Langmuir 2001, 17, 7439-7441
7439
Notes Measurement of Weak, Short-Range Attractive Forces Superimposed on Dominant, Longer Ranged Interactions George Maurdev* and Michelle L. Gee School of Chemistry, University of Melbourne, Victoria, 3010, Australia Received January 12, 2001. In Final Form: August 13, 2001
Introduction The surface forces apparatus1 (SFA) has long been the cornerstone for the direct measurement of surface forces between solid surfaces across a liquid medium. More recently, alternate techniques for measuring forces between adjacent surfaces have been developed. These include atomic force microscopy2,3 (AFM), and total internal reflectance microscopy.4,5 Surface forces have been measured across aqueous media, complex fluids, hydrocarbons, polymer melts, and even a gas phase.6 A variety of forces, both attractive and repulsive, have been detected using the SFA. The attractive interactions typically measured are van der Waals, bridging forces, and the hydrophobic interaction, each of which leads to a strong adhesion between the two interacting surfaces. In direct force measurements using the SFA, the strength of attractive forces is measured by applying a force that opposes the force of adhesion between the surfaces. The bottom surface is attached to a double cantilever spring of known spring constant. As the opposing force is gradually increased, the spring begins to deflect but the surfaces remain in contact by virtue of their mutual attraction. The extent to which the spring deflects depends on the stiffness of the spring and the force of adhesion. When the gradient of the opposing force is greater than the spring constant, the surfaces detach and jump apart. Typically, the spring is undeflected after the jump; i.e., it is at its position of mechanical equilibrium, so therefore the distance by which the surfaces separate on detachment is equal to the maximum deflection of the spring before the jump apart. Hence, by measuring the surface separation after the jump apart and knowing the spring constant, a force of adhesion can be calculated by simply using Hooke’s law. The simple procedure outlined above only holds true, however, if the surfaces jump out to a sufficiently large distance so that there is no residual force between the surfaces causing any spring deflection. That is to say, the calculation of the adhesive force from the experimental data is contingent on the spring attaining mechanical * To whom correspondence should be addressed. (1) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (2) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 2239. (3) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (4) Walz, J. Y.; Prieve, D. C. Langmuir 1992, 8, 3073. (5) Prieve, D. C.; Frej, N. A. Langmuir 1996, 6, 396. (6) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992.
equilibrium and so zero deflection. This means of calculating an adhesion fails if a residual force is present at the final position the surfaces come to rest. For example, consider the situation of a weak attractive force between two surfaces where the jump out distance is ca.
