Stalagmometric Method - Stalagmometer

Stalagmometer

A stalagmometer is a device for investigating surface tension using the stalagmometric method. It is also called a stactometer or stalogometer. The device is based on a capillary glass tube whose middle section is widened. In terms of the volume of the drop, it could be calibrated to the same size based on the design of the stalagmometer. The part of the bottom of the device is narrowed down to let the fluid fall out from the tube in a shape of drop. In the experiments, the drops of the specific fluid are flowing slowly from the tube in a vertical direction. The drops hanging on the bottom of the tube start to fall when the volume of the drop reaches the maximum value which is dependent on the characteristic of the solution. In this moment, the weight of the drops is in an equilibrium state with the surface tension. Based on the Tate’s law:

The drop is falling when the weight (mg) is equal to the circumference (2πr) multiplied by the surface tension (σ). The surface tension can be calculated when we know the radius of the tube (r) and the mass of the fluid droplet (m). On the other hand, on account the surface tension proportional to the weight of the drop, we can use a reference fluid (mostly using water as a reference) to compare with the fluid which we are interested in.

In the equation, m₁ and σ₁ can be the mass and surface tension of the reference fluid, and m₂ and σ₂ can be the mass and surface tension of the fluid we want to investigate. If we take water as a reference fluid, then:

If the surface tension of water is known, we can calculate the surface tension of the specific fluid from the equation. The weight of more drops we measure, the more precise we calculate the surface tension from the equation. One thing we need to notice is that keeping the stalagmometer clean is really important so as to get meaningful reading. There are commercial tubes for stalagmometric method in three kinds of size: 2.5, 3.5, and 5.0 (ml). The size of 2,5 (ml) is suitable for small volume and low viscosity, of 3.5 (ml) for relatively high viscous fluid, of 5.0 (ml) for large volume and low viscosity, 2,5 (ml) for small volume and high viscosity and are flexible for different size of most of the fluids. "Survismeter" produces ultra accurate results of surface tension along with viscosity, interfacial tension, wetting coefficient and density data. The survismeter is most accurate and safer for volatile, inflammable and carcinogenic liquids.