Droplet collisions in a liquid

The collision of two equally sized fluid droplets in a continouous phase consisting of a second liquid is investigated. The outcome of the collision, i.e. coalescence or bouncing depends, for two given fluids, on the relative velocity of the droplets just before impact, and the mis-alignment of the collision $\Delta x$, nondimensionialized in a Weber number, $We = \rho_{d} U_{rel}^2 d_{ave}/\sigma$ and the alignment parameter $B = \Delta x/d_{ave}$. We studied nearly head-on collisons by launching the droplets from two opposing capillaries. High speed dual-axis shadowgraphy is used to reconstruct the 3D-droplet trajectories. We found that if the Weber number exceeds a critical value, the droplets will coalesce; for lower values they bounce off again. For silicon oil droplets with a dynamic viscosity $4.6$ times that of the surrounding phase (water) the critical Weber number is found to be $We \approx 22.5$. For all bouncing droplets, the contact time was measured and found to be approximately equal to the theoretical droplet oscillation frequency period. Current work aims at investigating the influence of external continuous phase turbulence on the outcome of the collisions by placing the two-capillary system in between two counterrotating discs (``von K\'{a}rm\'{a}n flow'').