Anomalous transition to turbulence in microtubes

As the microfluidics field expands, required flowrates in microdevices are expected to span a large range of Reynolds numbers (Re), and the prediction of flow regime, namely laminar versus turbulent, is highly relevant. Recent measurements have been inconclusive in answering a fundamental question: Does microscale flow behave differently than macroscale flow? Previous measurements have suggested that the transition to turbulence occurs at Re much lower than 2000, the generally accepted lower limit of macroscale transition to turbulence. The current experiments use both bulk flow resistance measurements and micro-Particle Image Velocimetry (micro-PIV) results to show that, for Re < 1100-1500 and microchannel diameters 75 to 250 μm, the velocity profiles and flow resistance are well-predicted by macroscale laminar flow theory. For Re > 1100-1500, an initial departure from laminar behavior is noted both from the flow resistance and the micro-PIV experiments. Thus, some "microscale" effects are observed, though they are not as dramatic as those observed in previous studies. A brief literature review of transitional macroscale pipe flow is presented, and potential explanations are proposed for the possible "micro-scale" effects observed in these experiments.