Stability of confined jets


The transition from dripping (droplet formation close to the nozzle) to jetting (droplet formation farther downstream) in microfluidic co-axial injection devices has recently  been related to the transition from an absolute instability to a convective instability, as seen in figure 2.

Figure 2: Dripping (b) to jetting (c) transition in microfluidic coaxial injection devices (from Guillot et al. 2008).

Together we M.-L. Cordero and C. Baroud from LadHyX, we have studied how this system reacts to a synchronized harmonic forcing, using a pulsating laser actuation. This technique uses the selective dispersion relation of immiscible co-axial flows to reinforce the wavelength selection and thereby the monodispersity of the droplets formed.

Figure 3: Images of the jet perturbed with a laser beam at a constant frequency
flaser = 70Hz and different laser powers (Cordero et al., 2011)

However, we have also shown that the local instability theory could not predict correctly the frequency selection in the dripping regime. This has motivated further investigations funded by FNS through the PhD thesis of L. Augello.