Tip-multi-breaking in capillary microfluidic devices

We report tip-multi-breaking (TMB) mode of droplet breakup in capillary microfluidic devices. This new mode appears in a region embraced by Ca _i = 0 and lg(Ca _i)= 8.371(Ca ₀)7.36 with Ca₀varying from 0.35 to 0.63 on the Ca _i-Ca ₀ phase diagram, Ca_i and Ca₀being the capillary numbers of inner and outer fluids, respectively. The mode is featured with a periodic, constant-speed thinning of the inner liquid tip and periodic formation of a sequence of droplets. The droplet number n in a sequence is determined by and increases with outer phase capillary number, and varies from two to over ten. The distribution of both pinch-off time and size of the droplets in a sequence is a geometric progression of common ratio that depends exclusively on and increases monotonically with the droplet number from its minimum value of 0.5 at n=2 to its maximum value of 1 as n tends to infinity. These features can help identify the unique geometric morphology of droplet clusters and make them promising candidates for encryption and anti-fake identification.