Describe a method of determining surface tension in laboratory
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Molecules in a liquid have attractive forces that hold them together. The molecules on the surface are attracted to molecules from all sides and below, but not from above (Figure 1). This results in a downward and sideways pull on molecules on the surface layer. This property of liquid surfaces that causes the surface layer to behave like a thin elastic 'skin' is called surface tension.
Surface tension measurement is important in quality control and in the development of products and processes with better performances. It is an extremely sensitive indicator providing information on the washability, wetting, emulsification, foaming and other surface related processes. The progress of various chemical reactions and presence of solvents or surfactants in liquid systems can be monitored by surface tension measurement.
There are several methods available to measure surface tension. The best method depends upon the nature of the liquid being measured, the conditions under which its tension is to be measured, and the stability of its surface when it is deformed. The following are some methods:
Du Noüy Ring Method
The traditional and most common way of measuring surface tension. It utilizes a platinum ring with defined geometry that is immersed into the liquid and then carefully pulled out through the liquid surface (Figure 2). The measurement is performed by a tensiometer, an instrument incorporating a precision micro balance, platinum-iridium ring with defined geometry and a precision mechanism to vertically move sample liquid in a glass beaker.

Figure 2: Illustration of the Du Noüy Ring method to measure surface tension
Wilhelmy Plate Method
This method measures the weight of the liquid drawn by a plate when a plate is lifted from or through the surface of liquid. The weight of the liquid is proportional to the surface tension of the liquid.
Pendant Drop Method
This method is based on the fact that the shape of a drop of liquid hanging from a syringe tip is determined from the balance of forces which include the surface tension of that liquid. This method has advantages in that it is able to use very small volumes of liquid, measure very low interfacial tensions and can measure molten materials easily.
Bubble Pressure Method (Jaeger's Method)
This method is useful for determining surface tension at short surface ages. The maximum pressure of each bubble is measured.
Surface tension measurement is important in quality control and in the development of products and processes with better performances. It is an extremely sensitive indicator providing information on the washability, wetting, emulsification, foaming and other surface related processes. The progress of various chemical reactions and presence of solvents or surfactants in liquid systems can be monitored by surface tension measurement.
There are several methods available to measure surface tension. The best method depends upon the nature of the liquid being measured, the conditions under which its tension is to be measured, and the stability of its surface when it is deformed. The following are some methods:
Du Noüy Ring Method
The traditional and most common way of measuring surface tension. It utilizes a platinum ring with defined geometry that is immersed into the liquid and then carefully pulled out through the liquid surface (Figure 2). The measurement is performed by a tensiometer, an instrument incorporating a precision micro balance, platinum-iridium ring with defined geometry and a precision mechanism to vertically move sample liquid in a glass beaker.

Figure 2: Illustration of the Du Noüy Ring method to measure surface tension
Wilhelmy Plate Method
This method measures the weight of the liquid drawn by a plate when a plate is lifted from or through the surface of liquid. The weight of the liquid is proportional to the surface tension of the liquid.
Pendant Drop Method
This method is based on the fact that the shape of a drop of liquid hanging from a syringe tip is determined from the balance of forces which include the surface tension of that liquid. This method has advantages in that it is able to use very small volumes of liquid, measure very low interfacial tensions and can measure molten materials easily.
Bubble Pressure Method (Jaeger's Method)
This method is useful for determining surface tension at short surface ages. The maximum pressure of each bubble is measured.
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