1. The Importance of Surface Tensiometers in Dynamic Surface Tension Testing

1.1 The Physical Essence of Surface Tension

Surface tension is a key parameter that characterizes the properties of liquid interfaces and plays a crucial role in industrial applications. Using a surface tensiometer allows for the precise measurement of intermolecular cohesive forces in liquids—especially under dynamic conditions. For instance, pure water exhibits a surface tension of approximately 72 mN/m at 20°C, whereas the addition of surfactants can reduce this value to between 30–40 mN/m, thereby influencing droplet spreading behavior.

1.2 Time Dependence of Dynamic Surface Tension

Dynamic Surface Tension (DST) describes how surface tension changes over time as a new interface is formed. Modern surface tensiometers employ high-speed sampling to capture the millisecond-scale adsorption dynamics of surfactants at the gas–liquid interface. For example, when measuring an SDS solution with the Wilhelmy Plate method, insufficient time resolution in the instrument may result in missing critical early adsorption data.

2. Dynamic Testing Techniques and Their Advantages and Limitations

2.1 Comparison of Traditional Surface Tension Measurement Methods

Different measurement techniques vary in precision, time resolution, and suitability. The table below summarizes key aspects:

2.2 Technical Challenges in Dynamic Testing

Traditional surface tensiometers face several challenges in dynamic tests:

• Mechanical Delay: The Wilhelmy Plate method often employs stepper motors with a minimum step time of over 50 ms, which does not meet the demands of fast adsorption processes.
• Limited Sensor Bandwidth: Conventional pressure sensors usually have a sampling rate below 100 Hz, making it difficult to resolve millisecond-scale surfactant adsorption.
• Interface Disturbance: The immersion process may induce convection, interfering with the adsorption kinetics of surfactants and thus compromising measurement accuracy.

3. ADSA® Technology-Enhanced Wilhelmy Plate Surface Tensiometer

3.1 Intelligent Immersion Speed Correction

ADSA® technology introduces a real-time feedback mechanism to adjust the platinum plate’s immersion speed, ensuring synchronization between adsorption kinetics and interface formation. The algorithm used is:

where $v_n$ is the current immersion speed, $\gamma$ is the surface tension, and $k$ is a feedback coefficient. This method boosts the surface tensiometer’s time resolution from the traditional >100 ms to as low as 1 ms, enabling accurate capture of the adsorption process.

3.2 High-Precision Measurement with Piezoelectric Ceramic Sensors

Compared to conventional sensors, piezoelectric ceramic sensors offer faster response times and higher precision:

For instance, when testing a 0.1 CMC Triton X-100 solution, a surface tensiometer equipped with a piezoelectric sensor can detect a rapid tension drop (Δγ = 15 mN/m) at t = 2 ms—an event that traditional sensors would miss.

3.3 Enhanced Dynamic Surface Tension Testing Performance

The ADSA® technology-enhanced surface tensiometer delivers the following improvements:

• Time Resolution: With data sampling intervals of 1 ms, the instrument can resolve the exponential growth phase of the adsorption kinetics.
• Motion Control Accuracy: The piezoelectric actuator maintains a displacement error below 0.1 μm, minimizing interface disturbances.
• Dynamic Contact Angle Correction: Real-time fitting of the wetting curve reduces contact angle measurement error from ±5° to ±0.5°.

4. Limitations of Static Methods in Dynamic Surface Tension Testing

4.1 Mismatch Between Immersion and Retraction Times

Traditional methods like the Wilhelmy Plate and Du Nouy ring methods require a complete “immersion–equilibrium–retraction” cycle (lasting 10–30 seconds), which far exceeds the millisecond-scale surfactant adsorption time. For example, in a 0.1 mM SDS solution with an adsorption characteristic time ($\tau$) of 50 ms, the prolonged mechanical movement (t$_{move}$ = 10 s) only yields a quasi-static equilibrium value (38.2 mN/m), missing the dynamic drop (real minimum 36.5 mN/m) within the first second.

4.2 Measurement Distortions Due to Interface Disturbance

Numerical simulations show that at an immersion speed of 1 mm/s, the flow velocity near the interface can reach 10⁻³ m/s. This micro-convection accelerates surfactant diffusion by approximately 30% in low-concentration (0.01 CMC) solutions, leading to an overestimation of the surface tension.

4.3 Case Study: Comparative Testing of SDS Solutions

When comparing measurements on a 0.1 CMC SDS solution:

• Traditional Method: Equilibrium tension of 38.2 mN/m reached in 15 seconds.
• ADSA® Technology: Captured a rapid tension drop at t = 20 ms (Δγ = 12 mN/m) with a minimum tension of 36.5 mN/m, closely matching the no-flow method result (36.3 mN/m).

5. Experimental Data Analysis: Differences in Surfactant Adsorption Kinetics

5.1 Experimental Conditions

• Samples: Sample1 (branched nonionic surfactant) and Sample2 (linear anionic surfactant)
• Instrument: ADSA® technology-enhanced Wilhelmy Plate surface tensiometer (1 kHz sampling rate)
• Environment: Temperature at 25 ± 0.1°C; relative humidity 50% RH

5.2 Analysis of Adsorption Kinetics

The time–tension curves reveal:

• Initial Phase (t < 10 ms): Sample1’s tension drops rapidly from 72 to 58 mN/m, while Sample2 decreases to 55 mN/m.
• Stabilization Phase (t > 500 ms): Sample1 stabilizes at 40.3 mN/m, and Sample2 at 38.7 mN/m.

Fitting the data with the Rosen model yields key kinetic parameters:

These results indicate that the branched structure of Sample1 lowers the molecular ordering barrier, increasing the adsorption rate, while the sulfonate groups in Sample2 extend the relaxation time at the interface due to electrostatic repulsion.

6. Conclusions

1. A dynamic surface tensiometer must have a time resolution that matches the adsorption kinetics of surfactants.
2. The ADSA® technology-enhanced instrument, employing piezoelectric ceramic sensors and intelligent immersion speed correction, achieves a time resolution of 1 ms with an error rate below 1%.
3. This technique is applicable in industries such as coatings, inks, and cosmetics, providing critical data for dynamic surfactant behavior.