The following is an analysis of the key factors that affect the measurement results of the contact angle measuring instrument:
Surface characteristics of materials
1. Surface roughness: Microscopic geometric effects significantly alter the actual contact area of droplets. According to the Wenzel model, roughness amplifies wettability by increasing the solid-liquid contact area; The Cassie model describes the composite contact state when droplets penetrate into the voids of rough structures. A rough surface can also lead to an increase in the difference between the forward and backward angles, and the hysteresis interval needs to be tested by increasing or decreasing the droplet volume.
2. Chemical composition and heterogeneity: Uneven surface energy distribution can lead to uneven droplet spreading in different regions, requiring multiple measurements to take the average to reduce errors. The pollution layer or oxide film will change the local wettability and requires ultrasonic cleaning or nitrogen blowing pretreatment.
3. Dynamic changes in surface morphology: Soft materials undergo deformation under droplet pressure, resulting in a smaller apparent contact angle. It is necessary to control the droplet volume or use high-speed cameras to capture transient morphology. Porous materials can absorb liquids, causing dynamic contraction of three-phase contact lines, and images need to be collected immediately after dripping.
Environmental conditions
1. Temperature fluctuations: An increase in temperature will reduce the surface tension and viscosity of the liquid, affecting the droplet spreading kinetics. The thermal expansion of polymer materials may alter surface flatness and introduce errors.
2. Humidity effect: Hydrophilic surfaces adsorb water molecules in high humidity environments to form a pre wetting layer, resulting in a virtual low contact angle; Temperature difference may cause fogging of lenses or changes in droplet evaporation rate, and it is necessary to control the environmental humidity to be less than 50% RH.
3. Airflow and vibration: Forced convection accelerates droplet evaporation and changes the direction of three-phase contact line movement; Laboratory vibration may cause droplet deformation, and an isolation platform is required.
Instrument system parameters
1. Optical system accuracy: Non uniform illumination can produce shadow artifacts, which require the use of a diffuser plate to eliminate light intensity gradients. The edge distortion of low-quality lenses can cause deviation in contact angle calculation and requires calibration and correction.
2. Droplet system control: Droplet volumes that are too large are significantly affected by gravity, while those that are too small are easily dominated by evaporation. The optimal volume range is 2-5 μ L. The high-speed dripping impact on the surface generates kinetic energy interference, and it is necessary to use a flat head micro injector to avoid the "hanging effect".
3. Algorithm and fitting model: Threshold segmentation method is susceptible to background noise interference, while gradient edge detection is more suitable for complex lighting conditions. The Young Laplace equation is applicable to low viscosity liquids, and B-spline curve fitting can handle distortion of high viscosity droplets.
Operation and Sample Preparation
1. Levelness of the sample stage: An inclination angle greater than 0.5 ° can cause uneven force on the droplets, and the sample stage needs to be calibrated with a spirit level to control the error within ± 0.1 °.
2. Liquid purity and stability: Volatile solvents need to be sealed for testing, otherwise the contact angle will increase monotonically over time. Surface active agent contamination can significantly reduce the surface tension of liquids, requiring the use of fresh distilled water or ultra pure reagents.
3. Timeliness measurement: Reactive surfaces (such as PDMS) gradually become hydrophobic upon contact with water, and collection should be completed within 10 seconds after dripping. The degradation of biological samples requires controlling the testing time to be less than 5 minutes.
The measurement results of contact angle measuring instruments are influenced by various factors, and it is necessary to comprehensively consider all aspects when using them to obtain accurate and reliable measurement results.