Oil-film interferometry

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry, and shear stress-sensitive liquid crystal [5]. 

The optical-based shear stress sensor can be classified into two categories based on the measurement principle oil-film interferometry and shear-sensitive liquid crystal. These techniques are discussed in the following sections. 

Shear Stress Sensors, Fig. 5 Schematic showing (a) fringe formation for oil height measwement and (b) an experimental setup using oil-film interferometry... 

For oil-film interferometry, it is assumed that the oil film is so thin that it does not influence the flow above it and is driven by the skin friction distribution of the flow. Using a control volume analysis of the thin oil film with its height h in wall normal (y) direction as a function of streamwise (x) and spanwise (z) coordinate and assuming the shear stress contribution to be dominant compared to the pressure gradient and surface tension force, the governing equation for the thin oil-film flow is... 

Using the distribution of oil-film height from interferometry measurements and the integration of equation 16, the skin friction can be determined. The benefit of oil-film interferometry is that no calibration is required for measurement of shear stress and the basic analytical expression is... 

In microfiuidic devices the measurement of the wall shear stress is challenging in general. For a direct measurement, methods commonly used such as oil film interferometry or liquid crystal coatings cannot be applied since already the thin layer of oil alters the fluids behavior at the wall. Shear stress sensors, like micro-piUars, surface mounted and elevated hot wires, and Preston tubes, might be applicable in macroscopic flows but not for microscale flow investigations. The first reason is their size, which is often larger than the whole microfiuidic channel or structure on the surface. Furthermore, in some cases like the studies on endothelial cells mentioned above, the wall is a biological tissue that cannot be altered or replaced. Secondly, the sensors would... 

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry and shear stress sensitive liquid crystal. The Stanton tube is a rectangular shaped pitot tube located very close to the boundary wall and the mean velocity measured from this pitot tube pressure difference is directly related to the shear stress. The Preston tube is similar to the concept of the Stanton tube using a pitot static tube close to the surface and the difference between the stagnation pressure at the center of the tube from the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. A detailed discussion on the above three techniques can be found in Hanratty and Campbell [1]. These shear stress measurement techniques are not ideal MEMS-based techniques. 

For oil-film interferometry, it is assumed that the oil film is so thin that it does not influence the flow above it and is driven by the skin friction distribution of the flow. Using a control volume analysis of the thin oil film with its height... 

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