(333b) Quantification And Control Of Passive Flows To Minimize Sample Losses At Channel Junctions

In the absence of valves, sample material in microfluidic channels can be lost in channel junctions. These losses are strongly affected by ?passive? flows that result from an imbalance in the Laplace pressure due to liquid surface curvature in the sample reservoirs, or wells, and hydrostatic pressure resulting from differences in fluid level between wells. We present a systematic study of the behavior of these passive flows using a model microfluidic system and then demonstrate a control strategy to minimize their impact on sample loss. Based on direct observations of liquid menisci in the wells, we found that the surface curvature of the liquid in the well was strongly affected by the contact angle hysteresis and whether or not an electrode pin was inserted. Flow measurements indicated that the passive flow has a time-dependent behavior that may be of importance in microfluidic chip operation. The passive flow effects can be controlled using a pressure compensation strategy that balances the hydrostatic and Laplace pressures. We demonstrate this concept by controlling the passive flow at a channel junction in an isotachophoresis (ITP) chip. The loss of the stacked sample band can be significantly reduced when the passive flow is largely suppressed or directed out of the channel junction and is quantified by computer simulation.