Permittivity-based void fraction sensing for microfluidics

The design, fabrication and integration of capacitive void fraction sensors for microfluidic applications in silicon microchannels of 100 μm wide and 500 μm deep are described. Simulation data are presented and discussed and the most critical microfabrication steps involving the drilling of glass dice and the anodic bonding of glass to silicon via an intermediate sputtered Pyrex layer are elucidated.A read-out circuit was designed to convert capacitive input signals into a large-swing analog output signal. The signal-processing algorithm is explained. Measurements showing the variation of the output signal as a function of the gas content in the air/water-mixture, pumped through the microchannels, are presented.

► Simulations show fairly linear relationship between capacitance and void fraction.
► Anodic bonding of glass to silicon with intermediate Pyrex layer.
► Read-out circuit converts changing input capacitance to large-swing output voltage.
► Measurement results very closely approximate simulations.