Effects of spatial gradients of electrical conductivity on chip-based sample injection processes

The precise control of the loaded and dispensed sample amount in crossing microchannels is key to the performance of many lab-on-a-chip devices. The fundamental understanding of the complicated electrokinetic phenomena in such crossing type microchannels therefore is necessary. In the literature, a few theoretical models studying the transport phenomena in similar crossing microchannels did not consider the spatial gradients of conductivity due to its complexity. However, the spatial gradients of electrical conductivity of buffer solution often exist in many applications (i.e. pumping a large number of unknown species and stacking the interested sample species) and it was demonstrated that the presence of the conductivity gradients has significant influences on the on-chip microfluidic injection processes. A new theoretical model was developed in this paper to study the transport phenomena in crossing microchannels with the consideration of a large range of spatial gradient of electric conductivity. This developed model was used to simulate the potential field, flow field, and concentration field of the injection processes where the conductivity of the sample-carrying buffer differs significantly from that of the driving buffer. The transport phenomena are found to be very sensitive to the conductivity difference between the sample-carrying buffer and the driving buffer. The developed model here can be employed to find the optimal voltages for controlling the dispensed sample size and to provide guidance for designing such a crossing microchannel in lab-on-a-chip devices.