Dynamics of bubble formation in spontaneous microfluidic devices: Controlling dynamic adsorption via liquid phase properties.

Hypothesis: The interplay of interface evolution and surfactant adsorption determines the formation and stabilization of bubbles, and can be controlled by the liquid phase properties.

Experiments: We studied bubble formation in an Edge-based Droplet GEneration (EDGE) microfluidic device at relevant length and time scale, allowing investigation of sub-events in a single bubble formation cycle. We vary the properties of the continuous phase that contains whey proteins and study a range of trans-pore pressures (P).

Findings: The shallow pores highlight the crucial role of the Laplace pressure and dynamic adsorption of proteins to the meniscus. Bubble formation is divided into two regimes by the Laplace pressure of the bare meniscus inside the pore. At P<1400 mbar, pre-adsorption of proteins is required to lower the Laplace pressure; the bubble formation frequency f increases with increasing protein concentration and is hardly affected by velocity and viscosity. At P≥1400 mbar, bubble formation immediately occurs upon applying pressure, and f mainly decreases with increasing viscosity. In both regimes, the initial bubble size d mainly increases with the viscosity (~η). Bubble coalescence is only observed at P≥1400 mbar and can be effectively suppressed by raising protein concentration and viscosity within certain boundaries, yet ultimately this is at the cost of higher polydispersity of the bubbles. Our insights into the formation dynamics of micrometer-sized bubbles at time scales down to tens of microseconds can be used for effective control of bubble formation and stabilization in practical applications.