Investigation of electrocoalescence and non-coalescence behavior between unequal-sized droplets

Droplet manipulation is required to operate and control the droplet microfluidic devices as desired way like merging, mixing, and sorting. Electric force is one of the useful ways to merge the droplets, and it can be achieved by electrocoalescence. When the aqueous droplets exist in non-conductive medium under electric field, attractive force occurred, and it makes droplets merging. However, when strength of electric field becomes stronger than certain value, droplets do not merge even though they contact to each other, and it called as 'non-coalescence'. Previous studies suggested conditions to occur the non-coalescence such as strength of electric field, conductivity and so on. However, they used equal-sized droplets and it's still not clear when unequal-sized droplets show non-coalescence and why it shows different non-coalescence behavior compared to equal-sized droplets. In this study, we investigated the electrocoalescence of unequal-sized droplets. We found the unequal-sized droplets shows non-coalescence at lower electric field compared to equal-sized droplets. Also, we found that it is due to the different cone angle formation of unequal sized droplets, compared to that of equal-sized droplets. We believe these results can contribute to suggest an optimized electric field range for electrocoalescence in various situations.