Droplet-Droplet Interactions

The rational design of complex food emulsions requires an understanding of how the interfacial structure affects droplet-droplet interactions, a factor only obtainable by direct studies of deformable droplets pressed together. Feasibility studies, pioneered here at IFR, have shown how oil droplets can be attached to AFM cantilevers, positioned over surface-attached droplets and the force between these droplets in a liquid medium measured as a function of inter-droplet separation, interfacial composition and structure.The photograph shows oil droplets attached to a glass surface submerged in water. A tipless cantilever is positioned over a droplet and pressed down onto the droplet to attach it to the cantilever (see movie). The droplet can then be positioned over other surface-attached droplets to measure the droplet-droplet interaction.

The interfacial composition can be modified by adding components to the bulk aqueous phase. The studies of coated droplets provide the first realistic models of inter-droplet interaction in emulsions: they model correct unfolding and association of proteins at interfaces, allow for diffusion of interfacial components and deformation of droplets on approach.

Perceived wisdom suggests that protein adsorption should, by lowering the interfacial tension, make the droplets more deformable. Experimental studies show that as the proteins form an elastic network at the interface they actually make the oil droplets harder to deform. The interfacial elasticity determines deformation rather than the interfacial tension. Orogenic displacement weakens the protein network and the droplets are found to be more deformable when pressed together.

In the figure above the extent of cantilever deflection as two drops are pushed together provides a measure of droplet deformability. Initially the droplets become harder as the interfacial protein network is formed. Adding surfactant (arrow) weakens the protein network, causing the droplets to become softer.

Further reading

Gunning AP, Mackie AR, Wilde PJ, Morris VJ
Atomic force microscopy of emulsion droplets: probing droplet-droplet interactions.
Langmuir 20 (2004) 116-122.

Gunning AP, Mackie AR, Wilde PJ, Penfold RA & Morris VJ
Probing emulsion droplet interactions at the single droplet level
Food Colloids: Interactions, Microstructures and Processing (ed. Dickinson E) RSC Special publication No. 298, 85-96 (2005)

Mackie, AR, Gunning, AP, Wilde, PJ & Morris, VJ
The orogenic displacement of protein from the air/water interface by competitive adsorption.
J Colloid & Interface Sci. 210, 157-166 (1999).