Experimental and theoretical studies into the release of blood droplets from weapon tips

Abstract

The formation and release under gravity of blood droplets from simulated weapon tips has been investigated experimentally and the results analysed and interpreted using established theoretical models for detached pendent drops. Droplets were produced from a series of conical nozzles, manufactured with a range of cone angles and including a set of un-bored conical tips, under conditions where the tip was either non-wetted or pre-wetted with blood. For the former, radius-limited case, detached droplet volumes were found to agree well with the predictions of both the pendent drop weight and drop shape models. For pre-wetted tips, droplet volumes were found to increase with increasing cone angle and to be independent of whether the blood flow was through an orifice at the tip or across the tip surface itself for un-bored tips. Such angle-limited, detached droplet volumes were predicted well by applying the same contact angle correction factor as for a flat surface. The maximum droplet volume, formed from a horizontal flat surface, was found to be around 150 µL for horse blood. Similar theoretical approaches were then extended to droplets released under centrifugal force appropriate to swing cast-off activity and evaluated using previously published experimental data. Order of magnitude agreement for droplet diameters was fairly good but, more importantly, these were found to be proportional to the inverse of the tangential velocity thereby supporting a model where the blood droplet is released directly from a blood mass itself, such as a ligament, rather than from being pinned to some surface feature or orifice on the weapon. This work provides a sound theoretical and experimental understanding of blood droplet release under these conditions that can underpin both future research and the interpretation of blood evidence in case-work.