Bubble or Droplet Pressure Method

Illustration of the maximum bubble pressure method using two capillary 

A better method uses two capillaries of differing radius, at the same immersion depth, and involves measuring the differential maximum bubble pressure between the two capillaries. In this case, the two pgt) terms cancel out and the differential pressure is the difference between the two i ylb) terms where b is the radius of curvature at the apex of the bubble. 

A variation of this technique, the maximum droplet pressure method, can be used to determine interfacial tensions. This technique is especially useful in situations for which the density contrast between the two liquid phases is very low, or non-existent [18,19]. 

Relatively new are the developments of microfluidic surface or interfacial tension methods. Such methods offer the potential for rapid, online measurements on small volume samples. Typically, use is made of a property change (such as droplet deformation or pressure drop) associated with fluid flow through some kind of constriction in a microchannel. The fundamental principles are usually the same as in the examples just given above, such as shape or pressure changes. If the device is used to generate bubbles, then the bubble formation frequency can be used as the basis for surface tension calculation. In a multiple-channel device, multiple bubbles or droplets are usually sensed simultaneously. These kinds of approaches have been applied to the determination of both surface and interfacial tensions [20-22]. 

Extreme cleanliness of all surfaces and interfaces is required in all surface and interfacial tension methods. This is also required to achieve the condition 0 = 0 that some of the methods require. For most of the methods, the apparatus must be absolutely vibration-free. In all of the methods, the temperature must be carefully controlled. In the methods where densities are needed, these must be accurately known. 

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