Temperature dependence of inertial pumping in microchannels

Inertial pumping is a promising new method of moving fluids through microchannels but many of its properties remain unexplored. In this work, inertial flow rates are investigated for different channel lengths, operating temperatures, and resistor pulse energies. Flow in closed channels is visualized by adding fluorescent tracer beads to the test fluid (pure water). A robust methodology of extracting flow rates from high-resolution video recordings is developed. Flow rates are found to scale inversely with the channel length. The observed dependence is explained based on a simple phenomenological "kick" model of inertial pumping. Flow rates are also fitted to the more fundamental one-dimensional model of inertial pumping from which the intrinsic drive bubble strength is extracted. The measured flow rates vary strongly with temperature. For well-developed drive bubbles, flow rates at T = 70C are about 12x higher than at T = 30C. Three separate effects contribute to increasing flow rates at high temperatures: (i) lower viscosity of the test fluid, (ii) a stronger drive bubble, and (iii) increasing mechanical efficiency of the pump, i.e., better conversion of the drive bubble strength to unidirectional post-collapse kick. Relative contributions of the three effects are quantified. The energy dependence of flow rates exhibits a clear saturation behavior. The bubble strength is fitted to a phenomenological saturation model. In the end, a complete predictive length-temperature-energy model of flow rates is constructed. The observed strong temperature dependence of inertial pumping should be considered when designing microfluidic workflows. It also highlights the need for integrated flowmeters that could stabilize complex flow patterns via sensory feedback.