Surface tension is one of the important physical and chemical properties of liquids, which affects many natural phenomena and industrial processes. Among various measurement methods, the maximum bubble pressure method has become a commonly used method in scientific research and teaching due to its simple device, convenient operation, and no need to measure contact angle and liquid density.

　　1、 Core principle: Bubbles and pressure

The basic principle of the maximum bubble pressure method is the reverse process of the capillary rise method. Its core lies in measuring the maximum pressure difference generated when bubbles detach from the end of the capillary tube, and then calculating the surface tension.

During the experiment, the liquid to be tested is loaded into a surface tension meter so that the end face of the capillary tube is tangent to the liquid surface. Slowly extract air to reduce the pressure of the system, and the pressure on the liquid surface in the capillary tube is greater than the system pressure, forming a pressure difference (additional pressure, Δ p). This additional pressure is directly proportional to the surface tension and inversely proportional to the curvature radius of the bubble, following the Laplace formula: Δ p=2 σ/R (where σ is the surface tension and R is the curvature radius of the bubble).

The curvature radius of the bubble formation process varies:

At the beginning: the surface of the bubble is almost flat, with the maximum curvature radius.

2. In formation: The curvature radius gradually decreases.

When it is hemispherical, the curvature radius R of the bubble is equal to the radius r of the capillary tube, and at this point, the curvature radius is the smallest, and the additional pressure reaches the maximum value (Δ pmax).

4. Afterwards, the bubble further grows, R increases, and Δ p decreases until it escapes.

Therefore, the relationship between the maximum additional pressure Δ pmax and surface tension is: σ=(r/2) Δ pmax. For the same set of instruments, the capillary radius r is a constant. For the convenience of calculation, the instrument constant K is often introduced, and the formula is simplified as σ=K Δ pmax. The instrument constant K can be calibrated using a standard substance with known surface tension.

　　2、 Main features and advantages

The maximum bubble pressure method has its unique characteristics and advantages:

1. Dynamic measurement capability: This method can be used to measure dynamic surface tension, that is, the change of surface tension over time, and is particularly suitable for studying the adsorption kinetics of surfactants.

2. Easy to operate: The experimental setup is relatively simple, the operation is convenient, and there is no need to measure the contact angle θ and liquid density ρ.

3. Wide applicability: Especially suitable for liquids with low surface tension.

　　3、 Key experimental steps and precautions

To obtain accurate measurement results, it is necessary to follow standardized procedures:

1. Preparation and leak detection of surface tension tester: The instrument must be clean and dry. Check the airtightness by extracting air and maintaining a certain pressure difference for 2-3 minutes.

2. Control the rate of bubble formation: Adjust the pumping speed to allow individual bubbles to escape, and the optimal time for each bubble formation is 5-10 seconds. Excessive speed or escaping in a string can shorten the equilibrium time of bubbles, making it difficult to accurately read the maximum pressure difference and affecting the results.

3. Read the maximum pressure difference: It is necessary to record the maximum pressure difference of the pressure gauge at the moment when the bubble just leaves the pipe end, and take the average value after multiple consecutive measurements.

4. Measurement sequence: If measuring a series of concentration solutions, the measurement should be carried out from low concentration to high concentration in order to reduce the interference of pollution and concentration changes on the results.

5. Capillary state: The capillary must be clean, dry, kept vertical, and the opening of the tube must be exactly tangent to the liquid surface. If inserted below the liquid level, additional static hydraulic pressure will be generated, causing the measurement results to be biased.

　　4、 Main application areas

The maximum bubble pressure method and its surface tension tester are widely used:

1. Research on surfactants: Determine their critical micelle concentration (CMC), study adsorption kinetics and dynamic surface tension.

2. Industrial formula optimization: Used for formula development and quality monitoring of cleaning agents, inks, coatings, cosmetics, and other products, optimizing their wetting, spreading, and emulsifying properties.

3. Process analysis and control: Online monitoring of surface tension of electroplating solutions in the semiconductor industry to control concentration and additive dosage.

Conclusion

The maximum bubble pressure method provides an intuitive and effective means of measuring liquid surface tension by detecting the maximum pressure during the formation and detachment of bubbles. Its characteristics in dynamic measurement and relatively simple operation make it continue to play an important role in basic research and industrial applications.