants can exacerbate the fluctuation in contact angle. For example, residual fingerprints may cause local contact angle differences of more than 10°, and the instrument may mistakenly attribute this fluctuation to surface chemical diversity.

4. Solid Surface Cleanliness Testing Methods
Wilhelmy Plate Method (based on ADSA technology)
Principle: Measures the vertical force change on a platinum plate placed on the surface being tested, and calculates the surface tension of the liquid on the solid surface (dynamic method).
Standard Criterion: At 25°C, the surface tension of pure water should be 72±1.5 mN/m. If the measured value is lower than this range (e.g., 68 mN/m), it indicates the presence of contaminants.
Advantages: Directly quantifies surface tension, unaffected by droplet shape, and applicable to any surface structure.

Microdroplet Contact Angle Analysis Method (Supplementary Verification)
Top view axial symmetry analysis: If the droplet contact line profile is asymmetrical (ellipticity > 1.1), it suggests uneven chemical or cleanliness.
Side view left-right contact angle difference: If the left-right contact angle difference exceeds ±2°, the sample should be rotated multiple times to eliminate viewing angle errors. If a difference remains, it indicates surface non-uniformity.
Multiple droplet fluctuation method: After dripping 10 drops of 0.1 μL water, if the contact angle standard deviation exceeds 2°, a 3D morphology analysis (SEM/AFM) should be combined to distinguish chemical diversity or structural factors.

5. Industry Application Cases: Typical Problems Due to Cognitive Confusion
Semiconductor Industry: Wafer Surface Contamination and Lithography Process Failure
Problem: The contact angle of the same batch of silicon wafers fluctuates by 10° (target 60°±2°), leading to uneven photoresist coating.
Misjudgment: Engineers attribute this to crystal face orientation differences (chemical diversity), but SEM shows consistent crystal face orientation.
Reality: The Wilhelmy method measures a surface tension of 68 mN/m, confirming residual ammonium fluoride (NH₄F).
Solution: Introduce plasma cleaning and ultrapure water rinsing, improving lithography yield to 99.5%.

Coating Industry: Insufficient Adhesion of Hydrophobic Coatings
Problem: Fluorocarbon coating contact angles only range from 90° to 100° (target ≥120°), resulting in poor adhesion.
Misjudgment: Engineers believe insufficient fluorine content in the formula (high surface energy), but XPS shows the fluorine content is up to standard.
Reality: The top view contact line has an ellipticity of 1.12, and the side view shows a left-right contact angle difference of 5°–7°. The surface tension measurement is 65 mN/m, confirming cutting fluid residue.
Solution: Increase ultrasonic cleaning and high-temperature drying, raising the contact angle to 122° and meeting adhesion standards.

Biomaterials: Failure of Cell Culture Plate Modification
Problem: PLA-based culture plates show only a 30% cell spreading rate (target ≥90%).
Misjudgment: Plasma treatment is uneven (chemical diversity), but XPS shows that the hydroxyl group content is up to standard (5.2%).
Reality: The multiple droplet method shows a standard deviation of 2° in contact angle, indicating release agent residue.
Solution: Isopropanol ultrasonic cleaning and UV-ozone treatment result in a 95% cell spreading rate, with surface tension testing above 70 mN/m.

New Energy: Poor Wettability of Lithium Battery Separator
Problem: Contact angle at the edges of PE separators is between 12° and 15° (target ≤5°), leading to increased internal resistance of the battery.
Misjudgment: The surface modification layer is unstable, but infrared spectroscopy confirms successful modification.
Reality: The Wilhelmy method measures a surface tension of 60 mN/m, with acetone residue of 0.3%.
Solution: Increase drying temperature to 80°C and extend nitrogen purging, reducing internal resistance by 15%.

6. Cross-industry Solutions and Standardization Recommendations
Standardization of Testing Procedures
Pre-testing verification: All samples should be tested using the Wilhelmy method to confirm surface tension of 72±1.5 mN/m before analysis.
Dual-dimension analysis: Combine microdroplet top-view/side-view contact angle analysis (0.1 μL) to assess chemical diversity, along with 3D morphological analysis to exclude structural factors.

Equipment Integration Development
“Smart Contact Angle Meter”: Integrates an ADSA module, microdroplet generator, and AI image analysis system to achieve one-click detection of cleanliness and chemical diversity.

Industry Standard Promotion

7. Conclusion

The confusion between solid surface cleanliness and chemical diversity has led to product performance issues and wasted R&D resources across multiple industries. By quantifying surface tension using the Wilhelmy method, analyzing microdroplet shapes, and performing 3D morphological characterization, the two factors can be systematically distinguished. Future efforts should focus on enhancing interdisciplinary collaboration, driving surface science from “experience-based judgment” to “quantitative characterization,” providing reliable technical support for high-end manufacturing and new materials development.