Capillary driven movement of gas bubbles in tapered structures

This article presents a study on the capillary driven movement of gas bubbles confined in tapered channel configurations. These configurations can be used to transport growing gas bubbles in micro fluidic systems in a passive way, i.e. without external actuation. A typical application is the passive degassing of CO2 in micro direct methanol fuel cells (mu DMFC). Here, a one-dimensional model for the bubble movement in wide tapered channels is derived and calibrated by experimental observations. The movement of gas bubbles is modelled on straight trajectories based on a balance of forces. The bubble geometry is considered as three dimensional. In the development of the model, the effects of surface tension, inertia, viscosity, dynamic contact angle and thin film deposition are considered. It is found that in addition to viscous dissipation, the dynamics related to the contact line-dynamic contact angle and thin film deposition-are essential to describe the gas bubble's movement. Nevertheless, it was also found that both of these effects, as modelled within this work, have similar impact and are hard to distinguish. The model was calibrated against experiments in a parameter range relevant for the application of travelling gas bubbles in passive degassing structures for mu DMFCs.