Self-charging of sprays

Abstract

The charging of poorly conducting liquids due to flows is a well-known phenomenon, yet the precise charging mechanism is not fully understood. This is especially relevant for sprays, where the spray plume dynamics and maximum distance travelled of a spray dramatically changes for different levels of charging: charging of the droplets makes them repel, thereby preventing drop coalescence and altering the shape of the spray plume. As the charging depends on many factors including the flow and the interactions between the liquid and the nozzle, many models and scaling laws exist in the literature. In this work we focus on perhaps the simplest flow regime, laminar jets created by ultra short channels, and quantify the charging as a function of the different parameters. We present a simple model that collapses all the data for over 4 orders of magnitude difference in streaming currents for various nozzle sizes, flow velocities and surface treatments. We further show that the charging polarity can even be reversed by applying an oppositely charged coating to the nozzle, an important step for any application.

Electroless Ionization Mass Spectrometry Using a Compact Electrokinetic Ionization Source

“…For a true comparison, however, an ELI nozzle optimized for maximum charging and low flow rates should be used as a closed ionization source, as is done in ESI. This is beyond the scope of this work, where we only use separate ELI nozzles suitable for ambient MS. As it is well understood what determines the level of charging for ELI, theoretical considerations do, however, allow one to estimate how an ELI-type nozzle would perform under such circumstances. In electrospray, using a flow rate of 5 μL min –1 , the spray current can be 20 nA for solutions with a sample concentration on the order of 1 μM in a clean solvent .…”

“…While higher spray currents can be obtained if solutions with more electrolytes are used, such concentrated solutions are not suitable for ELI-MS. The expected induced electric current I s due to electrokinetic charging by a single ELI nozzle is given by I s = prefix− v ζ ϵ 2 R e − 1 where v is the liquid velocity, ζ the zeta potential between the nozzle charge transfer layer and the liquid, ϵ the electric permittivity of the liquid, and Re the Reynolds number given by Re = ρ vR /μ, where R is the nozzle hole radius, ρ the liquid density, and μ the viscosity of the liquid. For water, a single nozzle hole radius of 0.95 μm and a flow rate of 5 μL min –1 , I s = 6 nA, a value very similar to ESI.…”

“…Since ELI-MS does not require an external voltage supply and therefore does not contain a polarity switch, we adjusted the nozzle design and choose solvents or solvent mixtures to allow for either positive or negative ionization. The charging polarity and efficiency is mostly determined by the zeta potential between the nozzle’s charge transfer layer and the solvent or solvent mixture . For most common solvents or solvent mixtures such as water, acetonitrile, dimethyl sulfoxide, H 2 O/EtOH (v/v % water > 25), and H 2 O/MeOH (v/v % water > 25), the zeta potential between the liquid and the “positive” ELI nozzle is negative, resulting in positive ionization.…”

“…In ELI, the sample liquid is pushed through a microfabricated spray nozzle (Figure ). Due to the electrokinetic interaction with the nozzle surface, the liquid becomes charged . Combined with rapid evaporation, this charging results in a quickly expanding cloud of ionized particles for subsequent MS analysis.…”

“…Though this process is still not fully understood, the biggest uncertainty for these techniques in the ion formation is the droplet charge and size distribution. For ELI, however, the droplet size and expected level of charging can be accurately predicted if the zeta potential between the nozzle and liquid is known . Optimization of the charging mechanism by making use of different nozzle materials is still an active area of research.…”