Characterization of fluid dynamic behaviour and channel wall effects in microtube

Sometimes contradictory results available for fluid flow in micropipes show that much is yet to be verified in microfluid dynamics. In this study the influence of channel wall roughness and of channel wall hydrophobicity on adiabatic flow in circular microchannels is investigated, varying in diameter from 70 mu m to 326 mu m. The hydrodynamic behaviour of water in smooth tubes down to 30 mu m inner diameter (ID) is also ascertained. Within the current experimental accuracy it is found that the classical Hagen-Poiseuille law for friction factor vs. Reynolds number is respected for all diameters measured and Re>300. With degassed water, no effect of slip flow due to hydrophobic channel walls was noted even at 70 mu m ID, which might suggest that the liquid slip flow phenomenon is associated with local desorption of dissolved gases on the hydrophobic surface, as reported elsewhere in the literature. For roughened glass channels, an increase in Darcy friction factor above 64/Re was observed only at the smallest diameter measured, 126 mu m. Although the roughness levels of these channels were up to 10 times coarser than the untreated, smooth glass tubes, probably the higher factor was caused by actual deformation of channel circularity, rather than increased friction at the rougher wall, as similar behaviour was observed in a Teflon tube, also of imperfect circularity of cross-section. For all experiments, no anticipated transition to turbulent flow was observed, which means that the transitional Reynolds number was always found between Re approximate to 2000 and Re approximate to 3000. Finally, an introduction to the importance of viscous dissipation in microchannels is added, with quantitative indications of its influence on hydrodynamic properties. It is put forward as being an alternative to pressure measurements in the characterization of the behaviour of microscopic flow. (C) 2005 Elsevier Inc. All rights reserved.