Erik Freer

Research Summary:

Adsorption of proteins, asphaltenes, and other macromolecules at soft interfaces is well known to produce viscoelastic films. Upon compression of the interface, this film can be visually observed as shown in Figure 1. This interfacial gel-like network results from irreversible adsorption (relative to the time scale of interfacial compression), conformational changes, concurrent intermolecular aggregation of the macromolecules, and is believed to stabilize long-lived emulsions and foams as well as alter wettability in oil reservoirs. In this work, we utilize interfacial shear and dilatational rheology to noninvasively probe the structure of adsorbed macromolecules and the kinetics of network formation at the oil/water interface.

Figure 1. Interfacial Gel

Interfacial Rheology of Lysozyme and b-Casein Adsorbed at the Hexadecane/Water Interface: Comparison of Shear and Dilatation Deformation

To gain further understanding of interfacial transport processes and rheology, we undertook a collaborative project with the Fuller group at Stanford University. The aim of this project was to compare the interfacial rheological response in dilatational and shear deformations as a function of interface aging for adsorbed protein layers. Our work gives insight into the differences and similarities of the relaxation mechanisms that occur between the two principle modes of deformation and establishes a framework for interpretation of the viscoelastic properties of complex molecules at soft interfaces.

Asphaltenes at the Toluene/Water Interface: Diffusion Dissipation of the Reversibly Adsorbed and Ultraslow Dynamics of the Irreversibly Adsorbed

Upon adsorption at the oil/water interface, asphaltenes form a glassy interphase with very slow dynamics. This interphase is likely the reason for prolonged stability of crude oil/water emulsions. Here we utilize interfacial dilatation rheology using the oscillating pendant drop to investigate the relaxation mechanisms of asphaltenes adsorbed at the toluene/water interface. We compare classical viscoelastic models with the measured data and find that the frequency response of the dilatational moduli fit a combination of diffusion dissipation and in-plane surface relaxation models. We verify the model by washing out the asphaltenes from the organic phase and measuring the dilatational response of the irreversibly adsorbed species. The diffusion component of the frequency response disappears and the relaxation time of the interface increases by an order of magnitude after washout suggesting that the reversibly adsorbed species prevent formation of an asphaltene-gel/glass phase at the interface. These experiments show, for the first time, that most of the surface-active molecules are irreversibly adsorbed with respect to the oil phase.

The Role of Interfacial Rheology in Reservoir Mixed Wettability

(Freer, E. M.; Svitova, T. F.; Radke, C. J. Journal of Petroleum Science and Engineering 2003, 39, 137)

Since the early 1950's, industrial researchers have recognized that asphaltenic crude oil/ water interfaces form so-called "rigid skins". This work emphasizes the role that such oil/water interfacial microstructures play in establishing the mixed-wet state of reservoirs. We utilize a new oscillating-drop dynamic tensiometer that sinusoidally and infinitesimally expands and contracts a crude-oil droplet immersed in brine at a fixed frequency and measures the resulting dynamic tension from image analysis and axisymmetric drop-shape analysis. Linear viscoelastic theory permits evaluation of the dilatational interfacial elastic storage and viscous loss moduli. We find that for two crude oils, designated as Crude AS and Crude AH, immersed in synthetic sea water, the interface behaves primarily elastically and that the more asphaltenic the oil the stronger is the interfacial elasticity. Moreover, interfacial elasticity grows slowly in time over days and is clearly manifest even when "rigid skins" are not visible to the eye. Apparently, macroscopic, networked asphaltenic structures slowly evolve in time at the interface.