Contact angle/surface tension meter: comprehensive analysis from principle to application

Contact angle and surface tension are core parameters in material surface science. The former reflects the wettability of liquids on solids, while the latter characterizes the cohesion between molecules on the liquid surface. The contact angle/surface tension meter converts microscopic droplet morphology into quantitative data through optical imaging and mathematical modeling techniques.

1、 Testing principle: Precision quantification based on interface mechanics

Young's Equation and Wettability Determination

When a droplet contacts a solid surface, the interfacial tension between the solid liquid gas phase reaches equilibrium, forming a contact angle θ. The Young equation (γ - sv=γ - sl+γ - lv cos θ) reveals the relationship between contact angle and surface free energy: θ<90 ° is a hydrophilic surface (such as water spreading on glass), and θ>90 ° is a hydrophobic surface (such as water shrinking on lotus leaves). This equation is the theoretical basis for contact angle measurement.

Dynamic contact angle and hysteresis phenomenon

By controlling the spreading and retraction process of liquid droplets, the forward contact angle (non wetted area of the liquid) and backward contact angle (wetted area of the liquid) can be measured. The larger the contact angle hysteresis (Δ θ=θ - abv - θ - rec), the more significant the surface roughness or chemical non-uniformity. For example, the hysteresis angle of superhydrophobic surfaces (θ>150 °) is usually less than 5 °, while the hysteresis angle of rough metal surfaces can reach over 30 °.

Surface tension measurement method

Droplet suspension method: By analyzing the geometric parameters of the droplet profile (such as diameter ratio D/d), combined with the Young Laplace equation, the surface tension is calculated. This method is suitable for high-temperature molten metals or corrosive liquids.

Platinum plate method: The sandblasted platinum plate is immersed in a liquid, and when the surface tension is balanced with the reverse force of the instrument, the surface tension value is converted by the immersion depth. The testing of distilled water can be completed within 5 seconds with an accuracy of ± 0.1mN/m.

2、 Testing Process: Standardized Process from Sample Preparation to Data Analysis

Sample preparation and pretreatment

Block/film samples: The size should be greater than 2cm × 2cm, and the surface flatness error should be ≤ 5 μ m. For curved surfaces such as optical fibers, 3D contact angle testing technology is required.

Powder sample: Press into tablets using an infrared tablet press, with a thickness of ≥ 1mm. If the sample adheres to the mold, weighing paper can be placed on the surface.

Liquid sample: The surface tension test requires ≥ 1mL, and the purity requirement is ≥ 99.9% (such as secondary distilled water).

Data collection and fitting algorithm

Static contact angle: Young Laplace fitting method (accuracy ± 0.5 °) or tangent method (applicable for θ<120 °) is used. For non axisymmetric droplets (such as left/right contact angle difference>10 °), the ADSA RealDrop algorithm needs to be used.

Dynamic contact angle: Measure the rolling angle through a tilt table (0-90 °), or capture the forward/backward angle by controlling the droplet volume change rate (0.1-10 μ L/s).

Surface free energy calculation: Based on the Owens Wendt model (applicable to polar/non-polar surfaces) or Fowkes model (applicable to low-energy surfaces), it is necessary to measure the contact angles of at least three liquids with known surface tension, such as water, * * * *, and ethylene glycol.