Unidirectional bubble growth in microchannels with asymmetric surface features

The growth of vapor bubbles is studied numerically in a microchannel with asymmetric surface features. The channel design is chosen such that evaporation results in vapor bubbles growing only along a predefined direction. The principle relies on capillary forces and the pinning/depinning of three-phase contact lines at sharp edges of the wall geometry. Analytical expressions are derived predicting the direction of bubble growth and allowing to assess the robustness of a specific channel geometry in terms of supporting unidirectional bubble growth. From these expressions design rules for microchannels incorporating geometrical parameters and the wall contact angle of the liquid phase can be derived. The numerical calculations are performed based on an extended Volume-of-Fluid method accounting for phase change. The results confirm that under specific conditions, vapor bubbles only expand in one direction, thereby corroborating the analytical model. The presented concept may find applications in designing microchannels for stabilized flow boiling or micropumps/-actuators relying on phase change.