Sasha Yarin (University of Illinois at Chicago) - Atomization in Forensic and High Power Applications

The talk will cover the following three main topics associated with the acceleration-driven hydrodynamic instabilities and their suppression by elastic forces: 

(i) Atomization in forensic applications. A theoretical model describing the blood spatter pattern resulting from sharp and blunt bullet gunshots is discussed. These hydrodynamic problems belong to the class of the Wagner penetration problem or the impact hydrodynamics with the pressure impulse generating the blood flow. At the free surface, the latter is directed outward and accelerated toward the surrounding air. As a result, the Rayleigh-Taylor instability of the blood flow occurs, which is responsible for the formation of blood drops of different sizes and initial velocities. Then, the equations of motion of blood drops are solved, describing their trajectories in air accounting for gravity and air drag, as well as the collective effect of the drop-drop interactions through air, which diminishes air drag on the subsequent drops. The effect of viscoelasticity of blood could also be accounted for. Deposition of two-phase blood-drop/air jets onto surfaces is predicted and compared with experimental data. For the short-range shooting an interaction of blood backspatter with muzzle gases becomes important. Self-similar turbulent vortex rings are investigated theoretically in the framework of the semi-empirical turbulence theory for the modified Helmholtz equation. The vortex ring of muzzle gases moves toward the target and collides with blood drops in the backspatter resulting from the gunshot. This collision skews the distribution of blood stains on the ground and can either propel blood drops further from the target, or even turn them backward toward, and even behind, the target, which is confirmed experimentally. 

(ii) Reopening dentistry after COVID-19: Complete suppression of aerosolization in dental procedures by viscoelastic Medusa Gorgo. Ultrasonic cavitron scalers and high-speed dental drills are recognized as the worst aerosolization sources resulting in myriads of tiny droplets of water used as an irrigation fluid. The aerosolization at the cavitron scaler is driven by the Faraday instability, whereas the one at the drill - by the action of the centrifugal force.  The resulting airborne droplets entrain saliva and multiple bacteria and viruses from the patient mouth and spread them to significant distances, including open mouths of the other patients in dental clinics, personnel and surrounding surfaces. Similar situations arise in skin surgery, tattoo shops, etc. Droplets below 20 μm in size evaporate before settling down and are a source of airborne viruses (e.g., those of SARS-CoV-2, which are ~100 nm in size). Filtration of such aerosols is highly problematic. However, it is demonstrated here that dilute aqueous FDA-approved polymer solutions being used as the irrigation fluids can completely suppress droplet formation at the generating source without altering flow behavior in the supply line of standard dental chairs. This outcome results from significant elastic forces arising in the uniaxial elongational flow in the tails of droplets trying to detach from the liquid bulk. The products resulting from this work are licensed to the industry and enter the market in 2023. 

(iii) Liquid films (cavities) subject to pressure difference in the surrounding gas in rheology and large-scale explosions.  Cylindrical films with high gas pressure in the cavity resulting from an exploding electric wire or a chemical explosive are accelerated radially outward and, as is theoretically shown in this work, experience the Rayleigh-Taylor instability. This results in perturbation growth and ultimately, in breakup and debris formation. The results triggered rheological experiments with tremendously strong stretching of concentrated viscoelastic liquids, and are also of interest in relation to other practically important large-scale situations, which are illustrated experimentally.