Pressure-driven deformation with soft polydimethylsiloxane (PDMS) by a regular syringe pump: challenge to the classical fluid dynamics by comparison of experimental and theoretical results

Abstract

Pressure-driven flows through optically transparent polydimethylsiloxane (PDMS) microfluidic channels using a syringe pump are common in many experimental applications. However, because of small changes, the pressure-induced deformations due to soft materials and shapes and patterns in the microchannels, which generate untargeted flow velocity and pressure drop, have been easily ignored. In this paper, we present a simple experimental investigation with circular-shaped, periodically arranged objects of three different characteristic dimensions, as well as various channel heights ranging from 15 μm to 200 μm. 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. Channel deformation, measured by fluorescence intensity difference, was severe in shallow microchannels that had characteristically large obstacles. In addition, PDMS microchannel bulging effects in shallow microchannels exerted a greater deleterious effect on (ΔP/Δx)_QS values than upon V_QS for the experiments using a volumetrically controlled syringe pump. The discrepancy between experimental and theoretical (ΔP/Δx) increased with decreasing microchannel height. This means that the severe PDMS bulging observed in shallow microchannels using a syringe pump can generate large deviations from the desired experimental result. As a result, the slope of Darcy friction factors plotted against the Reynolds number indicates that the slope was strongly dependent on microfluidic channel height and the degree of PDMS bulging. The microfluidic channel with the greatest height exhibited the smallest bulging effect and was shown to be in relatively good agreement with predicted values, while the slope became much steeper than −1 as channel heights decreased.