Effect of muzzle gases on forward spatter of viscoelastic blood in close-range shooting

Abstract

Because bloodstain patterns resulting from close-range shooting are intrinsically different from those of long-range shooting, an accurate interpretation and delineation of these phenomena are essential in forensic science and crime scene analysis. Such a delineation would be helpful, for example, to distinguish whether a suicide or a homicide had happened. If the shooting was from a long-range (most likely a homicide), muzzle gases would not be able to influence blood spatter ejected from a victim. However, in the case of a close-range shooting, muzzle gases would greatly influence blood spatter. Herein, the effect of the muzzle gases on bloodstain patterns is studied. A de Laval nozzle is used to mimic an issue of supersonic “muzzle” gas from a gun barrel. The supersonic gas flow passes through a cylinder containing defibrinated sheep blood, which is blown off and atomized into numerous drops. These drops fly away and settle onto the floor or onto vertical walls at various distances from the cylinder exit. Viscoelasticity of the defibrinated sheep blood is enhanced by adding Xanthan to model different states of blood corresponding to different conditions. An impact of a vertically-released single drop onto an inclined substrate was also studied to elucidate splashing regimes/criteria of the blood drops of different viscoelasticity levels. We found that stronger elastic forces facilitated formation of bloodstains with higher ellipticity at a higher impact angles.