Jets and drops
Transition to turbulence in viscoelastic jets

We use Schlieren imaging to visualize jets of dilute polymer solutions injected into quiescent water. The two phases, i.e., the dilute aqueous polymer solution and water, have a very small density difference making their detection with the naked eye almost impossible. However, this small density difference provides sufficient refractive index difference that the mixing of the two phases can be followed using Schlieren imaging. Our visualization reveals a new mode of transition to turbulence that we call Elasto-Inertial Streaks. Unlike transition to turbulence for Newtonian fluids, in which intermittent turbulent bursts between laminar regions, termed turbulent puffs, are observed as the Reynolds number is increased, here the intermittency is eliminated and a gradual transition to turbulence occurs through growth and nonlinear evolution of an interfacial instability mode that leads to elongated streaks of the dilute polymer solution around the core of the jet.

With Gareth H. McKinley (MIT)​​

Airflows generated by an impacting drop

with Sidney Nagel (University of Chicago), Bahni Ray, Jeffrey Morris and Taehun Lee (CCNY)

A drop impacting a solid surface with sufficient velocity will splash and emit many small droplets. However, lowering the ambient air pressure suppresses splashing completely. This effect, robustly found for different liquid and substrate properties, raises the fundamental question of how air affects a spreading drop. In a combined experimental and numerical study we characterize the flow of air induced by the drop after it hits the substrate, using a modified Schlieren optics technique combined with high-speed video imaging and Lattice-Boltzmann simulations. Our experiments reveal the emergence of air structures on different length scales. On large scales, the airflow induced in the drop’s wake leads to vortex structures due to interaction with the substrate. On smaller scales, we visualize a ring structure above the outer edge of the spreading liquid generated by the spreading of the drop. Our simulations reveal the interaction between the wake vorticity and the flows originating from the rapidly escaping air from below the impacting drop. We show that the vorticity is governed by a balance between inertial and viscous forces in the air, and is unrelated to the splashing threshold.

Airflows generated by an impacting drop

I. Bischofberger, B. Ray, J. F. Morris, T. Lee, S. R. Nagel

Soft Matter, 12, 3013-3020 (2016)

 

Seeing the invisible – air vortices around a splashing drop

I. Bischofberger, K. W. Mauser, S. R. Nagel

Phys. Fluids 25, 091110 (2013)