Instruments direct force measurement 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). 

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]. 

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]. 

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. 

The pin-on-plate apparatus was instrumented to measure friction force via a bi-directional load cell. Readings of the frictional force were taken every ten minutes for 2s, which corresponds to two stroke cycles. The friction coefficient was then calculated and plotted as a function of time for the duration of the test. The average friction coefficient was also calculated for each test. 

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