Interfacial rheology studies the response of the interfacial layer to the external stimuli at air-liquid or liquid-liquid interfaces. Interfacial layers are typically composed of amphiphilic molecules with hydrophilic and hydrophobic moieties. When placed at the interface the molecules will orient themselves so that the hydrophilic part is in water and the hydrophobic part in the air (or oil if at the liquid-liquid interface). As molecules adsorb at the interface, they decrease the surface tension but in addition, the interfacial layer works as a barrier that prevents the breakage of the interface. To study the strength of the interfacial layer, different interfacial rheology techniques are used. The techniques can be divided into dilatational and shear methods.
Dilatational interfacial rheology studies in response to the area change
Dilatation methods change the surface area of the interface. As the size of the surface area oscillates, the molecules at the interface move further away or towards each other. This will change the surface tension at the interface accordingly. One common way to measure dilatation interfacial rheology is with oscillating drop. In this method, the volume of the droplet hanging at the end of the needle tip will oscillate with controlled frequency. If we would assume that the surfactants studied are soluble in the bulk phase, this would lead to an exchange of matter between the bulk and the interface when the oscillation frequency is similar to the time of the adsorption/desorption process.
Shear methods deform the shape of the interface
In a shear method, the shape of the surface is deformed. This can be done with different methods, but the most sensitive method utilizes a magnetic needle placed at the interface. The needle oscillates with the help of a magnetic trap, and the motion of the needle is recorded with a high-resolution camera. With shear methods, there should be no exchange between the bulk of the fluid and the interface as there is no area change.
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