Effect of the mixing region geometry and collector distance on microbubble formation in a microfluidic device coupled with ac-dc electric fields

In this work, we report a significant advance in the preparation of monodisperse microbubbles using a combination of microfluidic and electric field technologies. Microbubbles have been employed in various fields such as biomedical engineering, water purification, and food engineering. Many techniques have been investigated for their preparation. Of these, the microfluidic T-junction has shown great potential because of the high degree of control it has over processing parameters and the ability to produce monodisperse microbubbles. Two main lines of investigation were conducted in this work - the effect of varying the mixing region distance (M_x) and the influence of altering the tip-to-collector distance (D_x) when an ac-dc field is applied. It was found that when M_x was decreased from 200 to 100 μm, the microbubble diameter also decreased from 128 ± 3 to 88 ± 5 μm due to an increase in shear stress as a result of a reduction in surface area. Similarly, decreasing the tip-to-collector distance results in an increase in the electric field strength experienced at the nozzle, facilitating further reduction of the bubble diameter from 111 ± 1 to 86 ± 1 μm at an ac voltage of 6 kV P-P and an applied dc voltage of 6 kV. Experiments conducted with the optimal parameters identified from these previous experiments enabled further reduction of the microbubble diameter to 18 ± 2 μm. These results suggest that a unique combination of parameters can be employed to achieve particular microbubble diameters to suit various applications.