Clay platelets stabilising emulsions
Left: A schematic of an atomic force microscope combined with a laser scanning confocal microscope to simultaneously monitor the deformation of a clay coated emulsion droplet (10 μm in diameter) in aqueous solution during compression. Right: A cryo-scanning electron microscopy image of a clay coated oil droplet where the water has been removed by sublimation.
Emulsions are typically unstable, requiring energy input to form, such as shaking, stirring or ultrasounds, after which they quickly return to their separated state. Emulsions which are stabilised through the addition of solids to the liquid/oil package, have received increasing attention over the last two decades due to their superior stability when compared to traditional surfactant stabilsed emulsions. These emulsions can be found in a wide range of industries such as food, pharmaceutical, paint and petrochemicals.
Non-spherical particles with plate-like or needle-like shapes have had more recent attention and it has been shown that they can stabilise emulsions at a much lower solids volume fraction than spherical particles. Clay platelets, have been used as stabilisers in emulsions and foams by making ‘armoured’ drops or bubbles which are stabilised particles. In some areas however, the enhanced stability of these emulsions can also be detrimental. In solvent extraction, for example, these emulsions hinder separation processes that result in long, and therefore uneconomical, separation times. Thus, an understanding of the nano-mechanical proprieties of these types of coated drops are required to enhance their application in many products or find pathways to break them down in minerals processing separations.
The mechanical properties of clay-armoured emulsion droplets were investigated using laser scanning confocal microscopy with an in situ atomic force microscopy measurement. This novel combination of these two stand-alone instruments was achieved at MCN to the visualisation of droplet shape as a function of applied force. In addition, this combination of methods has many potential uses in the biotechnology area as well, in the study of individual living cells or in the compression of engineered particles and capsules in controlled applications.
In the case of emulsion droplets, they were found to be mechanically robust, stable against coalescence during drop collisions and able to recover from large deformations without disintegration. A Hookean constitutive law was used to extract the surface Young’s modulus of the clay-armoured droplets as a function of a range of solution conditions. The clay-armoured droplets were relatively insensitive to changes in solution ionic strength, pH and the addition of surfactant. The elasticity measurements in this study should help illuminate the impact of the clay-armoured droplets on macroscopic properties of emulsions including rheological properties and emulsion stability.
These measurements should help illuminate the impact of the clay-armoured droplets on macroscopic properties of emulsions including rheological properties and emulsion stability in areas including food, personal care products and some steps in the purification of minerals or pharmaceuticals.