How can the surface tension of a liquid be measured?
There are two types of methods for determining surface tension: static and dynamic. Dynamic methods, for example, the method of oscillating a jet flowing out of an irregularly shaped hole, make it possible to measure the surface tension of a newly formed surface immediately after its formation. Static methods measure the surface tension at the interface between the phases that have come to equilibrium. If the equilibrium between the phases is established quickly enough, then the static surface tension can be measured quite simply. If equilibrium is established over a very long period of time, especially when one of the components of the system passes through the interface, then none of the methods gives the true value of the equilibrium surface tension. In this case, one speaks of semi-static values of the surface tension and it is imperative to stipulate the duration of the formation of a new interface. The most widely used methods for determining the surface tension of liquid interphase boundaries:
• method for measuring the mass or depth of immersion of an object in the liquid under investigation (Wilhelmy’s method);
• method for measuring the geometric dimensions of free interfaces between phases (lying, hanging or rotating drop, sitting or hanging bubble);
• stalagmometric method (drop counting method);
• method for measuring the force of separation of an object from the interface (separation of a cylinder – Paddy’s method, separation of a ring – Du-Nui method);
• method for measuring capillary rise of liquid;
• method of measuring the maximum pressure in a gas bubble or liquid drop (Schrödinger method).