| For decades the fluid dynamic shear stresses associated with the flow of blood has been a topic of great interest in bioengineering. Use of a shear sensitive liquid crystal in air flow has been documented in the literature, but has not been used in liquids due to erosion of the coating. To enable the visualization of shear stress in flowing blood and other liquids, a liquid crystal and silicone rubber mixture was formulated that adheres to a surface and exhibits shear stress dependent photochromic changes.; This coating was placed within a parallel plate flow chamber and concentric Taylor cylinders with glycerol as a test fluid. A series of individually fabricated coatings were calibrated and evaluated for range (0–1500 dynes/cm 2), temporal resolution (50–170 ms), sensitivity resolution (3 dynes/cm2 in 0–120 dynes/cm2 range, 15 dynes/cm2 at high range), repeatability (96%), hysteresis (2%–4%), accuracy (96% in low range, 86% in high range), and variation between sensors (4%–17%). Since the sensor is partially translucent, the color output of the sensor was evaluated with the addition of an opaque background fluid color and blood. This evaluation led to a color combination prediction model based on the geometry of the hue circle, the known background fluid color, and the known reaction of the sensor to stimulus. Furthermore, the sensor was evaluated on a curved surface to study effects from the lighting and viewing angles, and led to a curved surface correction model. Finally, the sensor was applied to a diverging flow field and a narrow orifice to ascertain the direction-differentiation capabilities of the sensor. The results of these experiments show that a liquid crystal based shear stress sensor is a feasible means for obtaining spatial distribution and vector direction of biologically relevant shear stress levels. |
