CHARGE AND SIZE DISTRIBUTIONS OF ELECTROSPRAY DROPS

The distributions of charge q and diameter d of drops emitted from ele ctrified liquid cones in the cone-jet mode are investigated with two a erosol instruments. A differential mobility analyzer (DMA, Vienna type ) first samples the spray drops, selects those with electrical mobilit ies within a narrow band, and either measures the associated current o r passes them to a second instrument. The drops may also be individual ly counted optically and sized by sampling them into an aerodynamic si ze spectrometer (API's Aerosizer). For a given cone-jet, the distribut ion of charge q for the main electrospray drops is some 2.5 times broa der than their distribution of diameters d, with q(max)/q(min) similar to 4. But mobility-selected drops have relative standard deviations o f only 5% for both d and q, showing that the support of the (q, d) dis tribution is a narrow band centered around a curve q(d). The approxima te one-dimensionality of this support region is explained through the mechanism of jet breakup, which is a random process with only one degr ee of freedom: the wavelength of axial modulation of the jet. The obse rved near constancy of the charge over volume ratio (q similar to d(3) ) shows that the charge is frozen in the liquid surface at the time sc ale of the breakup process. The charge over volume ratio of the primar y drops varies between 98 and 55% of the ratio of spray current I over liquid flow rate Q, and decreases at increasing Q. I/Q is therefore a n unreliable measure of the charge density of these drops. (C) 1997 Ac ademic Press.