1. Introduction

1.1 Importance of Contact Angle Measurement in Materials Science and Industrial Applications
In the field of materials science, contact angle measuring instruments are essential tools for evaluating material wettability, accurately reflecting the interactions between material surfaces and liquids. By measuring the contact angle using a contact angle measuring instrument, the hydrophilicity or hydrophobicity of a material can be determined, which is crucial for developing new functional materials such as waterproof coatings and superhydrophobic materials. In industrial applications, contact angle measuring instruments help optimize production processes, such as in the printing and coating industries, by adjusting material surface wettability to improve product quality and production efficiency. In the pharmaceutical field, contact angle measuring instruments are used to study the interactions between drug carriers and biological systems, providing critical insights for drug development. In microelectronics, precise contact angle measuring instruments ensure the cleanliness and reliability of device surfaces, playing a significant role in advancing technological progress.

1.2 Concepts and Differences Between Chemical Diversity and Cleanliness of Solid Surface Free Energy
The chemical diversity of solid surface free energy refers to the non-uniform surface free energy caused by microscopic differences in chemical composition and functional group distribution. On the other hand, surface cleanliness refers to the state of a surface being free from organic pollutants and other impurities. These two concepts are closely related: chemical diversity affects the measurement and judgment of surface cleanliness, while cleanliness influences the accurate measurement of surface free energy. If pollutants are present on the surface, even with uniform chemical composition, contact angle measuring instruments may yield inaccurate results due to the influence of contaminants. Therefore, distinguishing between these two concepts is crucial for accurately measuring contact angles and understanding solid surface properties using contact angle measuring instruments.

2. Impact of Chemical Diversity of Solid Surface Free Energy on Contact Angle Measurements

2.1 Mechanism of Surface Chemical Heterogeneity Leading to Contact Angle Anisotropy
Surface chemical heterogeneity causes contact angle anisotropy. When a solid surface has variations in chemical composition, functional group distribution, or crystal orientation, the surface free energy differs across regions. For example, on a crystal surface with different crystal planes, the atomic arrangement and bonding strength vary, leading to differences in surface free energy. As a result, liquid droplets exhibit different contact angles on different crystal planes, showing anisotropy. Contact angle measuring instruments can capture this anisotropic behavior.

2.2 Relationship Between Contact Angle Hysteresis and Surface Chemical Diversity
Contact angle hysteresis refers to the phenomenon where the advancing and receding contact angles of a droplet are unequal during expansion and contraction. Surface chemical diversity is a significant factor contributing to contact angle hysteresis. On chemically diverse surfaces, liquid molecules must overcome resistance in high free energy regions during expansion, resulting in a larger advancing contact angle. During contraction, the adhesion force in low free energy regions is stronger, leading to a smaller receding contact angle. The greater the degree of chemical diversity, the more pronounced the contact angle hysteresis. Contact angle measuring instruments can precisely measure this hysteresis phenomenon.

2.3 Method for Evaluating Surface Chemical Diversity Using Top-View Contact Angle Analysis
Top-view contact angle contour analysis is an effective technique for assessing surface chemical diversity. By capturing top-view images of droplets on a solid surface, the contour of the contact line can be analyzed. If the surface is chemically uniform, the contact line contour will appear as a regular circle or ellipse. In contrast, chemically diverse surfaces exhibit irregular contours, such as distortions or deformations. Image processing software can analyze the contour and calculate contact angles in different directions. If significant variations in contact angles are observed, surface chemical diversity can be confirmed. Contact angle measuring instruments play a critical role in this process.

2.4 Reliability of Side-View and Left-Right Contact Angle Measurements
Side-view and left-right contact angle measurements are reliable for detecting surface chemical diversity but are susceptible to perspective errors. By rotating the sample and capturing side-view images from different angles, multiple side-view contact angles can be obtained. If these values fluctuate significantly, surface chemical diversity is indicated. Left-right contact angle measurements compare the contact angles on both sides of a droplet to assess surface chemical uniformity. Contact angle measuring instruments can help minimize perspective errors.

2.5 Multi-Droplet Contact Angle Fluctuation Method for Assessing Surface Chemical Diversity
The multi-droplet contact angle fluctuation method involves depositing multiple droplets on a solid surface and measuring the contact angle of each droplet. If the surface is chemically uniform, the contact angles of the droplets will be similar. However, if the surface is chemically diverse, the contact angles will vary significantly due to differences in surface free energy across regions. By statistically analyzing the standard deviation or coefficient of variation of the contact angles, surface chemical diversity can be determined. Contact angle measuring instruments provide precise data for this method.

3. Impact of Surface Cleanliness on Contact Angle Measurements

3.1 Mechanism of Organic Pollutants Reducing the Surface Tension of Test Water
Organic pollutants reduce the surface tension of test water through their colloidal properties and hydrophobic groups. For example, natural organic pollutants like humic acid adsorb onto surfaces through hydrophobic interactions, coordination exchange, and hydrogen bonding, forming micelles that lower surface tension. Synthetic organic pollutants, such as phenol and polychlorinated biphenyls, aggregate at the water surface due to their hydrophobic groups, reducing intermolecular interactions and decreasing surface tension. Contact angle measuring instruments can detect these changes in surface tension.

3.2 Impact of Surfactants on Contact Angle Measurements
Surfactants, composed of hydrophilic and hydrophobic groups, reduce water surface tension when dissolved. In contact angle measurements, surfactants facilitate droplet spreading on solid surfaces, resulting in smaller contact angles. The concentration of surfactants also affects contact angle measurements, with higher concentrations leading to smaller contact angles. Contact angle measuring instruments can accurately measure these changes.

