Principle
The surface wettability of paper is one of its key physicochemical properties, which directly affects the properties of printability, coating adhesion, liquid absorption and composite processing. The contact angle meter provides an accurate means for assessing wettability by quantifying the contact angle of droplets on the paper surface. Its core principle is based on Young's equation, which describes the surface tension equilibrium relationship at the solid-liquid-gas three-phase contact point, which can be expressed as: γsv = γsl + γlv cos θ, where γsv、γsland γlvIt represents the interfacial tension between solid-gas, solid-liquid and liquid-gas, respectively, and θ is the contact angle. The smaller the contact angle value, the better the liquid spreads on the paper surface, that is, the stronger the wettability. Conversely, the larger the contact angle, the weaker the wettability.
Measurement method
Measurements are usually taken using the drip method. A microsyringe is used to smoothly deposit a specific volume of the test liquid (commonly distilled water or standard test fluid) onto the surface of the paper, the droplet side profile is captured by a high-speed camera, and the contact angle is calculated by tangent or circle fitting method using instrument software. To ensure accurate measurement, the following technical points should be noted: paper samples should be flat and wrinkle-free, and balanced under standard temperature and humidity conditions (e.g., 23°C±1°C, 50%±2% RH); The droplet volume needs to be consistent (typically 2-5 microliters); Measurements should be done instantaneously after droplet deposition to avoid evaporation or absorption effects; For paper with a rough or porous surface, the influence of the Wenzel or Cassie-Baxter model on the contact angle should be considered, and the contact angle hysteresis phenomenon can be evaluated in combination with the measurement of the forward and retreat angles.
Influencing factors
The wettability of paper surface is affected by many factors, which can be mainly divided into two categories: material characteristics and measurement conditions. The material characteristics include fiber type, filler and sizing agent content, surface roughness and chemical modification, etc. The measurement conditions involve testing liquid properties, ambient temperature and humidity, and sample pretreatment methods. The following table summarizes the main influencing factors and their directions:
| Categories of influencing factors | Typical examples and influence trends |
| Paper composition | The increase of sizing agents (such as AKD) increases the contact angle; Changes in filler content (e.g., calcium carbonate) affect surface energy. |
| Surface structure | Increased roughness may amplify the intrinsic contact angle effect (Wenzel model). |
| Chemical treatment | Plasma treatment or coating reduces the contact angle and improves wetting. |
| Test the liquid | The surface tension is reduced (e.g. ethanol is added) and the contact angle is usually reduced. |
| environmental conditions | Increased humidity can alter the moisture absorption of the paper and affect measurement stability. |
Application and Significance
Contact angle measurement provides important data support for paper production and applications. During the R&D phase, sizing effects or coating formulations can be optimized by comparing the contact angles of paper for different formulations or processes to achieve the desired water or liquid absorption properties. In quality control, contact angle can be used as an in-line or offline monitoring indicator to ensure consistent wettability from batch to batch. For example, in the printing field, proper wettability helps inks spread and adhere evenly; In the field of packaging, contact angle data can be used to evaluate the compatibility of paper with liquids or adhesives. In addition, combined with surface energy calculation (such as the Owens-Wendt method), the polarity and dispersion components of the paper surface can be further analyzed, which provides a theoretical basis for surface modification.
Notes:
Although contact angle meters are effective tools, their limitations need to be recognized in practical applications. Non-homogeneity of the paper surface (e.g., uneven fiber distribution) can cause the contact angle measurement to fluctuate, and it is recommended to average the multi-point measurement. Porous paper will dynamically change the contact angle over time due to the rapid penetration of liquid, so it is necessary to use a high-speed camera to record the initial contact angle or use a penetration model to assist in the analysis. In addition, instrument calibration, reference level adjustment, and image analysis algorithm selection can all affect the accuracy of the results. Therefore, when comparing data from different studies, it is necessary to clearly report the measurement conditions, test liquids, and specific methods to ensure comparability of results.
References
1. Paper Surface Performance Analysis and Detection Technology, China Light Industry Press.
2. ASTM D724-99 Standard Test Method: Test method for wettability of paper surfaces.
3. Application of Surface and Interfacial Tension Measurement in Materials Science, Science Press.
4. Paper Chemistry and Technology, De Gruyter Publishing.