Flow regimes and instabilities in eccentric microjet impingement at low to moderate Weber numbers

We report a systematic experimental study on the impingement of two eccentric microjets at low to moderate Weber numbers. By conducting high-speed imaging and parametric scans, we identify several distinct film instability modes and construct a Weber number (We)-eccentricity (B) phase diagram to delineate transitions from closed to open films. In contrast to centric collisions, where jets are aligned, we find that eccentric impingement, with the jet axes offset, produces thinner films under comparable We. A momentum-based tilting model that incorporates a parabolic jet velocity profile, which reproduces measured film deflection angles and suggests the jets remain partially unrelaxed before collision. By adopting an equivalent merged-jet radius in Rayleigh-Plateau instability (RPI) scaling, we accurately predict the observed monodisperse droplet generation frequencies. These findings shed light on fluid dynamic mechanisms in eccentric microjet impingement and inform strategies for producing self-refreshing thin liquid films for liquid target or mirror applications.