Benchtop investigation of bubble nucleation in a pulse-driven microfluidic device

We have developed featherweight, insect-mimetic microfluidic infusion pumps driven by periodic pressure pulses. However, device performance has been challenged by bubble formation, which increases hydraulic resistance and induces variability in the flow rate. Here, we present a benchtop investigation of bubble nucleation in these devices. Three-layer devices consisting of a pump (flow) channel layer, an elastomeric membrane, and a pressurized actuation layer were fabricated using soft lithography. Each flow channel was connected to 100-mm-long, 0.3-mm-diameter inlet and outlet tubes and filled with water. Flow was actuated using a pressurized air pulse simulator that cyclically collapsed and re-expanded the elastomeric membrane using a square wave switching between vacuum and pressure, mimicking insect respiratory pumping mechanisms. Devices were tested across three actuation frequencies and four pressure levels. Video tracking software (Tracker) was used to analyze the fluid-air interface dynamics before and after bubble formation.

In multiple devices, activation of the vacuum immediately triggered bubble nucleation in the inlet and outlet tubes. This is likely due to abrupt pressure drops in the flow channel, potentially initiating cavitation or dissolved gas release. The square pressure waveform likely imposes steep pressure gradients that favor bubble growth. We also observed differences in bubble formation behavior across pressure-frequency combinations, which we modeled using system-level equivalent circuit theory to better understand the transient fluid response. To mitigate bubble formation, we replaced the square waveform with a sinusoidal input to assess whether smoother pressure gradients reduce nucleation. These results highlight the importance of actuation waveform design in minimizing two-phase flow disruptions in membrane PDMS microfluidic systems.