Whipping of electrified liquid jets

Abstract

We apply an electric field to a moderately conducting liquid surrounded by another coflowing liquid, all inside a glass-based microfluidic device, to study nonaxisymmetric instabilities. We find that the bending of the electrified jet results in a steady-state, helicoidal structure with a constant opening angle. Remarkably, the characteristic phase speed of the helicoidal wave only depends on the charge carried by the jet in the helicoidal region and its stability critically depends on the properties of the coflowing liquid. In fact, the steady-state helical structure becomes chaotic when the longest characteristic time is that of the inner liquid rather than that of the outer coflowing liquid. We also perform a numerical analysis to show that the natural preference of the jet is to adopt the conical helix structure observed experimentally.instability | charged jets | electro-coflow | electrospinning A liquid with finite electrical conductivity in the presence of a strong electric field can deform and adopt a conical shape resulting from the balance between electric and surface tension stresses (1). However, near the apex of the cone, this structure is not stable and the associated singularity is replaced by a thin jet (2-6). The resultant cone-jet structure, which is stable within certain values of the applied voltage and imposed liquid flow rate, is the workhorse of electrospray and all its associated applications (7-10).The jet that emanates from this structure always breaks into spherical droplets due to axisymmetric instabilities (11-13). However, in many cases, the jet bends off-axis due to a lateral instability that results from the electrostatic repulsion between bent and straight parts of the jet (14)(15)(16)(17)(18)(19). If the growth rate associated to this whipping instability is larger than that associated to jet breakup, the off-axis movement of the jet becomes the most significant aspect of its evolution. This is exploited in electrospinning, where the simple liquid is replaced by a polymer solution whose solvent evaporates before drop breakup takes place, thus resulting in the formation of polymer fibers (14,17,18,20). The presence of the lateral instability in this application results in thinner fibers, as bending stretches, concomittantly thinning the jet (20). Unfortunately, in most experimental realizations, whipping manifests in a chaotic fashion (15,16,18,19,21,22) preventing us from knowing and unraveling its detailed structure and properties.In this work, we apply an electric field to a moderately conducting liquid surrounded by a coflowing liquid to generate a steady-state whipping structure, which we find is helicoidal, with an amplitude that increases linearly along the downstream direction. Interestingly, the characteristic phase speed of the helical wave is determined by the electrostatic repulsion between the fluid elements of the jet in the whipping region. By performing a numerical analysis, we show that the conical helix structure is the natural configuration of electrified jets,...

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