GENERATION OF MONODISPERSE DROPLETS 0.3 TO 4 MU-M IN DIAMETER FROM ELECTRIFIED CONE-JETS OF HIGHLY CONDUCTING AND VISCOUS-LIQUIDS

The size distributions of droplets emitted from Taylor cones operating in the cone-jet regime are measured by sampling their electrosprays into an aerodynamic size spectrometer (API's Aerosizer). The sizing scheme is not affected by the large charge on the un-neutralized droplets in the range of diameters d explored, 0.3 mum < d < 4 mum. The diameters of the droplets electrosprayed from highly conducting liquids are found to be relatively insensitive to electrostatic variables, depending for a given liquid mostly on the flow rate Q pushed through the jet. At fixed Q, the size distributions consist of one or several fairly monodisperse classes of droplets with diameters d(i)(Q); i = 1, 2,..., N(Q). Near the minimum flow rate Q(min) at which the cone is stable, the spray tends to consist of ''primary'' and ''satellite'' droplets only, with N = 2. However, at larger flows, the modality of the distributions (N) increases. The largest size mode bifurcates into two branches at a critical flow rate Q1, coinciding with the onset of lateral oscillations of the jet. The diameter d1 of the largest drops scales approximately with r* = (Qtau)1/3, where tau is the electrical relaxation time of the fluid. Surprisingly, all the other size classes have diameters d(i) (i not-equal 1) nearly independent of flow rate, which scale as d(min) = (gammatau2/rho)1/3 (gamma = coefficient of surface tension; rho = liquid density). Although the jet diameter d(j) appears to be unaffected by viscosity, its breakup mechanism, and thus the diameters d(i) of all the droplet classes, do depend on the viscous parameters PI(mu) = (gamma2rhotau)1/3/mu (mu = coefficient of viscosity of the liquid). The diameters of the smaller droplets are given by functions d(i)/d(min) = G(i)(PI(mu)) (i not-equal 1), which depend steeply on PI(mu) for values of this parameter below 0.06, but appear to level off above PI(mu) = 0.15. An inviscid asymptote, in which d1/r* = F(eta), is approached also for d1 for sufficiently large values of PI(mu), where eta2 = rhoQ/gammatau. F is nearly constant below the bifurcation, and seems to tend to the asymptote F = 0.43 eta2/3 at large eta, in qualitative agreement with the behavior of d(j)/(Qtau)1/3 given by Fernandez de la Mora and Loscertales (J. Fluid Mech. 260, 155-184, 1994). It follows from the scaling laws found that, by varying the electrical conductivity of a given liquid, it should be possible to generate monodisperse droplets with initial diameters of the order of d(min), which may span the whole range between 100 mum down to a few nanometers. The flow rate must, however, be between Q(min) and its value at the bifurcation, which requires that eta approximately 1.