On the Relation between the Interfacial Charge of a Discharging Nozzle and Electrification of a Liquid Microjet

Abstract

A liquid microjet discharged from a fused silica capillary is electrified, and the polarity of this electrification is opposite to that of the net charge at the solidliquid interface of the capillary nozzle. Keywords: Liquid microjet | Fused silica | Capillary nozzleLiquid microjets, cylindrical streams of liquid with a diameter of tens of micrometers, are now widely employed in photoelectron spectroscopy of liquids 1 and gas-surface scattering experiments.2 The small surface area of the microjet minimizes evaporation of solvent molecules and facilitates introduction of volatile liquids into high-vacuum chambers. However, a liquid microjet is electrically charged, because the liquid flow shears off an electric double layer at the inner wall of a capillary nozzle to create unequal number densities of cations and anions in the mobile part of the flow (flow electrification). 3,4 In a previous study, 5 we showed that the electric potential of a liquid microjet of NaX solution (X = Cl, Br, and I) varies with the electrolyte concentration. A notable common feature of the variation was that the polarity of the potential reverses at around a concentration of 30 mM (Figure 1), suggesting that Na + plays a key role in determining the polarity. In the present work, we discuss the relation between the electric charge at the inner wall of a capillary and the microjet.The electric potential (º d ) in the diffuse region of an electric double layer is described by the PoissonBoltzmann equation as follows,where e is the elementary charge, ¾ 0 and ¾ r are respectively the permittivity of vacuum and the relative permittivity of a solution, Z « and N « are respectively the charge and number density of positive/negative ions, k B is the Boltzmann constant, and T is the temperature. Equation 1 cannot be solved analytically. However, if the electric potential is sufficiently small such that the following condition holds, i.e.,an approximate solution can be expressed aswhere l is the distance from the inner wall of the capillary, º OHP is the electric potential at the outer Helmholtz plane (OHP), and l OHP is the position of the OHP. ¬ is the inverse of the Debye length, defined aswhere c is the concentration of the solution. º OHP is related to the interfacial charge º 0 bywhere · surf is the original charge density of the solid surface, · ad is the charge density of the adsorbed ions, and l IHP is the position of the inner Helmholtz plane (IHP). The relative permittivity is assumed to be the same in all regions of the solution. Then, the continuity conditions for the electric field and the potential at the OHP yieldIf eq 2 holds, the charge density in the diffuse layer is given bySince Z þ ¼ 1 and Z À ¼ À1 for NaX, eq 7 simplifies to μðlÞ % À2ceº OHP e À¬ðlÀlOHPÞ ð8ÞThis equation shows that the excess charge in a diffuse layer is opposite in sign to both º OHP and · surf þ · ad . Although eq 3 is valid only for small º d , the opposite signs of μ and · surf þ · ad are expected to be general. In this study, we examine the relation bet...

Opto-mechanical expulsion of individual micro-particles by laser-induced shockwave in air

“…Although partially convincing, our results were still affected by residual charging of the droplets. The well-known parasitic charging of microdroplets by nozzles [28][29][30] could be evidenced previously while investigating second and third harmonics generation from microdroplets 31,32 . A perfect neutralization of the droplet stream was impossible in the present investigation and we consequently decided to use optical tweezers for a more precise measurement, inherently immune to charging and cumulative effects.…”