3.3 Interference of Chemical Residues on Contact Angle Measurements
Chemical residues alter the chemical properties of solid surfaces, affecting contact angle measurements. For example, residues of organic solvents like ethanol increase surface hydrophobicity, leading to smaller contact angles. Inorganic chemical residues, such as silica from tetraethyl orthosilicate hydrolysis, may form films that change surface roughness and chemical composition, impacting contact angle measurements. Contact angle measuring instruments can detect these changes.

4. Methods for Measuring Surface Cleanliness

4.1 Principle and Application of the Wilhelmy Plate Method
The Wilhelmy Plate method calculates liquid surface tension by measuring the force exerted on a solid plate immersed in the liquid. In surface cleanliness measurements, the surface tension of water droplets on a solid surface is measured to detect contaminants. If the surface tension deviates from the reference value, surface impurities are indicated. Contact angle measuring instruments provide accurate data for this method.

4.2 Application of ADSA Technology in Surface Tension Measurement
ADSA (Axisymmetric Drop Shape Analysis) technology automatically identifies and fits droplet contours to precisely measure surface tension. This technique is suitable for dynamic surface tension measurements and is widely used in petroleum, chemical, and pharmaceutical industries. Combining ADSA technology with contact angle measuring instruments enhances measurement accuracy.

4.3 Method for Judging Surface Cleanliness Based on Surface Tension Values
By measuring the surface tension of water droplets on a solid surface, surface cleanliness can be assessed. If the surface tension is within the reference range, the surface is clean. Deviations indicate the presence of contaminants. Contact angle measuring instruments provide precise data for this method.

5. Comparison of the Impact of Chemical Diversity and Cleanliness on Contact Angle Measurements

5.1 Differences in Contact Angle Changes Caused by Chemical Diversity and Cleanliness
Chemical diversity causes contact angle anisotropy, while cleanliness issues lead to overall shifts in contact angle values. Chemical diversity results in varying contact angles across different directions, whereas cleanliness problems uniformly reduce contact angles. Contact angle measuring instruments can capture these differences.

5.2 Analysis of Interference Mechanisms
Chemical diversity affects contact angles by altering surface chemical composition, while cleanliness impacts contact angles by reducing water surface tension. Contact angle measuring instruments can detect these distinct interference mechanisms.

5.3 Methods for Differentiating Sources of Contact Angle Measurement Errors
By observing contact angle patterns and using additional testing methods (e.g., XPS, AFM), the sources of errors can be identified. Contact angle measuring instruments provide accurate data for this analysis.

6. Reasons for Confusing Chemical Diversity and Cleanliness Concepts

6.1 Aspects Where Chemical Diversity and Cleanliness Are Easily Confused
Both factors influence contact angle measurements and may overlap in practical scenarios, leading to confusion. Contact angle measuring instruments can help differentiate their effects.

6.2 Measurement Errors Due to Lack of Understanding of Their Differences
A lack of understanding can result in experimental design flaws and data misinterpretation. Contact angle measuring instruments provide precise data to minimize errors.

6.3 Consequences of Ignoring Cleanliness in Contact Angle Measurements
Ignoring cleanliness can lead to inaccurate contact angle measurements, affecting material wettability assessments. Contact angle measuring instruments can detect cleanliness issues.

7. Differentiating the Impact of Chemical Diversity and Surface Structure on Contact Angle

7.1 Impact of Surface Roughness on Contact Angle Measurements
Surface roughness affects contact angles by altering the contact area between liquid and solid. Contact angle measuring instruments can detect these effects.

7.2 Comparison of Chemical Diversity and Surface Structure Impact Mechanisms
Chemical diversity changes surface chemical composition, while surface structure modifies physical morphology. Contact angle measuring instruments can distinguish these mechanisms.

7.3 Methods for Differentiating Their Impact Using 3D Surface Topography Testing
3D surface topography testing analyzes surface roughness and chemical composition to differentiate the effects of chemical diversity and surface structure. Contact angle measuring instruments provide accurate data for this analysis.

8. Comprehensive Methods for Considering Chemical Diversity and Cleanliness in Contact Angle Measurements

8.1 Methods for Correcting the Impact of Chemical Diversity and Cleanliness
Surface analysis techniques (e.g., XPS, AFM) and cleaning methods can correct the effects of chemical diversity and cleanliness. Contact angle measuring instruments provide precise data for these corrections.

8.2 Experimental Design to Minimize Interference
Uniform materials, controlled experimental conditions, and high-precision contact angle measuring instruments can reduce interference from chemical diversity and cleanliness.

8.3 Data Analysis to Identify and Exclude Their Impact
Statistical analysis of contact angle data, combined with other testing methods, can identify and exclude the effects of chemical diversity and cleanliness. Contact angle measuring instruments provide accurate data for this analysis.

9. Conclusion and Outlook

9.1 Summary of Key Findings
Chemical diversity and cleanliness significantly impact contact angle measurements, with distinct mechanisms. Contact angle measuring instruments play a vital role in distinguishing and correcting these effects.

9.2 Recommendations for Improving Contact Angle Measurement Accuracy
Strict surface cleaning, high-precision contact angle measuring instruments, and multiple measurement methods can enhance accuracy.

9.3 Future Research Directions

Future research should focus on advanced methods for assessing complex surface chemical diversity, studying the impact of various contaminants, and developing new surface treatment technologies. Contact angle measuring instruments will be essential in these studies.