PDMS Deformation Effects during Start-up Flow in Microchannels of Different Depths and Containing Various Obstacles

Abstract

New experimental data for the microfluidic flow of a water-based liquid at very low Reynolds numbers (10-4 to 10-3) are provided for an optically transparent polydimethylsiloxane (PDMS) microfluidic chip with rectangular flow channels obstructed by periodically arranged objects of varying shapes and sizes. Four obstacle shapes of circular, hexagonal, square and triangular contours with three characteristic dimensions of 132μm, 152μm and 172μm were fabricated within microchannels 243μm wide and depths of 15, 50, 100 and 200μm. The flow velocity and applied pressure during the start-up flow of a water-based, colored liquid solution were directly measured by optical microscopy and a pressure transducer. The resultant CCD images and pressure data were used to determine the extent of the start-up transients and evaluate the establishment of quasi-steady flow conditions. PDMS bulging was observed by fluorescence intensity differences at low flow rates. Microchannel bulging effects were found to be strongly affected by the pressure drop while being less affected by the flow velocity. As a result, the measured pressure drop increased less than 60% when the flow rate was doubled. The bulging modified mean hydraulic diameter, mean quasi-steady state flow velocity and pressure drop along the microfluidic channels were used to evaluate the Reynolds numbers and Darcy friction factors. The experimentally determined relationship between friction factor and Reynolds number indicated a much stronger dependence on Reynolds number than expected for the 15μm deep microchannels, even though the friction factor data from the current investigation were found to agree with literature data. Minimal elastic deformation of PDMS microfluidic channels was observed in the deepest channels and, the relationship between friction factor and Reynolds number agreed more closely with theoretical expectations for the deepest microchannels. Experimental measurements of bulging displacements of the flexible PDMS microchannels using fluorescence microscopy provided a cost effective alternative method to confocal microscopy. Measurements of bulging agreed with theoretical predictions using both the simplified scaling analysis of Gervais et al. (2006) as well as 2D finite element simulations.