1. Introduction
In the fields of materials science, semiconductor manufacturing, coating technology, etc., the wettability (contact angle) of solid surfaces is a key indicator for evaluating surface properties. In the application of contact angle measuring instruments, up to 99% of users and instrument manufacturers confuse the chemical diversity (surface energy difference) of solid surfaces with surface cleanliness (pollutant residue), resulting in inaccurate test results of contact angle measuring instruments, which affects product quality and research and development efficiency. Starting from theoretical mechanisms, testing methods, and industry cases, this article systematically analyzes this cognitive misconception and proposes standardized solutions.

2、 Core Concept Analysis: Chemical Diversity and Cleanliness
Definition of Chemical Diversity on Solid Surfaces: Refers to the differences in surface free energy caused by inherent properties such as material composition, crystal orientation, and functional group distribution on solid surfaces. For example, the surface energy of the (111) and (100) faces of a silicon wafer is different.
Influence: The formation of different contact angles on the surface of the same liquid is an intrinsic property of the material.

Definition of Cleanlines on Solid Surface: Refers to the presence of external impurities such as organic pollutants (fingerprints, grease), surfactants, and chemical reagent residues (mold release agents, cutting fluids) on the solid surface.
Impact: Pollutants can alter surface tension, leading to systematic deviations (usually smaller) in the contact angle values measured by contact angle measuring instruments, masking the intrinsic properties of the material.

Key DifferencesChemical diversity is an inherent property of materials, while cleanliness is a variable caused by external pollution. Confusing the two will lead to misjudgment in contact angle measurement, such as attributing the influence of pollutants to material defects.

3、 The Influence Mechanism of Cleanliness on Contact Angle Measuring Instruments
Surface tension reduction effect
Pollutants (such as surfactants) will adsorb onto solid surfaces, forming a low surface energy layer, resulting in a decrease in the surface tension of test water droplets (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 testing the surface tension of the liquid (water)$\\gamma_{lv}$When decreasing, contact angle$\\theta$Systematically small and unrelated to actual surface energy.

Amplification of contact angle measurement error
The uneven distribution of pollutants can exacerbate fluctuations in contact angle. For example, fingerprint residue may cause local contact angle differences of more than 10 °, and contact angle measuring instruments default to attributing fluctuations to surface chemical diversity.

4、 Solid surface cleanliness testing method
Wilhelmy Plate method (based on ADSA technology)
Principle: By measuring the vertical force change of the platinum plate on the surface to be tested, the surface tension of the liquid on the solid surface is calculated (dynamic method).
Standard criterion: At 25 ℃, the surface tension of pure water should be 72 ± 1.5 mN/m. If the measured value is below this range (such as 68 mN/m), it indicates the presence of pollutants.
Advantages: Directly quantifying surface tension, not affected by droplet morphology, suitable for any surface structure.

Micro droplet contact angle analysis method (supplementary verification)
Top view axis symmetry analysis: If the contact line contour of a droplet (<0.1 μ L) is an asymmetric ellipse (ellipticity>1.1), it indicates the presence of uneven chemical or cleanliness on the surface.
Side view left and right contact angle difference: If the difference in left and right contact angles exceeds ± 2 °, the sample needs to be rotated multiple times to measure and eliminate viewing angle errors. If there is still a difference, it indicates uneven surface.
Multi drop fluctuation method: When continuously dropping 10 drops of 0.1 μ L water droplets with a standard deviation of contact angle>2 °, it is necessary to combine 3D morphology analysis (SEM/AFM) to distinguish chemical diversity or structural factors.

5、 Industry application case: Typical problems caused by 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 °), resulting in uneven photoresist coating.
Misjudgment: Engineers believe that there is a difference in crystal orientation (chemical diversity), but SEM shows that the crystal planes are consistent.
Truth: The Wilhelmy method measured a surface tension of 68 mN/m, confirming residual ammonium fluoride (NH ₄ F).
Solution: Introduce plasma cleaning+ultrapure water rinsing to increase the lithography yield to 99.5%.

Coating industry: Insufficient adhesion of hydrophobic coatings
Problem: The contact angle of fluorocarbon coating is only 90 °~100 ° (target ≥ 120 °), and the adhesion is poor.
Misjudgment: The formula has insufficient fluorine content (high surface energy), but XPS shows that the fluorine content meets the standard.
Truth: The ellipticity of the top view contact line is 1.12, the left and right contact angle difference is 5 °~7 ° when viewed from the side, and the surface tension measurement is 65mN/m, confirming residual cutting fluid.
Solution: Add ultrasonic cleaning+high-temperature drying, increase the contact angle to 122 °, and achieve adhesion standards.

Biomaterials: Cell culture dish modification failure
Problem: The cell spreading rate in PLA medium culture dishes is only 30% (target ≥ 90%).
Misjudgment: Uneven plasma treatment (chemical diversity), but XPS hydroxyl content meets the standard (5.2%).
Truth: The standard deviation of the contact angle for the multi drop method is 2 °, and there is residual release agent.
Solution: Isopropanol ultrasonic cleaning+UV ozone treatment, cell spreading rate of 95%, surface tension test above 70mN/m.

New energy: Poor wettability of lithium battery separators
Problem: The contact angle of the PE diaphragm edge is 12 °~15 ° (target ≤ 5 °), and the internal resistance of the battery increases.
Misjudgment: The surface modification layer is unstable, but infrared spectroscopy confirms successful modification.
Truth: The Wilhelmy method measured a surface tension of 60mN/m and residual acetone of 0.3%.
Solution: Increase the drying temperature to 80 ℃, extend nitrogen purging, and reduce internal resistance by 15%.

6、 Cross industry solutions and standardization recommendations
Standardization of testing process
Pre validation: All samples must be confirmed to have a surface tension of 72 ± 1.5 mN/m using the Wilhelmy method before testing.
Dual dimensional analysis: Combining top/side view contact angle analysis of microdroplets (0.1 μ L) to evaluate chemical diversity, supplemented by 3D morphology analysis to exclude structural factors.

Equipment integration development
Intelligent Contact Angle Meter ": Built in ADSA module, droplet generator, and AI image analysis system to achieve one click detection of cleanliness diversity.

Promote industry standards

VII. Conclusion
The confusion between solid surface cleanliness and chemical diversity has led to product performance issues and wasted research and development resources in multiple industries. By quantifying surface tension, micro droplet morphology analysis, and 3D morphology characterization using the Wilhelmy method, the two can be systematically distinguished. In the future, it is necessary to strengthen interdisciplinary collaboration and promote the transformation of surface science from "empirical judgment" to "quantitative characterization", providing reliable technical support for manufacturing and new material research and development.