The oscillating jet method is used to determine the static and dynamic surface and interfacial tensions of liquids (Couper 1993). The apparatus used for this type of measurement consists of an elliptical orifice through which the liquid to be analyzed is pumped. The stream of liquid that emerges changes its shape from a vertical elliptical cross-section to a spherical cross-section as it moves away from the orifice because this enables it to reduce its surface area. Nevertheless, the liquid will continue to change its shape because of its momentum
until it gains a horizontal elliptical cross-section (Figure 5.19). As the liquid travels away from the orifice, its shape therefore oscillates between these different cross-sections. These oscillations have a characteristic wavelength which depends on the surface or interfacial tension of the liquid. The wavelength of the oscillating jet is measured using special optical techniques. The surface or interfacial tension can then be determined from the following equation:
2 n2 r 3pv2
where r is the radius of the orifice, v is the velocity of the liquid stream, g is the gravitational constant, and X is the wavelength of oscillation. Surface tensions are measured by having the stream of liquid travel through air, whereas interfacial tensions are measured by having it travel through an immiscible liquid. The age of the surface can be varied by altering the flow rate of the liquid emerging from the elliptical orifice: the faster the flow rate, the shorter the age of the interface. Dynamic surface or interfacial tensions can be measured down to times of 1 ms using this method, and so it is particularly suitable for characterizing the adsorption kinetics of rapidly adsorbing emulsifiers. Nevertheless, there is some debate about whether the movement of the emulsifier molecules to the surface is due to convection or diffusion under the conditions prevailing within the instrument (Couper 1993). Pure liquids can be analyzed at any flow rate, because their surface or interfacial tensions should be independent of surface age. The major limitation of this technique is that it can only be used to analyze low-viscosity liquids (i.e., those with a viscosity less than about 5 mPa s).
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