Instruments surface forces apparatus

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

For surfaces that deform plastically, the contact area A is proportional to the applied load. A single elastic contact deforms as as load increases, and would not be expected to follow this rule. However, when considering an exponential surface height distribution, which leads to a multiplicity of elastic asperity contacts, a linearity between load and contact area is recovered. Using instrumentation developed in the last 15yr, notably the atomic force microscope (AFM) and surface forces apparatus (SFA), researchers have explored the universality of friction-load proportionality over a much wider range of dimensions and surface characteristics. Indeed, SFA experiments have shown friction-load proportionality between atomically smooth mica surfaces in dry air over square micrometers of contact area. A contact mechanics expression for elastic contacts that incorporates the effects of adhesion was used. Similarly, AFM experiments of... 

Many of the instruments used in nanotribology experiments are limited in terms of the surfaces and materials used. The surface forces apparatus accommodates soft materials such as... 

The Surface Force Apparatus (SFA), designed by J. Israelachvili is a commonly used instrument for the measurements of surface forces. The... 

The force between two adjacent surfaces can be measured directly with the surface force apparatus (SEA), as described in section B1.20 [96]. The SEA can be employed in solution to provide an in situ determination of the forces. Although this instrument does not directly involve an atomically resolved measurement, it has provided considerable insight into the microscopic origins of surface friction and the effects of electrolytes and lubricants [97]. 

The AFM is a force measuring instrument [2]. It operates on broadly similar principles to the surface force apparatus [3], except that instead of probing the interaction forces between two macroscopic surfaces, the forces measured are those between a very sharp tip and a surface. The tip is attached to a cantilever - the spring - which, as the sample is scanned under the tip (or the tip scanned with respect to the sample), or moved in a direction normal to the tip, deflects in accordance with the force experienced between the tip and the surface. This basic concept is depicted in Fig. 1. Importantly, the AFM can be used to image any surface irrespective of sample conductivity - this is in contrast to STM where the substrate must be (semi)conducting. AFM probes are typically microfabricated from Si3N4 or Si [4]. 

Surface Forces Apparatus. The experiments were carried out with a modified version of the Mk 3 SFA (Surforce, Santa Barbara, CA). 2 The instrument is equipped with a fully automated data acquisition and evaluation program based on the fast spectral correlation algorithm and is mounted in a box with precise temperature control. All experiments were carried out at 25 °C. 

Surface Force Measurements. Another method to measure the thickness of adsorbed layers is by the surface force apparatus (SFA) (17). In this method two freshly cleaved mica sheets are glued to the surfaces of two crossed cylindrical lenses. Polymer chains are then allowed to adsorb on the mica sheets. In order to measure the thickness of the adsorbed layers the two cylinders are brought in contact and the force between them is measured as a fimction of separation. The onset of the repulsive force associated with compression of the adsorbed layer can be related to the thickness of the adsorbed layer. On the other hand, in the event of bridging between the adsorbed layers the force will be attractive. Recent advances in the instrument have made it possible to probe the effect adsorption has on the flow of fluid past a surface (18). 

It has recently become common to use the JKR theory (Johnson, Kendall Roberts, 1971) to extract the surface and inteifacial energies of polymeric materials from adhesion tests with micro-probe instruments such as the Surface Force Apparatus and the Atomic Force Microscope. However the JKR theory strictly applies only to perfectly elastic solids. The paper will review progress in extending the JKR theory to the contact mechanics and adhesion of linear viscoelastic spheres. The observed effects of adhesion hysteresis and rate-dependent adhesion are predicted by the extended eory. 

A different approach yielding values of work of adhesion involves sensitive measurements of detachment force and contact radius when two surfaces are carefully moved towards and apart from another. The instrument is known as surface force apparatus (SFA) and was first developed by Tabor and Winterton " for direct measurement of van der Waals forces between molecularly smooth surfaces. Further improvements were carried on by Israelachvih and Tabor. ... 

The first instrument to directly measure the van der Waals forces between molecularly smooth cylindrical mica surfaces was the Surface Force Apparatus (SFA) developed by Tabor and Winterton (1969) and Israelachvili and Tabor (1972, 1973). The results from SFA were the first to confirm the predictions of Lifshitz theory of van der Waals forces down to surface separations as small as 1.5 nm. The SFA technique has since then been modified and has found applications in many different areas, both biological and non-biological to provide information on the different forces acting between the surfaces and molecules. However, a limitation ofthis technique is that it measures interactions between membranes, proteins or various metal oxide layers or films, which have to be deposited/adsorbed onto the molecularly smooth mica surfaces. 

Fundamental rmderstanding of friaion on the atomic and nanometer scales is critical for design of micro- and nanoe-lectromechanical systems. There are two powerful instruments in nanotribology the surface force apparatus and the atomic force microscope. AFM experiments allow high-resolution mapping of friction properties between solids on the nanometer scale and usually indude measurements of friction forces by friction force microscopy (FFM) and adhesion by CFM. In FFM, as a sharp tip scans across a surface in the... 

The force as a function of surface separation between glass substrate surfaces was measured with a MASIF instrument [28], This apparatus is based on a bimorph force sensor to which one of the surfaces is mounted (Fig. 5). The other surface is mounted on a piezoelectric tube. The bimorph (enclosed in a Teflon sheath) is mounted inside a small measiuing chamber, which is clamped to a translation stage that serves to control the coarse position of the piezoelectric tube and the upper siuface. 

TRI has developed a different method for low-load friction (43), which uses a modification of the interfiber fiictional apparatus mentioned above. In this instrument, the forcemeasuring device is a Kahn recording microbalance on which the vertical fiber is mounted. The horizontal fiber can be moved by a micrometer to establish the exact angle of deflection between the fibers. The system is shown schematically in Figure 21. The apparams is an attachment to the TRI/SCAN surface force analyzer, which was described earlier in the discussion of wetting force measurements with the appropriate software. 

A force Is applied to the gel s surface by a syringe piston powered by compressed air. Measurements can be Influenced by a skin on the jelly s surface or uneven application of pressure (3 ). Swenson et al. (48) modified the Tarr-Baker apparatus to produce a balance-plunger type Instrument. Although they found a linear stress-strain region within the elastic limits of the gel they also found elasticity was somewhat dependent on the rate of loading. 

Bremond Porosimeter. Apparatus for the evaluation of pore size distribution by the expulsion of water from a saturated testpiece. (P. Bremond, Bull. Soc. Franc. Ceram. (37), 23,1957). Brenner Gauge. An instrument for the non-destructive determination of the thickness of a coating of vitreous enamel it depends on the measurement of the force needed to pull a pin from contact with the enamel surface against a known magnetic force acting behind the base metal. 

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