Mist Flow Visualization for Round Jets in Aerosol Jet Printing

With the microdroplets of water serving as light scattering particles, the mist flow patterns of round micro-jets can be visualized using the Aerosol Jet® direct-write system. The visualization images show that the laminar mist jet appears to extend to more than 20 times the diameter of nozzle orifice, D, for jet Reynolds number Re < 600, especially with D = 0.3 mm and less. For smaller jets (e.g., with D = 0.15 mm), laminar collimated mist flow might be retained to 40xD for Re < 600 and for Re ~ 1500 within 20xD from the nozzle. The laminar part of mist flow associated with larger jets (e.g., with D = 1.0 mm for Re < 600) tends to exhibit noticeable gradual widening due to viscous diffusion. For free jets, their breakdown length–the distance from nozzle where transition from laminar to turbulent mist flow takes place as signaled by the inception of a rapid widening of mist stream–is shown to decrease with increasing Re. The presence of impingement wall tends to prevent turbulence development, even when the wall is placed further downstream of the free-jet breakdown length for a given Re . The critical Re for an impinging jet to develop turbulence increases as the standoff S is reduced. The mist flow of impinging jet of D = 1.0 mm seems to remain laminar even for Re > 4000 at S = 12 mm.