Droplet charging regimes for ultrasonic atomization of a liquid electrolyte in an external electric field

Abstract

Distinct regimes of droplet charging, determined by the dominant charge transport process, are identified for an ultrasonic droplet ejector using electrohydrodynamic computational simulations, a fundamental scale analysis, and experimental measurements. The regimes of droplet charging are determined by the relative magnitudes of the dimensionless Strouhal and electric Reynolds numbers, which are a function of the process ͑pressure forcing͒, advection, and charge relaxation time scales for charge transport. Optimal ͑net maximum͒ droplet charging has been identified to exist for conditions in which the electric Reynolds number is of the order of the inverse Strouhal number, i.e., the charge relaxation time is on the order of the pressure forcing ͑droplet formation͒ time scale. The conditions necessary for optimal droplet charging have been identified as a function of the dimensionless Debye number ͑i.e., liquid conductivity͒, external electric field ͑magnitude and duration͒, and atomization drive signal ͑frequency and amplitude͒. The specific regime of droplet charging also determines the functional relationship between droplet charge and charging electric field strength. The commonly expected linear relationship between droplet charge and external electric field strength is only found when either the inverse of the Strouhal number is less than the electric Reynolds number, i.e., the charge relaxation is slower than both the advection and external pressure forcing, or in the electrostatic limit, i.e., when charge relaxation is much faster than all other processes. The analysis provides a basic understanding of the dominant physics of droplet charging with implications to many important applications, such as electrospray mass spectrometry, ink jet printing, and drop-on-demand manufacturing.

Desorption electro-flow focusing ionization of explosives and narcotics for ambient pressure mass spectrometry

“…For the solvents and electric eld strengths considered here, the electric timescales, i.e., charge relaxation and ion transit, were sufficiently shorter than the hydrodynamic timescales, leading to a quasi-electrostatic limit of the electrohydrodynamics and direct correlation between electric eld and droplet charge. [49][50][51] The signal saturation relationship experienced at elevated potentials has also been demonstrated with emitted current measurements and found to be a result of droplet discharging effects at the DEFFI orice for high applied potentials, as the electrospraying regime was approached. 23,47 The maximum achievable emitted current due to orice discharging effects corresponded directly with the local maxima cocaine signal intensity (Fig.…”

Rapid detection and isotopic measurement of discrete inorganic samples using acoustically actuated droplet ejection and extractive electrospray ionization mass spectrometry

“…For these conditions, excitation signal amplitudes below 10 V pp did not yield the necessary liquid velocity in the nozzle orifice to achieve ejection. The positive polarity of the measured current was due to net positive charge transport through advection and migration of hydrogen ions in an alternating current electric field induced by the signal actuating the piezoelectric transducers . Figure (b) also displays the ejected droplet stream below the ultrasonic nebulizer for a range of signal amplitudes.…”

“…These systems readily atomize liquid samples for further droplet desolvation and introduction of analyte into the gas phase, and require little to no sample preparation. They also induce droplet charging and analyte ionization through inherent electric fields generated by the piezoelectric element actuation, by the application of external electric fields, or through coupling techniques such as extractive electrospray ionization (EESI) . EESI enables the ionization of neutral analytes from the gas phase, liquid phase, or from atomized aerosols .…”

Mechanisms, Capabilities, and Applications of High‐Resolution Electrohydrodynamic Jet Printing

“…Although the behavior of liquid droplets under the action of electrohydrodynamic forces has been studied since the early 20 th century, several issues remain topics of theoretical study. [ 70,71 ] Basaran and co-workers developed modeling approaches for EHD tip-streaming from a fi lm [ 72 ] and a droplet [ 22 ] to capture the mechanisms of drop formation and the associated scaling laws. In a recent study, [ 22 ] they demonstrated the existence of three different regimes for the size and charge of droplets depending on the conductivity of the fl uid.…”

“…An enhanced fundamental understanding and computational models that can simulate the jet formation process will be needed for the practical use of e‐jet printers in routine nanomanufacturing. Although the behavior of liquid droplets under the action of electrohydrodynamic forces has been studied since the early 20 th century, several issues remain topics of theoretical study . Basaran and co‐workers developed modeling approaches for EHD tip‐streaming from a film and a droplet to capture the mechanisms of drop formation and the associated scaling laws.…”