In this work, we examine the influence of viscosity on breakup and coalescence of a swarm of large drops in a wall-bounded turbulent flow. We consider several values of surface tension and a wide range of drops to fluid viscosity ratios $\lambda ={\eta }_d/{\eta }_c$ (with ${\eta }_d$ the viscosity of the drops and ${\eta }_c$ the viscosity of the carrier fluid), from $\lambda =0.01$ to $\lambda =100$, while we maintain the same density for drops and carrier fluids. Drops can coalesce and break following a complex dynamics that is primarily controlled by the interplay between turbulence fluctuations (measured by Reynolds number, ${\text{Re}}_{\tau }$), surface tension (measured by Weber number, $\text{We}$), and $\lambda$. We use direct numerical simulation of turbulence coupled with a phase field method to describe the drops dynamics. We consider three different values of $\text{We}$ (which is the inverse of the surface tension): $\text{We}=0.75$, 1.5, and 3. For each value of $\text{We}$, we assume five values of $\lambda$: $\lambda =0.01$, 0.1, 1, 10, and 100. We observe a consistent action of increasing $\lambda$, which, especially for the larger Weber numbers, decreases significantly the breakup rate of the drops. Qualitatively, an increase of drop viscosity decreases the breakup rate, very much like an increase of surface tension does. The mechanism by which drop viscosity acts is a modulation of turbulence fluctuations inside the drop, which reduces the work surface tension has to do to preserve drop integrity. We believe that this may give important indications in many industrial applications to control drop coalescence and fragmentation via the ratio of drop to fluid viscosity.

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• Drop breakup
• Drop coalescence
• Multiphase flows