The accuracy of the contact angle measuring instrument directly determines the reliability of the liquid solid surface wetting performance test results. Its measurement accuracy is influenced by four core factors: sample state, instrument hardware configuration, operation method, and environmental conditions. These factors are interrelated, and any deviation in any link will lead to distortion of the measurement results. The following is a specific analysis of influencing factors:
1、 The influence of sample status
The surface characteristics of the sample are the basis for determining the accuracy of contact angle measurement and are also the most easily overlooked key factors, mainly including the following points:
The core of measuring the contact angle of the flatness and cleanliness of the sample surface is to capture the boundary line (three-phase contact line) between the droplet and the solid surface. If there are scratches, dents, protrusions, or large roughness on the sample surface, it will cause uneven droplet spreading, deformation of the three-phase contact line, and the instrument cannot accurately recognize the true contact angle value. For example, droplets on rough surfaces may exhibit a "pinning effect", resulting in high or low contact angle measurements and poor repeatability. Meanwhile, pollutants on the surface of the sample, such as dust, oil stains, and residual solvents, can alter the surface's free energy, leading to changes in wetting properties. For example, oil stains on metal surfaces can significantly increase the contact angle, while residual hydrophilic solvents can reduce the contact angle. The presence of these pollutants can cause measurement results to deviate from the true performance of the sample. Therefore, the sample must be cleaned (such as ultrasonic cleaning, plasma cleaning) before testing, and the surface must be dry and free of residue.
The uniformity and stability of the sample. If the sample is a composite material, coating, or gradient material, the unevenness of its surface composition and structure can lead to significant differences in contact angles at different positions. For example, for samples with uneven coating thickness, the surface energy of the thin coating area and the thick coating area is different, and the measured contact angle will show significant fluctuations. In addition, the chemical stability of the sample can also affect accuracy - if the sample undergoes a chemical reaction with the test liquid (such as dissolution, swelling, hydrolysis), the droplets will gradually deform over time, and the contact angle will continue to change, making it impossible to obtain stable measurement values.
The conductivity of the sample (for dynamic contact angle measurement) can affect the detection accuracy of the instrument for three-phase contact wires when conducting dynamic contact angle measurements (such as forward angle and backward angle tests) or using the William plate method. If the insulation sample is not subjected to conductive treatment, it may cause droplet displacement due to electrostatic effects or affect the photoelectric detection signal of the instrument, resulting in measurement errors.
2、 The impact of instrument hardware configuration
The core hardware components of the instrument directly determine the ultimate accuracy of the measurement, and devices with different configurations have significant differences in resolution and stability
The performance of the imaging system's contact angle measuring instrument relies on the imaging system (camera+lens) capturing the contour image of the droplet, and the imaging quality is the prerequisite for accurate analysis of the contact angle.
Camera resolution: High resolution cameras (such as over 2 million pixels) can clearly capture the details of droplet edges, avoiding contour fitting errors caused by insufficient pixels; Low resolution cameras can blur the edges of droplets and result in inaccurate recognition of three-phase contact lines.
Magnification and depth of field of the lens: When measuring small droplets (such as 1-5 μ L), a high-power macro lens is required, and the lens must have sufficient depth of field to ensure clear imaging of the droplet as a whole. If the depth of field is insufficient, the top and bottom of the droplet cannot be displayed clearly at the same time, resulting in deviation in contour fitting.
Stability and uniformity of the light source: The light source should provide a uniform and soft backlight to avoid reflection or shadows at the edges of the droplets. If the brightness of the light source is unstable or the illumination angle is improper, it can cause deformation of the droplet contour, and the image analysis algorithm of the instrument cannot accurately extract edge information.
The accuracy of the injection system (microinjector+automatic sampler) controls the volume and droplet position of the liquid droplet, and its accuracy directly affects the shape of the droplet:
The accuracy of droplet volume: There is a certain correlation between contact angle and droplet volume (small droplets are less affected by gravity, and the contact angle is closer to the true value; large droplets are prone to sagging due to gravity, resulting in lower measurement values), so precise control of droplet volume is required (usually recommended 1-3 μ L). Manual injection can easily cause fluctuations in droplet volume due to operational errors, while high-precision automatic injectors (with an accuracy of ± 0.1 μ L) can ensure consistent droplet volume for each drop, improving measurement repeatability.
Stability of droplet position: The droplet needs to be dropped vertically to the center position of the sample surface. If the verticality deviation of the injection needle or the droplet position is offset, the droplet will slide on the sample surface, resulting in irregular three-phase contact lines and a decrease in measurement accuracy.
The advantages and disadvantages of image analysis algorithms depend on the software's image analysis algorithms for calculating contact angles, and the accuracy of different algorithms varies greatly
The commonly used algorithms include ellipse fitting (applicable to axisymmetric droplets), Young Laplace equation fitting (applicable to droplets affected by gravity), and tangent fitting (manual fitting, with large errors). Among them, the Young Laplace equation fitting method considers the surface tension and gravity balance of droplets, making it the most accurate algorithm and suitable for high-precision measurements; The tangent method relies on the subjective judgment of the operator, and the error can reach ± 2 ° or more.
High quality analysis software also has edge enhancement and noise filtering functions, which can effectively process blurry images and improve the accuracy of contour recognition; However, the algorithms of inferior software are rough and susceptible to image noise interference, leading to fluctuations in measurement results.
3、 The impact of operating methods
The level of operational standardization of operators is the key to ensuring measurement accuracy, and improper operation can introduce significant human errors:
The settling time of the droplet is controlled by allowing it to settle on the surface of the sample for a certain period of time (usually 3-10 seconds) to reach thermodynamic equilibrium. If the settling time is too short, the droplets may not fully spread and the measured contact angle may be higher; If the standing time is too long, the droplets may deform due to evaporation (volatile liquid) or adsorption (porous sample), resulting in measurement drift.
The selection of measurement points and the number of repetitions are often not representative due to the unevenness of the sample surface. Repeated measurements need to be taken at different locations (at least 5) of the sample, and the average value should be taken as the final result. If measured only at a single location, it is prone to result deviation due to local defects or pollutants. At the same time, it is advisable to avoid selecting the edge area of the sample during measurement, as the surface energy of the edge may differ from that of the central area, resulting in abnormal contact angles.
Normative instruments for calibration operations need to be calibrated before use, including lens distortion calibration, droplet volume calibration, and contact angle standard sample calibration:
Lens distortion can cause geometric deformation of droplet images, and a standard calibration plate is required for distortion correction;
The calibration of droplet volume requires verification of the actual liquid output of the sampler through weighing method;
Contact angle standard samples (such as polished silicon wafers and polytetrafluoroethylene sheets with known contact angles) can be used to verify the measurement accuracy of the instrument. If the calibration result deviates beyond the accuracy range indicated by the instrument, the instrument parameters need to be adjusted.