Effect on Droplet Deformation

The mere presence of a surfactant, apart from its effect on the magnitude of y, may affect the deformation and break-up of a drop. This will now be briefly discussed. 

If the y-gradient can become large enough, it will virtually arrest the interface. The largest value attainable for dy equals about Ileq, i.e. yo - y q. If it acts over a small distance, a considerable stress can develop, say 10 kPa. 

On the other hand, if surfactant is applied at one site on an interface, a y-gradient is formed that will cause the interface to move, roughly at the velocity given by 

The magnitude of the y-gradients and of the Marangoni effect will depend on the surface dilational modulus d see eqn. (2.37). For a plane interface in contact with one surfactant-containing phase, in the absence of streaming, and for a small expansion or compression of the interface, the following relations have been derived by Lucassen  

Some theoretical studies reveal differences in the mode of deformation and the relative deformation at breaking of the drop, but the effects seem to be moderate. See also a discussion of the results by Janssen et aV Currently, the importance for practical emulsification is not too clear. 

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