Splashing. How does a drop of liquid splash when it hits a smooth dry solid surface? Our intuition tells us it must splash and eject thousands of tiny droplets. We would expect the same behavior anywhere - here on Earth, on Mars and on the Moon. We would be wrong! In collaboration with Wendy Zhang, we have found that we can suppress splashing completely by removing the surrounding atmosphere. A drop which splashes in Chicago would not necessarily splash on the top of Mt. Everest where the pressure is less and would definitely not splash on the Moon which has no atmosphere.
Recently, we discovered the splashing of a liquid drop on impact with a dry substrate is a strongly air-dependent phenomena. We find at sufficiently low pressures, splashing can be completely suppressed, even for highest impact velocities we can achieve (about 8 m/s).
1 atm
0.2 atm
“Creation of prompt and thin-sheet splashing by varying surface roughness or increasing air pressure”, A. Latka, A. Strandburg-Peshkin, M. M. Driscoll, C. S. Stevens, and S. R. Nagel, arXiv: 1203.2718v1 (2012).
“Ultrafast Interference Imaging of Air in Splashing Dynamics”, M.M. Driscoll and S.R. Nagel, Phys. Rev. Lett., 107, 154502 (2011).
“Thin Film Formation During Splashing of Viscous Liquids”, M.M. Driscoll, C.S. Stevens, S.R. Nagel, Phys. Rev. E 82, 036302 (2010).
“Splashing of liquids: Interplay of surface roughness with surrounding gas,” L. Xu, L. Barcos, and S. R. Nagel Phys. Rev. E 76, 066311 1-5 (2007).
“Liquid drop splashing on smooth, rough, and textured surfaces,” L. Xu, Phys. Rev. E 75, 056316 1-8 (2007).
“Drop Splashing on a Dry Smooth Surface,” L. Xu, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. 94, 184505 1-4 (2005).
Active Research Areas:
Leidenfrost drops. When a drop of liquid is placed on a very hot surface, an amazing phenomenon occurs. Above a certain temperature, instead of boiling rapidly, the drop will levitate on a thin cushion of vapor. This effect is known as the Leidenfrost effect, and can be easily visualized when small water drops float across a hot frying pan. Underneath the drop, there is a small vapor pocket created by the evaporation of the liquid. We are currently studying this thin vapor layer using laser light interference coupled with high-speed video. Using this technique, we can clearly see the neck where the drop is closest to the interface, as well as many fluctuations and structure that is not visible in the images from the side or in the simple model of the vapor cushion.
Pictured at left is a water drop floating above an aluminum surface at T = 285C. The reflection of the drop can be seen in the metal surface. Underneath the drop, there is a small vapor pocket created by the evaporation of the liquid. We are currently studying this thin vapor layer using laser light interference coupled with high-speed video, pictured at right.
“The geometry of the vapor layer under a leidenfrost drop.”, J. C. Burton, A. L. Sharpe, R. C. A. van der Veen, A. Franco, S. R. Nagel, arXiv:1202.2157 (2012).
Singularities in Free-surface Flows. A drop falling from a faucet is a common example of a liquid fissioning into two or more pieces. The cascade of structure that is produced in this process is of uncommon beauty. As the drop falls, a long neck, connecting two masses of fluid, stretches out and then breaks. What is the shape of the drop at the instant of breaking apart? Something dire must happen to the mathematical description of the liquid at that point since the drop undergoes a topological transition where it starts out as a single, connected fluid and ends up in two or more separate pieces. This is an example of a finite-time singularity since the drop breakup occurs a short time after the drop becomes unstable and starts to fall. At the transition, a singularity occurs since the radius of the neck holding the drop to the nozzle becomes vanishingly thin. As its radius goes to zero, the curvature diverges and the surface tension forces become infinite. How can such dramatic dynamics occur in something which had such smooth and innocuous initial conditions and forcing terms? Using photographic techniques, we have been studying transitions such as these to understand how the non-linearities in the governing equations (in this case the Navier-Stokes equations) can be tamed and understood. Singularities of this kind occur in many areas of physics from stellar structure to turbulence to bacterial colony growth. This drop breakup problem is one of the simplest places to start an experiment that directly probes the singularity itself. In collaboration with Wendy Zhang, we have uncovered a variety of different singularities - some of which surprisingly retain a memory of their initial conditions throughout the entire breakup process.
The break-up of an air bubble in water. Recent experiments have shown that air bubble break up exhibits memory of its initial condition until the moment of pinch off. This is in stark contrast with the break up of water in air (a dripping faucet) and many other generic pinch off phenomena, which completely forgot their initial conditions near the break up.
“Perturbed breakup of gas bubbles in water: Memory, gas flow, and coalescence”, N.C. Keim, Phys. Rev. E 83, 056325 (2011).
“Memory-encoding vibrations in a disconnecting air bubble”, L. Schmidt, N.C. Keim, W.W. Zhang, and S.R. Nagel, Nature Physics, DOI: 10.1038/NPHYS1233, (2009).
“Breakup of Air Bubbles in Water: Breakdown of Cylindrical Symmetry,” N. C. Keim, P. Møller, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. 97, 144503 (2006).
Coalescence. When fluid drops merge, a dramatic transformation occurs: the topology changes as the fluid masses, originally separated, merge into a single entity. At first, the drops are separated by only a small distance. Then a thin fluid bridge is formed between them which rapidly widens due to surface tension forces as shown in the figure. We have employed an electrical method and high-speed imaging to study the coalescence of two drops over a wide range of time-scales and fluid viscosities. We have shown that at low approach velocity, where the drops coalesce as undeformed hemispheres, the viscous-to-inertial crossover is unexpectedly late and inconsistent with the theory. We have presented a new picture of the flow field near the singularity based on an unappreciated length-scale, and it correctly predicts the late crossover. Recently, in collaboration with Osman Basaran's group at Purdue University, we have identified a new regime in coalescence that dominates the asymptotic dynamics of coalescence for drops of any finite viscosity.
Initial moments of coalescence for two water drops with A = 2 mm. Frames are 120 μs apart. The central white spot is due to the light source located behind the drops.
“The inexorable resistance of inertia determines the initial regime of drop coalescence”, J.D. Paulsen, J.C. Burton, S.R. Nagel, S. Appathurai, M.T. Harris, and O.A. Basaran, PNAS 109, 6859(2012)
“Viscous to Inertial Crossover in Liquid Drop Coalescence”, J.D. Paulsen, J.C. Burton, S.R. Nagel, Phy. Rev. Lett. 106, 114501 (2011).
“Coalescence of low-viscosity fluids in air”, S.C. Case”, Phy. Rev. E, 79, 026307, 1-10 (2009).
“Coalescence in low-viscosity liquids,” S. C. Case and S. R. Nagel, Phys. Rev. Lett. 100, 084503 1-4 (2008).
