Alternative method for determining the original drop volume of bloodstains on knit fabrics Jingyao Li, Xingyu Li, Stephen Michielsen * Textile Engineering, Chemistry and Science, College of Textiles, North Carolina State University, Raleigh, NC 27695, USA 1. Introduction Bloodstain pattern analysis, BPA, is a forensic discipline that has developed into a science since Piotrowski’s pioneering studies in 1895 [1], incorporating mathematics, physics and fluid mechanics [2]. The application of mathematics and physics to the formation and distribution of airborne blood drops has enabled analysts to estimate the area of convergence of the spatter. The traditional method for determining the point of origin by stringing (or by virtual stringing using computer programs) often leads to points of origin that are much too high [3]. If one introduces fluid mechanics to account for air resistance (drag) effects, it is possible to determine point of origin more accurately if both the drop volume and velocity are known. Unfortunately, these are difficult to obtain from a dried bloodstain and thus the analyst is left with a family of drop volumes and velocities that are consistent with the stain size. If the drop volume could be determined, the drop velocity could then be found, which would provide valuable information about the forces (energies) imparted to the blood source to create and propel the drops. If the Weber number of the drop moving through air is higher than 13 (We > 13), the air resistance exceeds the surface tension forces holding the drop together and thus causes the drop to break up (We = rD 0 v 2 /g; where r is the density of air, D 0 is the drop diameter, v is the drop velocity, and g is the liquid-vapor surface tension). For ballistic drops, if the diameter of the drop is known, the maximum velocity and maximum distance the drop could travel from the blood source to the location of the stain can be determined [4]. Hulse-Smith et al. have shown that the size of a stain and the number of spines on a non-absorbent surface depends on the drop volume, We and the Reynolds number (Re = rD 0 v/h and h is the viscosity) [5]. Both the Reynolds number and the Weber number of an airborne drop depend linearly on the drop diameter. The Reynolds number depends linearly on the drop velocity while the Weber number depends on the velocity squared. Unfortunately, there is a continuum of combinations of drop diameter and velocity that are consistent with stain sizes used to determine the Reynolds number. If the drop diameter could be determined, this ambiguity would be eliminated. This has lead to several authors attempting to determine the volume of the original drop from the resulting stain. For example, Forensic Science International 263 (2016) 194–203 A R T I C L E I N F O Article history: Received 27 October 2015 Received in revised form 5 April 2016 Accepted 11 April 2016 Available online 20 April 2016 Keywords: Bloodstains BPA Textiles Drop volume Knit Porcine blood A B S T R A C T Bloodstains are often observed at violent crime scenes and on the skin and clothing of persons involved. The diameters of the blood drops that created these stains are related to the force or energy that caused these drops to become airborne. This has resulted in several attempts to determine the diameter of the original drops, beginning with the methods reported in the pioneering work of Henry Lee [6]. However, his methods destroyed the bloodstain during the measurement. Other methods described in the literature cannot be applied to bloodstains on textiles. A new, rapid, reliable, non-destructive method for determining the diameter of the original drop of blood that results in a stain has been developed for bloodstains on cotton single jersey knit (tee-shirt) fabrics, which is one of the most common fabrics analyzed for BPA both at crime scenes and in forensic laboratories. In this method, a drop of known volume of an appropriate artificial blood substitute is applied to a region similar to the stained region but in an area away from any stains/areas of interest. The areas of the original stain and the artificial blood substitute stain are determined, from which the original drop diameter can be calculated. Errors in the drop diameters, the Reynolds numbers and the Weber numbers resulting from this procedure are less than approximately 6%. This procedure has only been verified on cotton single jersey knit fabrics with 30 mL  drop volume  80 mL. It should not be applied to other materials. ß 2016 Elsevier Ireland Ltd. All rights reserved. * Corresponding author. Tel.: +1 919 606 8657. E-mail addresses: jli42@ncsu.edu (J. Li), xli39@ncsu.edu (X. Li), smichie@ncsu.edu (S. Michielsen). Contents lists available at ScienceDirect Forensic Science International jou r nal h o mep age: w ww.els evier .co m/lo c ate/fo r sc iin t http://dx.doi.org/10.1016/j.forsciint.2016.04.018 0379-0738/ß 2016 Elsevier Ireland Ltd. All rights reserved.