Adhesion and short-range forces between surfaces. Part I: New apparatus for surface force measurements
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A new miniature Surface Forces Apparatus (SFA Mark III) is described for measuring the forces between surfaces in vapors and liquids. The apparatus employs similar techniques to those used in current SFAs, but it is easier to operate and is generally more user-friendly. Four stages of increasingly sensitive distance controls replace the three control stages of previous apparatuses. The first three stages allow for rapid manual control of surface separation to within 10 A, while the fourth piezo-control stage has a sensitivity of better than 1 A. All four distance controls have been specially designed to produce perfectly linear displacements of the surfaces. In addition, the SFA Mk III is more robust, less susceptible to thermal drifts, easier to clean, and requires smaller quantities of liquid than conventional SFAs. The high performance of this new instrument is illustrated in the succeeding paper (Part II), which describes the subtle effects of surface lattice mismatch on the oscillatory forces in water in the distance regime from 0 to 10 A.
I. INTRODUCTION A. Early surface forces apparatuses
We briefly review the current state of direct surface forces measuring techniques, describe their limitations, and discuss why it was felt necessary to develop a new apparatus. In 1969 Tabor and Winterton,1 and later Israelachvili and Tabor,2 developed Surface Forces Apparatuses (SFAs) for directly measuring the van der Waals forces between surfaces in air or vacuum. These were successfully used for measuring van der Waals forces in the distance regime 1.5-130 nm with a distance resolution of 0.1-0.2 nm. In 1976, Israelachvili and Adams3 designed a new apparatus (later known as SFA Mk I) for measuring the forces between surfaces in liquids as well as in vapors, allowing control and measurement of the surface separation to within 0.1 nm. The SFA Mk I enabled the first detailed measurements to be made of the two fundamental forces of colloid science. These are the repulsive electrostatic doublelayer forces and the attractive van der Waals forces, which exist between any two charged surfaces immersed in an electrolyte solution. These two forces make up the so-called DLVO Theory, which forms the basis of analyzing the long-range interactions of colloidal and biological structures in solution.4 The principles on which Mk I operates are simple3: one of the surfaces (the upper) is rigidly mounted at the end of a piezoelectric crystal tube while the lower surface, which faces the upper, is suspended at the end of a force-measuring spring. The surfaces can be moved toward or away from each other using a three-stage sysJ. Mater. Res., Vol. 5, No. 10, Oct 1990
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tem of controls of increasing accuracy. First, a coarse control micrometer drive allows for positioning to within about 500 nm over a range of 1 cm. The second level of control, the medium control, employs a micrometer-driven differential spring having an accuracy of 1 nm over a range of 10 ^tm. The third or fine control
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