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

Abstract

Recent studies have successfully measured surface forces using 8 atomic force microscope (AFM) and modeled 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 upon aging, due to gradual formation of rigid interfacial networks. AFM was used to measure the interactions between a small silica sphere (D ≈ 8 m) and a deformable water droplet (D ≈ 70 m), which exhibits time-dependent interfacial viscoelasticity in NPAM solutions. Unlike the systems studied previously, the measured deformation shown as a repulsive force over the region of constant compliance could not be modeled 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 in NPAM-in-toluene solution), interfacial deformation is well predicted by the SRYL model. However, upon further exposure to the NPAM solution, droplet deformation is over predicted by the model. Physical properties of this mechanical barrier as a function of interfacial aging were further investigated by measuring interfacial tension, dilatational rheology, and interfacial "crumpling" (non-smooth, non-Laplacian interface) upon droplet volume reduction. By introducing a viscoelasticity parameter to account for interfacial stiffening and using experimentally determined elasticity, we are able to correct this discrepancy and predict droplet deformation under AFM cantilever compression. This parameter appears to be important for modeling non-Laplacian systems of significant viscoelastic contributions, such as biological cell membranes or polymer blends.

Accelerated spreading of inviscid droplets prompted by the yielding of strongly elastic interfacial films

“…n = 0.5, = 0 -115 o 9 n ~ 0.5 then 0.06 < n < 0.1, = 0 - Of particular interest in the current study is the contribution from surface elasticity which can be influenced by surface active species partitioning at the liquid-liquid interface. [31][32][33][34] To the authors' knowledge, interfacial effects on the spreading dynamics of droplets are rarely explored except for surfactant systems, with particular focus given to changes in surface tension. Generally, surfactants reduce the equilibrium three-phase contact angle and increase the solid-liquid contact area.…”

Surface Interaction of Water-in-Oil Emulsion Droplets with Interfacially Active Asphaltenes

“…17,19,30 The mechanical and rheological properties of this protective layer were recently studied using different techniques, with implication on the stabilization effects of asphaltenes on W/O emulsions. 31,32 It has been reported that the environmental conditions (e.g., solvent type, 18,33 temperature, 34,35 presence of stabilizer 36,37 ) could significantly change interaction behaviors of W/O emulsion droplets with asphaltenes. 36 Generally, strong repulsion between water droplets corresponds to enhanced W/O emulsion stability, while interfacial adhesion can result in droplet agglomeration and flocculation.…”

Probing Mechanical Properties of Water–Crude Oil Interfaces and Colloidal Interactions of Petroleum Emulsions Using Atomic Force Microscopy

“…For 0.588 g/L bitumen-intoluene the slope of the constant compliance region after 15 min aging is 32 mN/m and increases to 45 mN/m after 1 hr aging, comparable to the results for the asphaltene systems previously reported. 44 The increasing slope of constant compliance region in bitumen solution is also attributed to an increase in the interfacial dilatational elasticity (Figure 3(a)), since the change in water-bitumen interfacial tension, shown in Figure 3 Firstly, considering the asphaltene system, the slope of the constant compliance region following 1 hr aging and in the absence of EC-4 is 41 mN/m, and the associated adhesion force between the particle and water droplet is 5.6 nN. Upon addition of EC-4, the slope of the constant compliance region and the particle-water droplet adhesion force continually decreases with demulsification time.…”

“…The elasticity of the oil-water interface, measured by dilatational rheology (Figure 3(a)) increased to 17 mN/m after ~1 hr aging, confirming the formation of a rigid asphaltene network. 44 To account for this high elasticity contribution, an experimentally measured viscoelasticity parameter ( /E') is included as an extra term inside the brackets of Eqn. 1.…”

“…29,32,[35][36][37][38][39][40] However, there is limited research which considers industrially relevant systems, such as water droplets in oil, 42 or interfaces with significant elasticity, for which the criterion of a Laplacian response no longer holds. 43 Our previous study 44 explored viscoelastic effects encountered in water-in-crude oil emulsion systems. Oil-water interfacial stiffening and "skin" formation was observed upon water droplet aging in asphaltene solution, leading to the gradual formation of a viscoelastic oil-water interface.…”