Optical Shear Stress Sensors

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

The local height of the oil film can be obtained from the fringe patterns. The height of the oil film at kth black fringe is given by 

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 (12.17), the skin friction can be determined. The benefit of oil-film interferometry is that no calibration is required for the measurement of shear stress and the basic analytical expression is used. It can be used for 3-D flow situation and provides both magnitude and direction of shear stress. The disadvantage is that it does not possess any temporal resolution and therefore cannot be used for fluctuating skin friction measurement. 

Liquid crystal is a phase of matter that exists between the liquid and solid phase. It exhibits optical properties similar to the solid crystalline material. The molecular arrangement of liquid crystal is a function of either temperature or shear stress. When the molecular arrangement is sensitive to temperature, the liquid crystal coating can be used for temperature measurement, and when the molecular arrangement is sensitive to shear stress, the liquid crystal can be used for shear stress measurement. 

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Mechanical nanosensors possess comparative advantages over optical nanosensors and electromagnetic nanosensors for the measurement of nanoscale mechanical properties [2]. Examples of mechanical nanosensors include CNT-based fluidic shear-stress sensors [3] and the nanomechanical cantilever sensors [4]. 

Padmanabhan A, et al (1996) A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes. J Microelectromech Syst 5(4) 307-315 Jiang F, Tai Y-C, Huang J-B, Ho C-M (1995) Polysilicon structures for shear stress sensors. In TENCON 95. IEEE Region 10 International Conference on Microelectronics and VLSI Qiao Lin YX, Tai Y-C, Ho C-M (2005) A parametrized three dimensional model for MEMS thermal shear stress sensors. J Microelectromech Syst 14(3) 625-633 Rouhanizadeh M, et al (2006) MEMS sensors to resolve spatial variations in shear stress in a 3D blood vessel bifurcation model. IEEE Sens J6(l) 78-88... 

Slit rheometers are more difficult to build and use but are preferred for research studies, because the flat flow channel makes it possible to mount pressure sensors and to make optical measurements. It has been proposed that measurement of the exit pressure or hole pressure [129] might be used to infer the first normal stress difference using a slit rheometer [9,p. 309], but these approaches have been little used because of the difficulty of measuring the small pressures or pressure differences involved. As in the case of capillary rheometers, there are established methods for calculating the true wall shear stress and shear rate from experimental slit data [9, 81]. 

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