microflows and microscale heat transfer laboratory

Abstract 

Two microfluidic transistors for analog flow con- trol and amplification for lab-on-a-chip applications are presented. The transistors are based on the elastic mem- brane microchannel, where the flow in the microchannel between the substrate and the membrane is controlled by the pressure differences along the channel and across the membrane. Reduced-order models that capture the low- inertia dynamic behavior of the coupled fluid–structure interaction were developed to enable fast small-signal analysis of large circuits. The accuracy of the models is assessed by comparing to numerical simulations of the coupled fluid–structure interaction problem. Analog behavior (based on analytical modeling and numerical simulation) of the two devices is characterized in terms of dependence of the volume flow rate on the source–drain and gate–source pressure differences, analogous to the characterization of MOSFET operation. The characteristic curves are then used to extract the small-signal parameters (transconduct- ance and intrinsic output resistance), characterizing the dynamic response to small time-varying pressures at the gate and/or drain. The characterization enabled identifica- tion of the various static and dynamic operation regimes of the devices, including the transistive regime where the device operates as amplifier, and the capacitive (positive and negative) regimes. Finally, the dual-membrane transis- tor is used to showcase its use as a diode and a common- source amplifier in the design of a micropump that, in turn, is used for mixing of two species using pulsating flows.

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Conclusion 

Two microfluidic transistors that employ the elastic mem- brane microchannel channel are presented.

Reduced-order models that capture the low-inertia dynamic behavior of the coupled fluid–solid problem were developed to enable fast small-signal analysis of large cir- cuits. The models, when compared to detailed numerical simulations of the coupled fluid–structure interaction prob- lem over a wide range of operating pressures, proved to be most accurate when the inertia of the flow in the channel is small.

Characterization of the analog behavior of the two devices is expressed in terms of dependence of the volume flow rate versus the source–drain and gate–source pressure differences, analogous to the characteristic curves of MOS- FET operation. The characteristic curves are then used to extract the small-signal parameters (transconductance and intrinsic output resistance), characterizing the dynamic response to small time-varying pressures at the gate and/ or drain.

For the single-membrane device, the characterization revealed that, depending on these operating pressure differ- ences, the device operates as a capacitor when the mem- brane is deflected away from the channel and as a transis- tor when it is deflected into the channel. When the flow inertia is sufficiently high, a negative capacitive behavior is observed at high gate pressures. In the transistor regime of operation, the device is capable of providing large gains. Due to the narrowness of the saturation regime, the linear- ity is poor and distortion is high.

The dual-membrane transistor proposed is shown to improve linearity and reduce distortion by expanding the saturation zone. In addition to its operation as a transistor, the dual-membrane design is also shown to operate as a diode and a shutdown safety switch.

Finally, the dual-membrane transistor is used to drive a micropump equipped with two diodes. The micropump is then employed in a larger circuit that performs mixing of two species using pulsating flows.