What is Surface Tension?
Intermolecular Force works between the molecules and the molecules of the liquid try to minimize the surface area of the liquid. If Surface Tension is this force, the magnitude of the force that is exerted is the significant phenomena such as Wettability.
Measurement of Surface Tension
When measuring unit (Wilhelmy plate) makes contact with the surface of the liquid, the liquid will wet the Wilhelmy plate upwards. In this case, the surface tension acts along the perimeter of the plate and the liquid pulls in the plate. This method detects the pulling force is read and determines the surface tension.
As the name suggests, a ring, made of platinum, is used in this measurement method. The method has a longer history after establishment than do the other modern measurement methods and has been adopted by some industrial standards. First, a ring that is hanging parallel to the liquid surface is sunk into the liquid. Then, the ring is gradually drawn apart from the surface in a vertical direction. In this process, the surface tension of the liquid membrane that is hanging by the ring generates a force on the ring. This force changes as the ring is drawn farther. Using the maximum value of this force, surface tension is determined by the following formula:
C in the formula is a correction factor and a constant that corresponds to the ring size and liquid sample density. This is a factor to correct the effects of the tension direction and liquid membrane shape. Our DyneMaster series corrects them automatically with the aid of the Zuidema-Waters formula.
When the liquid is pushed out from the tip of the capillary toward the vertical direction, the droplet hangs from the tip of the capillary. This hanging droplet is called the "pendant drop".
Since the amount of liquid depends on the extruded amount of the liquid, density, surface and interfacial tension, the surface and interfacial tension can be determined by analyzing the shape of this pendant drop.
The pendant drop method is suitable for highly viscous liquids that do not wet plates easily, molten polymers, and for measuring the interfacial tension between the two liquids.
This method calculates the surface tension by measuring the diameter ds of a pendant drop in a position where only de has risen from the lowest end and when de is the maximum diameter of the pendant drop (diameter equatorial plane).
We calculate the surface tension by fitting the Young-Laplace equation by image processing from the value of the difference between the contour shape and density of droplets created from the needle tip.
It is possible to determine the precise interfacial tension by fitting the theoretical curve and the many coordinates (several hundred) of the Young-Laplace contour curve.
Surface tension that causes surface tension to go towards the equilibrium with the passing of time is called Dynamic Surface Tension. It tends to be higher and less wet than the Static Surface Tension by the Wilhelmy Method.
To calculate the surface tension, we measure the maximum pressure when the probe (capillary) inserted into liquid is continuously generating air bubbles.
When pressurized air is flown continuously through the probe, the pressure inside the probe changes periodically. →(1)-(4)
When the radius r of the probe tip and the radius of curvature R of the bubble are equal, the pressure becomes maximum. →(3)
After that, the pressure drops rapidly by expansion of the bubbles (4)
Maximum bubble pressure technique is a method for determining the surface tension from the maximum pressure.
The above (1)-(3) refers to the lifetime (lifetime of bubbles), from the time that the new interface has been generated in the probe tip until the maximum bubble pressure is reached.
Surfactant adsorbed during the lifetime will determine the surface tension.