Rheology surface forces apparatus

P. Bordarier, B. Rousseau, A. Fuchs. Rheology of model confined ultrathin fluid films. I. Statistical mechanics of surface forces apparatus experiments. J Chem Phys 70(5 7295-7302, 1997. 

Another powerful tool for investigating a rheology of liquid films on nano-scale is Surface Force Apparatus (SFA)... 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

For the sake of concreteness of the following developments, we consider a fluid confined to a slit-pore such that the solid surfaces representing the pore walls are planar, parallel to one another, and perpendicular to the 2-axis of a Cartesian coordinate system. The separation between the pore walls will be denoted s. In addition, the two solid surfaces can be manipulated by external agents normal to the fluid-solid interface sucli that s, may be altered. EventuaJl), these planar surfaces will come to rest at some equilibrium separation Sj,. As we shall see later in Section 5.3.1, the situation just described is akin to laboratory experiments in which the rheology of confined fluids is investigated by means of the so-called surface forces apparatus (SFA). 

The surface forces apparatus (SFA) measures forces between atomically flat surfaces of mica. Mica is the only material that can be prepared with surfaces that are atomically flat across square-millimetre areas. The SFA confines liquid films of a few molecular layers thickness between two mica surfaces and then measures shear and normal forces between them (figure C2.9.3)b)). In essence, it measures the rheological properties of confined, ultra-thin fluid films. The SFA is limited to the use of mica or modified mica surfaces but can be used to study the properties of a wide range of fluids. It has provided experimental evidence for the formation oflayered structures in fluids confined between surfaces and evidence for shear-induced freezing of confined liquids at temperatures far higher than their bulk freezing temperatures [14. 15]. 

Abstract The structure and mechanics of very thin hquid crystal films depend on the intermolecular interactions in confined dimensions. The rheology of such films has been investigated by a shear force apparatus constructed as an attachment to the surface forces apparatus. The novelty of this method is that the rheological parameters are extracted from the amplitude and the phase of the output signal as a function of the resonance frequency. The apparent viscosity of the liquid crystal film is calculated from the damping coefficient by using a simple theoretical model. The viscosity of nanometer thin films of 4-cyano-4-... 

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