Electrospray cone-jet breakup and droplet production for electrolyte solutions

The production of charge droplets via the electrospray of an electrolyte is experimentally and theoretically found to be a Coulombic fission phenomenon that occurs when induced space charge at the microjet interface reaches a critical density as the charge separation approaches the Bjerrum length. The field surrounding the cone is dominated by the Taylor harmonic, which is responsible for inducing space charge within an interfacial Debye layer, such that the jet breakup length and plume angle are strong functions of both the ionic strength and interfacial surface tension. editor’s choice Copyright c © EPLA, 2012 When a sufficiently high direct current (DC) electric field is applied to liquid exiting a capillary, it deforms into a conic structure called a Taylor cone [1]. In typical practice, the tip of this cone extends to form a liquid microjet (cone-jet mode), and charged droplets are ejected from the end of the microjet [2]. Among other applications, electrospray ionization has become a workhorse in mass spectrometry because it enables soft ionization of large biomolecules, such as proteins, from an aqueous solution [3]. However, the droplets are ejected in a large, expanding plume that negatively impacts transport into a mass spectrometer, with injection efficiencies less than 1% [4]. Moreover, the size and charge of each aerosol directly influences the all-important degree of ionization of the analyte molecules within the droplets. Therefore, understanding the phenomena that lead to microjet breakup and droplet production is essential to controlling and manipulating the droplets and enhancing electrospray performance. This work reports a theory, verified by imaging experiments, that elucidates the mechanisms of microjet breakup and plume formation for an electrolyte and provides quantitative estimates of the various important parameters that govern the phenomena. (a)E-mail: [email protected] Taylor [1] offered the first theoretical description of the conic meniscus for an electrospray, which has been studied since the pioneering work of Zeleny [5]. The cone’s universal half-angle of θ= 49 for any high permittivity (or high conductivity) liquid corresponds to a unique and discrete spherical harmonic of the Laplace operator for the potential φ in the gas phase. The Legendre function Pn is of order n= 1/2 in the polar angle θ direction and r in the radial direction, such that this specific harmonic gives rise to a Maxwell pressure that scales as 1/r and hence offsets the singular capillary pressure of the cone that has the same 1/r scaling. It is a very elegant real-life example of a discrete spectrum in (semi-)unbounded domains that can only occur near geometric singularities, such that the local electric field has a generic functional dependence that is insensitive to far-field conditions. Because of the universal functional dependence of the harmonic potential near the cone, in the limit of high liquid/gas permittivity ratio, parametric effects on the field are contained in the coefficient of the potential. Balancing the Maxwell pressure ε0 |∇φ| 2 with the singular azimuthal capillary pressure of a cone γf/r gives rise to the coefficient of the potential, which must scale as (γf/ε0) . Therefore, the potential along the cone axis (θ= 0) away from the tip is φTaylor =B(γf/ε0) r, (1)