Functional Molecules and Materials Research Theme Seminar
Speaker - Professor Daniel Bonn, Amsterdam Universty
Title - CSI Amsterdam: the science of (blood) drop impact
Bloodstain Pattern Analysis is a forensic discipline in which, among
others, the position of victims can be determined at crime scenes on
which blood has been shed. To determine where the blood source
was investigators use a straight-line approximation for the trajectory,
ignoring effects of gravity and drag and thus overestimating the
height of the source. To include gravity and air drag into the models,
the maximum diameter a droplet that impacts on a surface needs to
be determined. However for a long time this was subject of
controversy, notably for high-velocity impacts of low-viscosity
liquids such as water or blood.
I will present our work on the drop impact for simple and complex
liquids and show that the controversy can be resolved by understanding that the spreading
behavior cannot simply be predicted by equating the inertial to either capillary or viscous forces,
since, for most situations of practical interest, all three forces are important. We determine the
correct scaling behaviors for the viscous and capillary regimes and, by interpolating between the
two, find a universal rescaling. This allows to include gravity and drag in the trajectory
calculation of blood drops on the crime scene also; the origin’s location can be determined
roughly four times more accurately than with the straight-line approximation.
Academic Host - Paddy Royall