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 i nto an aerodynamic size spectrometer (API's Aerosizer). The sizing sch eme 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 diamet ers of the droplets electrosprayed from highly conducting liquids are found to be relatively insensitive to electrostatic variables, dependi ng 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 o nly, with N = 2. However, at larger flows, the modality of the distrib utions (N) increases. The largest size mode bifurcates into two branch es at a critical flow rate Q1, coinciding with the onset of lateral os cillations of the jet. The diameter d1 of the largest drops scales app roximately with r = (Qtau)1/3, where tau is the electrical relaxation time of the fluid. Surprisingly, all the other size classes have diam eters d(i) (i not-equal 1) nearly independent of flow rate, which scal e as d(min) = (gammatau2/rho)1/3 (gamma = coefficient of surface tensi on; rho = liquid density). Although the jet diameter d(j) appears to b e unaffected by viscosity, its breakup mechanism, and thus the diamete rs d(i) of all the droplet classes, do depend on the viscous parameter s 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), i s approached also for d1 for sufficiently large values of PI(mu), wher e eta2 = rhoQ/gammatau. F is nearly constant below the bifurcation, an d seems to tend to the asymptote F = 0.43 eta2/3 at large eta, in qual itative agreement with the behavior of d(j)/(Qtau)1/3 given by Fernand ez de la Mora and Loscertales (J. Fluid Mech. 260, 155-184, 1994). It follows from the scaling laws found that, by varying the electrical co nductivity of a given liquid, it should be possible to generate monodi sperse droplets with initial diameters of the order of d(min), which m ay 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 bifurc ation, which requires that eta approximately 1.