Dynamic Properties of Molecularly Thin Liquid Films JACOB N. ISRAELACHVILI, PATRICiA M. MCGUIGGAN, ANDREW M. HOMOLA An experimental technique is described for simultaneously measuring the static and dynamic interactions of very thin liquid films between two surfaces as they are moved normally or laterally relative to each other. Film thickness can be measured and controlled to 1 angstrom. Initial results are presented of the transition in the physical properties of liquid films only one molecular layer thick to thicker films whose properties are practically indistinguishable from the bulk. In particular, the results show that two molecularly smooth surfaces, when close together in simple liquids, slide (shear) past each other while separated by a discrete number of molecular layers, and that the frictional force is "quantized" with the number of layers. T HE PROPERTIES OF SUBMICROSCOP- ic quantities of matter, and especially of liquids confined to very small spaces, are of increasing interest, both for a fundamental understanding of molecular systems as well as from a practical need to understand and control many colloidal, in- terfacial, and material engineering processes. Examples abound, ranging from the forma- tion of small liquid droplets, polymer clus- ters, or surfactant micelles in solution, to the properties of liquids in very narrow pores or trapped between two solid surfaces (essen- tial to understanding interfacial adhesion, the properties of concentrated dispersions of particles, and lubrication), and to the prop- erties of thin monolayer or bilayer films of surfactants and lipids (the basic structure of biological membranes). Fundamentally, it is important to ascertain how the physical properties of such submicroscopic structures differ from those of the bulk materials, and in particular the size dependence of this variation. Certain bulk, or continuum, prop- erties such as refractive index, dielectric con- stant, and surface energy appear to be al- ready applicable to individual molecules or to very small clusters of molecules (1). In contrast, the results of direct measurements of interaction forces, such as the van der Waals forces between surfaces in liquids, are well described by continuum theories only at large distances, but fail drastically when two surfaces are closer than a few molecular diameters of the intervening liquid (solvent) molecules (1-5). This effect is due to the induced (or enhanced) ordering into dis- crete liquid molecular layers brought about J. N. Israelachvili and P. M. McGuiggan, Department of Chemical and Nuclear Engineering, and Materials De- partment, University of California, Santa Barbara, CA 93106. A. M. Homola, IBM Research Division, Almaden Re- search Center, San Jose, CA 95120. by the approach of the second surface, which leads to a nonuniform variation of the liquid density in such thin films and to an oscillatory force law (5-7). All the above examples refer to equilibri- um, or static, properties. Concerning dy- namic properties such as viscosity, measure- ments of the shear viscosities of liquid films down to 8 to 10 molecular diameters thick between two molecularly smooth surfaces show that the first, or at most second, layer of liquid molecules at each surface already exhibits its bulk viscosity (8, 9). However, as in the case of the van der Waals interactions, deviations appear to set in for thinner films (10). We investigated these deviations (i) to establish whether there are any correlations between the static interactions and dynamic properties of very thin liquid films and (ii) to establish how certain fundamental physi- cal properties vary during the transition from very thin monomolecular films (only one layer thick) to thick liquid films whose properties are practically indistinguishable from the bulk. The experimental techniques are based on an extension of the surface forces apparatus (4, 11, 12) that has previously been used to measure the forces between surfaces in liq- uids and vapors, wherein two curved mica or mica-coated surfaces are moved normally toward or away from each other and the forces directly measured from the deflection of a force-measuring spring. Figure 1 shows the newly developed lateral sliding mecha- nism that can be attached to the apparatus and which allows for two surfaces to be sheared past each other at various sliding speeds while simultaneously controlling the normal (compressive or tensile) load and measuring the transverse (frictional) force between them, all these being continuously variable during an experimental run. In ad- dition, the distance between the two sur- faces, their shape and exact molecular con- tact area, and the lateral motion can all be monitored in real time by recording the optical fringe pattern with a video camera and recorder (8, 10). This new attachment is similar to one developed by Israelachvili and Tabor (13) and used in earlier studies (13, 14) of the shear properties of surfactant- coated surfaces in air (that is, exposed to atmospheric conditions). The present device greatly extends the versatility of the tech- nique in the ways described above in addi- tion to having the surfaces in an enclosed chamber where the environment (vapor or liquid) can be controlled. Sliding was carried out with the initially curved surfaces under various loads that Fig. 1. Lateral sliding mechanism. To enable two TO VOLTMETER surfaces to be slid laterally (transversely) past each other, a new attachment replaced the piezoelectric MICROMETER TRANSLATION crystal tube mount supporting the upper silica STAGE disk of the basic surface forces apparatus [see MOUNTING PLATE figures in (4, 11, 12)]. Lateral motion is initiated by a variable speed motor-driven micrometer HORIZONTAL SPRING 0 screw that presses against the translation stage, LEVELLING SCREWS LIGHT TOMICROSCOPE AND BOLT ~~~SPECTRIOMETER OBJECTIVE TUBE which is connected through two horizontal dou- ANO BOLL ble-cantilever strip springs to the rigid mountingm_ plate. The translation stage also supports two - CHAMBER WALL STRAIN SAVGES ON vertical double-cantilever springs, which at their VERTICAL SPRING lower end are connected to a steel plate support- CURVED MICA _ DOVUBLE-CANTILEVER ing the upper silica disk. One of the springs acts as SILICA DISKS MOVABLE CLAMP a frictional force detector by having four resist- f n =9 _ _ ~~~~~FIXED CLAMP ance stram gauges attached to it, which form the MOVABLE CLAMP four arms of a Wheatstone bridge, and electrically ADJUSTING ROD MAINSUPPORT connected through a hole to a sensitive voltmeter WHITE LIGHT or chart recorder. Thus, by rotating the micrometer, the translation stage deflects, causing the upper surface to move horizontally and linearly at a steady rate. If the upper mica surface experiences a transverse frictional or viscous shearing force (due to its contact or proximity to the lower surface), this will cause the vertical springs to deflect, and this deflection can be measured by the strain gauges. The main support, force-measuring double-cantilever spring, movable clamp, white light, and so forth are all parts of the original (unmodified) basic apparatus whose functions are to control the surface separation, vary the externally applied normal load, and measure the separation and normal force between the two surfaces, as previously described in detail (4, 11, 12). REPORTS I89 8 APRIL I988 on December 24, 2014 www.sciencemag.org Downloaded from on December 24, 2014 www.sciencemag.org Downloaded from on December 24, 2014 www.sciencemag.org Downloaded from