Interfacial rheology studies the flow properties of the interfacial layer that is formed between two immiscible fluids, such as oil and water. The interfacial layer can be composed of any surface-active material such as surfactant, polymers, or even nanoparticles. The behavior of this layer is important especially when studying the stability of emulsions or foams. Interfacial rheology measurements are used for these studies.
Surface active molecules stabilize the interface
When two immiscible fluids, such as oil and water are mixed, the droplets of oil can be seen in water. Quickly after the mixing is stopped, the oil will, however, again separate from water and migrate on top (when the oil is the lighter phase). When surface active material such as surfactant, polymer or even particles are added into the mixture, stable emulsions can be formed. Hydrophilic to hydrophobic balance of the molecule dictate the nature of the emulsions (O/W or W/O) being formed.
There are two mechanisms by which the surface-active molecules are stabilizing emulsions. First, they decrease the interfacial tension between the oil and water. When oil and water are mixed and small oil drops are formed, there is a significant increase in the interfacial area in the system. This will increase the total interfacial free energy and result in thermodynamically unstable system when surfactants are not present. With the addition of surfactants, the total interfacial free energy increase is not as high. In addition, the surfactant molecules will form a mechanical, steric and/or electrical barrier between the droplets and the continuous phase which will decrease coalescence of the droplets.
Pulsating drop offers a quick method to evaluate surface elasticity
The properties of the interfacial layer have significant effect on the stability of the emulsion. Surface elasticity has been proposed as one of the parameters that can be used to predict emulsion stability. Interfacial elasticity can be measured by using optical tensiometer equipped with the pulsating drop module.
In the pulsating drop measurement, the volume of the drop is pulsated with the known frequency. The area variation as well as the surface tension change as a function of frequency is detected. This information is used to calculate the surface elasticity.
To read more about how interfacial rheology can be utilized to predict stability of the droplets, please download the case study through the link below.