  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(R) 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.
