Universal cone angle of ac electrosprays due to net charge entrainment.

The half-angle of an ac cone is shown to exhibit a universal (length-scale independent) asymptotic value of 12.6 degrees at high-permittivity ratio, which is quite distinct from the 49.3 degrees of dc Taylor cones. Our theory and experiments suggest that ac entrainment of low-mobility anions, generated by field-assisted ion dissociation, sustains a net charge at the ac cone. Mutual Coulombic repulsion between these free charges compensates the singular azimuthal capillary force and elongates the cone with an axial force that does not exist in dc cones.