The temperature dependence of fluid viscosity in Poiseuille flow through microchannels was examined. Results are presented in the form of the Poiseuille number, a dimensionless parameter equal to the product of the Fanning friction factor and the Reynolds number. Experimental data on 1-propanol, 2-propanol, 1-pentanol, 3-pentanol, and water flows through triangular and trapezoidal microchannels with hydraulic diameters of 25, 12, and 5 {dollar}mu{dollar}m indicate a significant temperature dependence for the Poiseuille number. Specifically, the Poiseuille number increases with temperature. Furthermore, this dependence appears to be stronger for the smaller channels. These results are contrary to theoretical expectations based on the Navier-Stokes equation, which shows that the Poiseuille number is a constant independent of temperature, pressure, channel size, fluid velocity, and fluid material properties. The Poiseuille number is solely a function of the channel cross-sectional shape.; The temperature dependent Poiseuille number suggests that the liquid viscosity within the microchannels decreases more slowly with increasing temperature than the bulk, macro viscosity. A detailed uncertainty analysis sets the precision of the measurements in the range of 2% or better, so that the observed deviations from theory are not readily explained by experimental errors. No theory has been developed to account for these effects.
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