Bloodstain Patterns on Textile Surfaces: A Fundamental Analysis

Despite the fact that Bloodstain Pattern Analysis (BPA) is now a well-established forensic discipline, there remains much to do to establish the under-pinning science of the discipline, especially for textiles. So far most of the research effort has been on patterns formed on hard, non-porous surfaces. Surprisingly, no comprehensive and fundamental research on the more complex problem of bloodstain patterns on textile materials has been undertaken to date. When blood is deposited on fabrics it tends to migrate along the fibers of the fabric (wicks). Bloodstains on these surfaces have not lent themselves to the same degree of analysis due to the strong interaction between the blood deposition process, the fabric structure, and the surface properties. Further complicating BPA on textiles is the wide range of textile materials and their surface treatments. These include fabric construction, yarn construction, and fiber types. A fundamental analysis of the deposition of blood onto fabric structures and the subsequent movement of blood into or across the fabric is critically needed to enable models to be developed that could ultimately define the scope and limitations of BPA on textiles. The overall goal of this study is: To provide a fundamental understanding of the complex interactions between blood and textile materials. The basis for this project is the hypothesis that an understanding of the fundamental mechanics of the bloodstain pattern formation process is essential to the reliable classification of the resulting bloodstain pattern. This is particularly true for the more complex problem of classifying bloodstain patterns on fabric surfaces. The overall experimental strategy therefore will be to study the mechanics of the formation of a series of bloodstain patterns on a range of the most common fabric surfaces used in the clothing and home furnishing industries under a range of controlled and relevant conditions. Three workpackages along with detailed fabric characterization are proposed. These are organized to address the three main bloodstain pattern types: drip, spatter and transfer. Each of these workpackages will be executed in two phases. In the first phase, only woven fabrics (e.g., plain woven cotton used in bed sheets) and cotton twill (e.g., denim) will be studied. In the second phase knit fabrics (e.g., those used in cotton jersey knit T-shirts) will be studied. These tend to be much more open, but also much less dimensionally stable than plain weave textiles. The more open structure of most knits should enhance wicking of blood into the fabric, but their dynamic behavior may greatly alter the shape and dimensions of a blood drop. Blood will be dripped, spattered, and transferred onto the fabric and the motion of the blood will be recorded using high speed imaging to capture the initial deposition phase and time-lapse photography to characterize the subsequent spreading and drying phase. The final pattern will be analyzed based on the fabric structure and the expected wicking behavior, with the aid of both conventional and laser scanning confocal microscopy. In addition, since the fabric behaves as a filter, the motion of the cells will also be analyzed to determine direction of flow and whether other liquids interfere or alter the BPA. Finally, fluid mechanics and the capillary structures inherent in the fabric structure will be analyzed to develop fundamental models of BPA. The key deliverable of this study will be new data that characterizes bloodstains on fabric, including a new understanding of the fundamental science behind the mechanics of blood drip, spatter and transfer on the most common woven and knit apparel fabrics, and quantitative models to predict the wicking behavior of blood into these fabrics. ca/ncf