Quantifying flow through thin and flexible electroosmotic micropumps with droplet shape analysis*

Electroosmotic (EO) micropumps transport fluid without any moving parts. The ease of miniaturizing these pumps into thin flexible devices enables novel applications in implantable drug delivery and lab-on-a-chip systems. However, it also complicates conventional methods of measuring pump performance. It is not trivial to attach fluidic interconnects to these microscale devices for mass flow measurements. Also, these pumps may be optically incompatible with fluorescent particle tracking. In this study, we introduce a connected droplets technique to measure pump performance by quantifying pump parameters such as zeta potential, velocity, and flow rate. We fabricated a thin biocompatible EO pump consisting of an epoxy SU-8 passive layer sandwiched between two polymer electrodes. The pumping action takes place through a microchannel passing through the multilayer stack. We placed droplets on either side of the microchannel and observed the evolving droplet shapes as the pump drove fluid from one droplet to the other. By comparing these observations to theory and multi-physics simulations, we deduced the key parameters describing the flow. The advantages of this technique include simplicity, low cost, and measurement of zeta potential without specialized instruments.