Physical Characteristics of Spatter Stains on Textiles

Overview

Bloodstain pattern analysis (BPA) is a forensic technique for crime scene reconstruction, through analyzing bloodstains (e.g., size, shape, etc.) and their patterns (e.g., distribution, location, etc.) to recreate the blood shedding event. BPA practice on textiles has been hindered due to the highly distorted bloodstains resulting from complex surface properties and multi-physical processes involved. Here we report the findings regarding interpreting bloodstains on textiles for deriving the volume and impact speed of corresponding airborne porcine blood drops. 

“Dynamic” stains were created by gravity-driven perpendicular impact of airborne blood drops upon three types of cotton fabrics (i.e., plain-woven, twill, jersey knit), with seven different impact speeds between 0.3 and 6.2m/s. On the other hand, seven different volumes of blood drops (0.1 - 16µL) were dispensed by using a micropipette to create “static” stains on those three distinct fabrics. For “dynamic” stains, the complete stain formation processes were also recorded by utilizing a customized multiscale imaging system, which enables a transient analysis of the stain characteristics and underlying physics. In total, a database of 75 “dynamic” bloodstains, 63 “static” bloodstains, and affiliated video footages of the stain formation process has been constructed, for investigating the yet missing link between the airborne blood drops and resultant bloodstains on textile surfaces at the scene.  

The acquired data set provides the forensic community benchmark data for validating BPA on specific fabrics and new perspectives on the role of drop volume and impact speed in bloodstain formation on textiles. Analysis validates, for the three types of fabrics, that the bloodstain area is independent of the airborne blood drop impact speed when no splashing occurs upon impact. The wicking process significantly altered the bloodstain formation on absorbent textiles, making it hard to retrieve the information of airborne blood drops from the observed stains. The observed final bloodstain area or stain factor was strongly affected by the fabric type but weakly dependent on the blood drop impact speed.    

A new irregularity factor was proposed to quantify the bloodstain irregularity, of which the dependence on the blood drop impact speed was demonstrated. This suggests the utilization of a new quantitative parameter (i.e., irregularity factor) to reconstruct the drop impact speed when practicing BPA on absorbent textiles. Analysis of the “static stain” data evaluates the appropriateness of extending a nondestructive approach, which was proposed to determine the original drop volume for large drops (drop volume > 30µL) only on knit fabrics, to typical spatter drops (drop volume < 16µL) on a broader range of fabrics. This analysis also suggests that it may be possible to estimate the impact velocity by counting the number of ejected drops, when they occur.

Detailed Learning Objectives

1. Attendees will become aware that at the moment a drop impacts a textile, the behavior is the same as on other surfaces. Thereafter, wicking masks this detail. 
2. Attendees will learn that the final area of a bloodstain is directly proportional to the original drop volume. 
3. Attendees will recognize that the number of drops ejected by the impact event is proportional to the square root of the Weber number. 

Presenters

• Tiegang Fang, Ph.D. | Professor, North Carolina State University
• Stephen Michielsen, Ph.D. | Professor, School of Fashion and Textiles, Royal Melbourne Institute of Technology (RMIT) University