Interaction forces, deformation and nano-rheology of emulsion droplets as determined by colloid probe AFM

Atomic force microscopy (AFM) has been used to determine the interaction forces between a spherical silica probe and immobilised colloidal droplets of polydimethylsiloxane (PDMS) in aqueous solution. Under mildly alkaline conditions where PDMS droplets and the silica probe are both negatively charged, a repulsive force is evident that increases less rapidly on surface approach than expected for electrical double layer interaction of rigid particles. The departure from hard-sphere behaviour enables the extent of droplet deformation to be determined and by varying the extent of internal cross-linking within the PDMS droplets, the influence of bulk rheology and interfacial tension on droplet deformation and nano-rheology has been isolated. For highly cross-linked droplets, the extent of deformation is controlled by the bulk rheology rather than the droplet's interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force, and has been theoretically fitted to obtain nano-rheological parameters, which describe the droplet relaxation processes. For liquid-like droplets, with a low level of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant. The spring constant is proportional to the surface tension (as controlled by surfactant adsorption) and the reciprocal of the droplet radius, i.e. the Laplace pressure exclusively controls droplet deformation. These colloid probe AFM studies offer due insight into the deformation and interaction of emulsion droplets and have implications when considering the stability, adhesion and processing of emulsions.