(552a) Ehd Tip Streaming: Size and Charge of Electrospray Droplets

When subjected to strong electric fields, pendant (sessile) drops, free drops, and thin films form conical tip structures and emit thin jets, i.e. tip stream, from these conical points that subsequently break into small droplets.  This explosion of liquid drops and films subjected to an electric field into a spray consisting of charged droplets is common to mass spectrometry, printing and coating processes, and raindrops in thunderclouds.  Despite being one of the oldest and most celebrated problems in science, there exist conflicting theories and measurements on the size and charge of these small electrospray droplets.  While simulation can provide definitive answers to such questions, theoretical analysis of the temporal development of  electrohydrodynamic (EHD) tip streaming phenomena has heretofore been elusive given the large disparity in length scales between the macroscopic drops/films and the microscopic (nanoscopic) jets.  Here, simulation is used to investigate EHD tip streaming from drops and films of finite conductivity.  In the simulations, the full Taylor-Melcher leaky-dielectric model, which accounts for charge relaxation, is solved to probe the mechanisms of cone formation, jet emission, and breakup of the jet into small drops.  First, simulations show that tip streaming does not occur if the liquid is perfectly conducting or perfectly insulating, in accord with theory.   Simulation results are used in concert with theory to develop scaling laws for the sizes of and charges carried by the micro (nano) scale drops that produced from the breakup of the thin jets.  The computations at last reveal unequivocally whether electrospray droplets are Coulombically stable or unstable at the instant that they are created.