Coexistence of different droplet generating instabilities: new breakup regimes of a liquid filament

Abstract

The coexistence of multiple droplet breakup instabilities in a Step-emulsification geometry is studied. A liquid filament, which is confined in one dimension by channel walls and surrounded by a co-flowing immiscible continuous phase, decays into droplets when subject to a sudden release of confinement. Depending on the filament aspect ratio and liquid flow rates, an unexpectedly rich variety of droplet breakup regimes is found. All of these breakup regimes are composed of two basic instabilities, i.e. a step- and a jet-instability, that coexist in various combinations on the same filament. Surprisingly, even an asymmetric breakup regime is found, producing droplet families of significantly different diameters, while the filament is subject to a fully symmetric flow field. We suggest key physical principles explaining the spontaneous symmetry breaking and the transitions between individual droplet breakup regimes. The particular ability to produce distinct droplet families from a single filament is demonstrated to allow for simultaneous concentration and encapsulation of particles into one droplet family while excess bulk liquid is released into another family of droplets.

On capillary self-focusing in a microfluidic system

“…We attribute the double jet-breakup to the strong confinement. The transition from double to single jet-breakup is discussed in details in (Hein et al 2015b). Here we also numerically show the existence of these breakup regimes.…”

“…We also fix b(x, y) = b. Experimental results in (Priest et al 2006) and (Hein et al 2015b) show that the transition of jet-to step-breakup regime is independent of the viscosity ratio when the results are presented in terms of the rescaled filament width w 1 (0)/b as a function of Ca. We therefore do not consider unmatched viscosities in this work.…”

A passive microfluidic system based on step emulsification allows the generation of libraries of nanoliter-sized droplets from microliter droplets of varying and known concentrations of a sample

“…15 Above a certain critical value of the capillary number, the step emulsification regime turns into a jetting regime, and the emulsification no longer occurs. 16 Because of that, MSE suffers from low frequencies of generation of droplets when compared to flow focusing. However, MSE can be readily parallelized for an improved frequency of generation of droplets.…”

“…[17][18][19] Step emulsification techniques have recently gained significant attention with important new developments. [14][15][16][17][18][19][20][21][22][23] Emulsions generated at the step are more tightly monodisperse than emulsions obtained in T-junction or flow focusing systems due to almost the same physical conditions at the step for each generated droplet. 12 However, if the step is blocked by the emulsion, the generated droplets begin to exhibit higher variation of sizes.…”

Fragmentation mechanisms of confined co-flowing capillary threads revealed by active flow focusing

“…Discrete droplets indeed represent individual digits for (bio-)chemical analyses [10] as well as material synthesis [11], motivating the investigation on techniques for dispensing calibrated droplets at high throughput [12]. The main approaches explored to this end relied on the geometrical properties of the channel, for example by focusing a stream of fluid to be dispersed into another immiscible fluid with a lateral constriction [13,14], or by abruptly releasing a thread confined vertically when reaching a thicker terrace [15][16][17]. Many active approaches were also prompted to actuate the detachment of individual droplets from a fluid reservoir [18][19][20][21][22][23], such as electric-field induced ejection of droplets from a Taylor cone [24], or a mechanical stimulation of a liquid thread by a transducer [25,26].…”