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We study a liquid jet that breaks up into drops in an external co-flowingliquid inside a confining microfluidic geometry. The jet breakup can occurright after the nozzle in a phenomenon named dripping or through the generationof a liquid jet that breaks up a long distance from the nozzle, which is calledjetting. Traditionally, these two regimes have been considered to reflect theexistence of two kinds of spatiotemporal instabilities of a fluid jet, thedripping regime corresponding to an absolutely unstable jet and the jettingregime to a convectively unstable jet. Here, we present quantitativemeasurements of the dripping and jetting regimes, both in an unforced and aforced state, and compare these measurements with recent theoretical studies ofspatiotemporal instability of a confined liquid jet in a co-flowing liquid. Inthe unforced state, the frequency of oscillation and breakup of the liquid jetis measured and compared to the theoretical predictions. The dominant frequencyof the jet oscillations as a function of the inner flow rate agreesqualitatively with the theoretical predictions in the jetting regime but not inthe dripping regime. In the forced state, achieved with periodic laser heating,the dripping regime is found to be insensitive to the perturbation and thefrequency of drop formation remains unaltered. The jetting regime, on thecontrary, amplifies the externally imposed frequency, which translates in theformation of drops at the frequency imposed by the external forcing. Inconclusion, the dripping and jetting regimes are found to exhibit the mainfeatures of absolutely and convectively unstable flows respectively, but thefrequency selection in the dripping regime is not ruled by the absolutefrequency predicted by the stability analysis.

Quantitative analysis of the dripping and jetting regimes in co-flowing capillary jets