Dynamic Interactions between a Silica Sphere and Deformable Interfaces in Organic Solvents Studied by Atomic Force Microscopy

Recent studies have successfully measured surface forces using atomic force microscope (AFM) and modelled surface deformations using the Stokes-Reynolds-Young-Laplace (SRYL) equations for particle-droplet, particle-bubble, droplet-droplet and bubble-bubble systems in various solutions. The current work focuses on interactions between spherical silica particles and a viscoelastic interface of water droplets in crude oil. The self-assembly of surface active natural polyaromatic molecules (NPAMs) at the oil-water interface has previously been shown to change a viscous dominant oil-water interface to an elastic dominant interface, with interfacial aging due to gradual formation of rigid interfacial networks. AFM was used to measure the interactions between a small silica sphere (D ≈ 8 µm) and a deformable water droplet (D ≈ 70 µm), which exhibits time-dependent interfacial viscoelasticity in NPAM solutions. Unlike the systems studied previously, the measured deformation shown as a repulsive force over the constant compliance could not be modelled adequately by the conventional SRYL equations which are applicable only to purely Laplacian interfaces. As the water droplet ages in NPAM solutions, a rigid "skin" forms at the oil-water interface, with the interface exhibiting increased elasticity. Over a short aging period (up to 15 min), interfacial deformation is well predicted by the SRYL model. However upon further exposure to the NPAM solution, droplet deformation is under predicted by the model. Physical properties of this mechanical barrier as a function of time were further investigated by measuring interfacial tension, dilatational rheology and interfacial "crumpling" (non-smooth interface) upon droplet volume reduction. By introducing a viscoelasticity parameter to account for interfacial stiffening, we are able to correct this discrepancy and predict droplet deformation under AFM cantilever compression using experimentally-determined elasticity. This parameter appears to be important for modelling non-Laplacian systems with significant viscoelastic contributions, such as biological cell membranes or polymer blends.