Monodisperse droplet stream

Keywords Discrete polydisperse spray Electric droplet charging Extension nozzle Ink-jet printing Monodisperse droplet stream Monodisperse spray Multihole orifice Modulated jet excitation Nozzle hole shapes Rapid prototyping Rayleigh-type jet break-up Solder ball production... 

K. Anders Monodisperse droplet streams and their application in space. Proceedings of the Symposium on Fluid Dynamics and Space, VKl, Rhode-Saint-Genese, pp. 119-125 (1986). 

The primary stream of droplets is monodisperse. Walton and Prewett [49] demonstrated this principle of uniform droplet generation. An improved version with a spinning top was developed by May [50,51]. Further improved versions have been developed since then, and droplet sizes that are produced range from 15 to 150 pm (e.g., see Ref. [52]). A recent version produced monodisperse droplets between 10 and 60 pm in diameter [53]. Similar particles can be obtained using solutions. Toivonen and Bailey produced solid particles up to 40 pm in diameter [54]. Rotation speeds used are up to 70,000 rpm. Improvements in particle concentrations have been obtained by Cheah and Davies [55] but are still nominally less than 100 cm-3. A detailed study of the mechanism of droplet formation was provided recently by Davies and Cheah [56]. Eisner and Martonen [57] demonstrated that if the primary and satellite droplets can be effectively separated, then two monodisperse aerosol streams can be generated simultaneously. 

Fig. 26.1 Stream of monodisperse propanol-2 droplets with highly controlled size, as produced with a droplet stream generator of the presently discussed kind. The oscillations are caused by the deformations upon pinch-off from the jet...

This part of this review reports about techniques for enforcing monodisperse droplet formation. The techniques must essentially allow for the formation of droplets with controlled sizes. One requirement may be that the droplet generators produce monodisperse streams of droplets. However, essentially the same technique appropriate for this purpose may be used for the controlled formation of sprays with discrete polydisperse drop size spectra also. We will also account for techniques that achieve the monodispersity of the drop streams at least approximately. 

A limited number of polyanion-polycation systems were tested using a droplet/falling annulus method (Fig. 4). This technique, which has been described elsewhere [64] reduces the net impact velocity between the droplet with the oppositely charged counterion fluid. A stream of droplets was directed into a collapsing annular liquid sheet. By matching the velocities of the droplet and sheets, the impact conditions can be moderated. It has been shown to produce monodisperse spherical capsules, though it requires several days of calibration for each new system and is obviously not practical for a massive screening such as was carried out herein. 

The mechanisms of formation of discrete segments of fluids in microfiuidic flow-focusing and T-junction devices, that we outlined above point to (i) strong effects of confinement by the walls of the microchannels, (ii) importance of the evolution of the pressure field during the process of formation of a droplet (bubble), (iii) quasistatic character of the collapse of the streams of the fluid-to-be-dispersed, and (iv) separation of time scales between the slow evolution of the interface during break-up and last equilibration of the shape of the interface via capillary waves and of the pressure field in the fluids via acoustic waves. These features form the basis of the observed - almost perfect -monodispersity of the droplets and bubbles formed in microfiuidic systems at low values of the capillary number. 

Mulbolland, J., Srivastava, R., and Wendt, J. Influence of droplet spacing on drag coefficient in nonevaporating, monodisperse streams. AIAA J. 26(10), 1231-1237, 1988. 

---