A fabric’s physical properties, such as its wettability and porosity, affect the dynamics of an impacting droplet. Understanding these effects can help forensic scientists draw a better picture of a crime scene. In a paper that appeared in Physics of Fluids, UvA-physicist Thijs de Goede and collaborators from UvA, ETH Zürich, Empa (Switzerland) and the University of Sherbrooke (Canada) develop a physics-based approach to blood droplet analysis.
Picture a crime scene: Different types of fabrics, all stained with drops of blood. Some of the stains are little, splattered threads; some have soaked through the cloth. How can forensic scientists decipher meaning from the shapes of these stains?
To help solve this problem, de Goede et al. experimentally and computationally observed droplets impacting onto fabrics. They identified a number of key parameters in determining how and how much the droplet will spread: its velocity, the fabric’s wettability, and the size and density of the pores in the fabric.
The team compared droplet impact on three polyester materials with different pore sizes and yarn thicknesses, as well as a solid steel surface and a steel surface patterned similarly to one of the fabrics. After verifying their numerical simulations with experimental data, they also examined the impact of a drop on a fabric placed atop another surface, and one suspended in air.
“This research was expressly done to try and answer an important forensic question: Can you distinguish – for instance, on a piece of clothing – a blood stain from the impact of a blood drop from a stain that originates from contact with a bloody object?” said author Daniel Bonn.
Since the present work only looks at one particular type of weaving, additional studies are needed in order to obtain more definite conclusions.
“The question remains how our findings can be generalized to any type of clothing or fabric – there are often blood stains on carpets also – and how this can be used in practice by forensic scientists on a crime scene,” said Bonn.
Droplet impact of Newtonian fluids and blood on simple fabrics: effect of fabric pore size and underlying substrate, Thijs Christiaan de Goede, Ali Mazloomi Moqaddam, Karlijn Limpens, Stefan Kooij, Dominique Derome, Jan Carmeliet, Noushine Shahidzadeh and Daniel Bonn, Physics of Fluids 33, 033308 (2021) .