GB/Z 166-2026Paper and board - Measurement of water contact angle by optical methods (English PDF)

3.1 Baseline

The contact line between the droplet and the surface of the paper or board is determined optically and serves as the reference plane for measuring the contact angle.

NOTE: Accurate measurement of the contact angle relies on an accurate baseline. To obtain the optimal baseline, adjust the camera's tilt angle to view the droplet at a slight inclination, typically at an angle of 0° to 3° relative to the horizontal, rather than viewing it directly from the side (0°). Experience shows that slight variations in the viewing angle do not affect the measurement of contact angle but do improve the precision of baseline identification.

3.2 Three-phase point

The intersection point of the three boundary lines. liquid/solid, liquid/gas, and solid/gas.

3.3 Contact angle; theta

Within the specified contact time, draw a tangent to the gas-liquid interface at the three-phase point (3.2) of gas, liquid, and solid; the angle formed between this tangent and the baseline (3.1) is the angle enclosing the droplet.

NOTE: The unit is degrees (°).

3.4 Contact time

The duration of contact between the droplet and the specimen surface, that is, the time elapsed from the moment the timing device is triggered to the moment the measurement is completed.

3.5 Droplet height; H

Within the specified contact time (3.4), the distance from the baseline (3.1) to the top of the droplet in contact with the specimen surface.

NOTE: Droplet height is expressed in mm.

3.6 Droplet base diameter; D

Within the specified contact time (3.4), maximum width from the baseline (3.1) to the base of the droplet in contact with the specimen surface.

NOTE: The unit is in mm.

5.1 Water. Grade 3 in GB/T 6682

NOTE 1: This Document is also applicable to other liquids, provided that they are dispensed as droplets using the liquid delivery system (6.2.3) described in this Document and do not cause significant swelling of the substrate within the measurement timeframe that would prevent the clear identification of the baseline. Furthermore, for liquids with high vapor pressures, the evaporation rate may be too rapid for the method to be applicable, unless measurements are conducted in a saturated vapor environment (e.g., a closed glass cuvette containing a few drops of the solvent). Contact angle values obtained using other liquids are not comparable to those obtained using water.

NOTE 2: Some contact angle meters (6.1) have the capability to heat the liquid prior to droplet formation.

Heating alters the liquid's surface tension and viscosity, both of which affect contact angle measurement results. Heating the liquid constitutes a deviation from this Document.

NOTE 3: Many contact angle meters (6.1) are capable of measuring liquid surface tension and are used to verify the surface tension of the water. Regularly check for water contamination, as contaminated water directly affects contact angle measurement results.

6.1 Contact angle meter

The contact angle meter consists of various components (see Figure 2), each complying with the specific requirements set forth in 6.2.1 ~ 6.2.7. The contact angle meter needs not be identical to the schematic diagram; the meters without a specimen stage, or those using a separate liquid container, and those featuring different structural designs or observation systems may also be used.

6.2 Components of the contact angle meter

6.2.1 Specimen stage with fixture (Component 1)

The specimen stage is used to secure the specimen; the specimen shall be positioned horizontally and flat during the test. The specimen stage may be a platform, and the fixture may be the equipment housing.

6.2.2 Light source (Component 2)

The light source illuminates the droplet and is equipped with a suitable filter, if necessary, to prevent heating of the specimen or the droplet. The illumination system shall provide uniform lighting across the entire field of view.

6.2.3 Liquid storage and delivery system (Component 5)

The liquid storage and delivery system consists of a reservoir and a metering device capable of delivering the droplet of a specific volume with an accuracy of ±10%. If the droplet volume is determined by the pumped volume, care must be taken to ensure the droplet is free of air bubbles, as bubbles can cause significant volume errors.

NOTE 1: The droplet volume refers to the volume of the droplet formed at the tip of the dispensing needle, not necessarily the volume of the droplet deposited on the specimen surface. After the droplet is released, a small amount of water remains on the needle tip; the residual amount depends on water quality, needle diameter, needle type and material, and the method of droplet application.

NOTE 2: The dispensed volume and reproducibility of the metering device are verified using a non-absorbent sample.

NOTE 3: Some contact angle meters can calculate the volume directly from a pendant droplet; in such cases, air bubbles do not affect the final droplet volume.

6.2.4 Droplet delivery system (Component 6)

6.2.4.1 Under specified conditions, the droplet delivery system can suspend a droplet of a specific volume at the tip of an injection needle and transfers it to the surface of the paper or board. The diameter and material of the injection needle shall be selected in accordance with Clause 5 and the specifications of the droplet delivery system. The droplet delivery system shall be adjusted to ensure that the droplet has minimal kinetic energy upon impact with the specimen surface.

6.2.4.3 The material of the injection needle shall prevent moisture from creeping up the outer wall of the needle; be chemically inert, having no effect on the liquid's surface tension; and allow the droplet to detach with minimal force.

6.2.4.4 If the specimen is a highly hydrophobic material, the test must be conducted with caution; the droplet delivery system may impart kinetic energy to the droplet, causing it to bounce briefly and resulting in significant fluctuations in the contact angle value. Such values shall be discarded. This phenomenon does not occur when testing hydrophilic materials.

6.2.4.2 The specific method of applying the droplet varies depending on the instrument. In this

Document, the droplet must detach from the injection needle tip after remaining there for a maximum of 20 ms ~ 40 ms, so as not to interfere with the contact angle measurement.

6.2.5 Image acquisition system (Component 7)

The image acquisition system includes an optical magnification device capable of accurately measuring the shape and dimensions of the droplet in contact with the paper or board specimen.

The camera shall have a capture rate of at least 50 frames per second, with an exposure time per frame not exceeding 1 ms. The ratio of image width (in mm) to the number of horizontal pixels in the displayed image shall not exceed 0.02 mm/pixel to ensure sufficient detail for image analysis. The depth of field shall be sufficient to capture the entire profile of the droplet on the paper or board surface.

6.2.6 Timing system

The timing system is triggered at one of the following moments.

a) the instant the droplet detaches from the dispensing needle;

b) the instant the droplet makes contact with the specimen surface;

c) based on video initiated before the droplet reaches the paper or board specimen.

The timing system shall start either before contact or no later than 2 ms after contact; and shall assign a unique timestamp to each digital image with an accuracy of ±2 ms.

NOTE: The nature of the triggering mechanism varies depending on the instrument. Generally, the trigger is either camera-based or software-based. For example, in some instruments, the droplet is released from the dispensing needle via a mechanical pulse that simultaneously starts the timer. In other instruments, the timer starts when the falling droplet approaches a point directly above the specimen surface.

Additionally, there is a method involving continuous video recording prior to the droplet contacts with the paper or board surface, where the contact time is calculated retrospectively by designating the point of liquid-solid contact as "time zero."

6.2.7 Suitable image analysis software

The image analysis software processes each digital image captured by the camera and calculates the values for droplet height (H), droplet base diameter (D), and contact angle (theta); in some cases, it may also estimate the droplet volume.

NOTE: Various algorithms can be used to analyze droplet shape or profile and determine contact angle values. The determined contact angle value is influenced by the chosen mathematical method. Many instrument manufacturers employ proprietary algorithms based on the shape of a single droplet on the sample surface, leading to differences between instruments. Therefore, this Document selects the circle equation [see Formula (1)] as the sole compliant algorithm. […]