There is an interface between phases in a multiphase system, and the molecules at the interface are subjected to different pressures than the molecules in the bulk of the phase. For gas-liquid interface, solid-liquid interface, and liquid-liquid interface, due to the unbalanced force on the interface molecules, the force acting on the surface of the molecule toward the interior of the liquid is caused. The force perpendicular to the liquid surface is the surface tension. The surface tension of pure water is about 72mN/m depending on the temperature.
If a surfactant (such as wetting agent, leveling agent, etc.) is added to the water, it will affect the surface tension of the liquid. When a new interface is formed, it takes time for the surfactant to migrate to the new interface. For example, it only takes ten milliseconds (or less) for the print head of an inkjet printer to spray ink onto the paper, and it may take tens to thousands of milliseconds for the latex paint to be sprayed on the workpiece to be applied to the wall, depending on the surface. The time required for the active agent to migrate to the new interface is different, so the impact on the wetting and leveling properties of the product is also different. As shown in the figure below, Figure 1 shows that when the capillary of the Sita tensiometer just forms new bubbles (that is, a new interface), only a small amount of surfactant gathers on the new interface. The longer the interface is formed (i.e., the longer the bubble lifetime), the more surfactant aggregates to the interface. The Sita tensiometer can measure dynamic surface tension from 15 milliseconds to 15 seconds.
The ordinary ring method or plate method can only measure the static surface tension of the liquid after reaching equilibrium in a few minutes (such as the last data in the figure above: 29nN/m), but cannot test the critical surface when a new interface is just formed Tension, nor can the curve of dynamic surface tension be drawn.
The Sita Surface Tensiometer uses the bubble pressure method to continuously generate new boundary surfaces, so that the surface active substance diffuses during the measurement process. At this time, the surface tension value depends on the diffusion ability of the surface active substance in the system over time, and of course it is also affected by temperature, etc. influence of other factors.
Therefore, the conditions and results of many wetting and leveling processes are completely affected by dynamic surface active substances, so it is very important to be able to measure and record the changes in surface tension over time.
The Sita Surface Tensiometer has a capillary tube. Immerse the capillary tube in the liquid surface, let the air flow through the capillary tube, increase the air pressure in the tube, and bubbles will be generated at the lower end of the capillary tube. The radius of the bubble sphere decreases continuously until the radius of the bubble is the same as the radius of the capillary tube. At this time, the pressure of the bubble is at its maximum value. According to the Laplace-relation formula, the maximum and minimum pressure difference is related to the surface tension. Therefore, the surface tension at this time can be calculated by measuring the pressure difference of the bubble.
The static water pressure is related to the depth of the capillary submerged in the liquid surface and the density of the liquid. These parameters can be eliminated by the relative measurement pressure difference, so the measurement of the surface tension is not affected by the depth of the capillary submerged in the liquid surface.
Bubble life is called the bubble generation time, which is defined as the time from when the capillary starts to generate bubbles (at the minimum bubble pressure) to when the radius of the bubble sphere is equal to the radius of the capillary (at the maximum bubble pressure), after which the bubble spherical surface expands rapidly, making the bubble sphere radius increase , so the bubble pressure gradually decreases.
We do not need to know the radius of the capillary in detail, because this online measuring instrument can be calibrated with pure water at any time. At this time, the radius of the capillary (or the calibration parameters of the instrument) can be automatically obtained from the pressure and the surface tension of pure water.
In the online measurement mode of this instrument, a fixed frequency of bubble generation can be set to measure the change of the tension value. In this way, the change characteristics of the surface tension of the liquid to be tested over time can be recorded and monitored online for up to several days.
Note: When wetting agent is added to the plating solution as anti-pitting agents, the surface tension measurement can be used as a reference for controlling the wetting agent. Others such as surfactants in the cleaning solution can also It can be controlled by surface tension measurement.
