"The Measurement of Surface Forces at Deformable Interfaces"

Raymond R. Dagastine (Advisors: Dennis C. Prieve and Lee R. White, Dept. Chemical Engineering)

Understanding the behavior of colloidal particles in processes such as flotation, de-inking of papers, oil recovery and a variety of other applications, is complicated not only by the importance of surface forces on the colloidal scale, but also by the deformation of the liquid/liquid or liquid/gas interfaces that can make quantifying surface forces difficult. A large body of work exists on theoretical and experimental investigations of surface forces at rigid interfaces, but very little work has been done in measuring surface forces at deformable interfaces. Macroscopic properties such as contact angle and surface energy typically have been the means to characterize interactions at deformable interfaces with little emphasis on the surface forces at hand.

There have been several experimental investigations on the forces between a rigid particle (normally a sphere on the order of 5 microns) and the bubble-water or oil-water interface using Atomic Force Microscopy (AFM). The quantity of interest is the surface force, F, as a function of axial separation distance, D_o, (see Fig 1) acting across the thin film of water created by the close proximity of the rigid particle to the interface. The deflection of a thin cantilever and the motion of the piezo tube, the distance actuator, are the measured quantities in the AFM experiment. Converting raw AFM data to force versus separation data requires further analysis, and the deformation of the interface during the measurement complicates this process. To interpret or quantify the results from an AFM experiment requires an appropriate theory modeling the physical situation. We have developed a semianalytic theory^1,2 to predict the force curve of an AFM measurement at a liquid interface using a colloidal probe for a general force law with both attractive and repulsive forces. For a given material system and disjoining pressure, P(D), we can predict an AFM force curve as a function of scaled piezo movement to within a system constant of the experimental measurement.

We are using this theory to quantitatively interpret AFM measurements at the oil/water interfaces. In Fig 2 the AFM force measurement of a glass probe above a tetreadecane/water interface as a function of scaled piezo distance, DX, is plotted for a variety of concentrations of sodium dodecyl sulfate(SDS). The slope of the force curves decreases with increasing SDS concentration and then increases at a concentration above the CMC (~8 mM). The addition of surfactant changes surfaces tension, screening length, and surface potential with concentration, so this is a somewhat complicated system. We are currently working to compare the theory to experiments by independently measuring the electrostatic and wetting parameters (surface tension and contact angle) to independently construct the disjoining pressure and compare theoretical predictions to the experimental measurements. We are also developing an iterative method to use the theory to determine the disjoining pressure without an independent measure of P(D).

Future work will include the use of Total Internal Refection Microscopy (TIRM) to probe deformable interfaces. In TIRM the potential energy profile of a particle undergoing Brownian motion above an interface is measured using a light scattering technique. The forces measured in a TIRM experiment are commonly 1-3 orders magnitude less, probing a different region of the interaction than AFM.

References

1 Chan, D. Y. C., Dagastine, R. R., and White, L. R., J. Colloid Interface Sci. 236, 141 (2001).

2.Dagastine, R. R., and White, L. R., submitted J. Colloid Interface Sci. May (2001).