Digital In-line Holography of blood atomization

Abstract

<p>Bloodstain Pattern analysis (BPA) has been widely used as a forensic tool for crime scene reconstruction by law enforcement agencies worldwide. The ultimate pattern left behind by a blood-letting event has been well described in the literature but the connection between the pattern and the fluid dynamic origin of the droplets that caused the stain remain uncertain. A variety of bloodstain patterns could be traced to the flight of the droplets, the resulting size and velocity distributions of cast droplets, and wetting and drying on a surface. In this dissertation, a study of the atomization mechanisms in the immediate moments after impact on a film of blood is presented. Both blunt and projectile impacts have been examined using high-speed imaging, which show distinct drop breakup characteristics due to ligament formation and high velocity impact. Digital in-line holography (DIH), in conjunction with high-speed imaging at kHz-rates, was used to quantify drop diameters and velocities milliseconds after impact. The Sandia HOLOSAND code was utilized to process holograms and identify droplet trajectories over multiple frames, thus improving overall out-of-plane accuracy of position and velocity estimation. The merits of DIH over traditional backlit imaging in terms of three dimensional measurement is significant. The temporal evolution of blood droplets in blunt impact (flat-to-flat surface) at up to 4 m/s is reported and comparisons are made with water as a reference fluid. The characteristic velocities and diameters of blood droplets from a bullet impact are also reported. This quantitative data-set would be helpful in verification of theoretical models of droplet trajectories, which is a positive step towards connecting BPA and fluid dynamics communities.</p>