Experimental and theoretical studies of the spreading of bloodstains on painted surfaces

The spreading of bloodstains on matt, vinyl silk and gloss painted surfaces, following both perpendicular and non-perpendicular impact, has been studied with a view to understanding whether surface-specific properties affect the size and shape of the final stain. In all cases the dimensions of the stains on the rougher, matt paint are up to 5% less than on the other painted surfaces though this difference decreases with impact velocity. Both the diameter of circular stains and the width of elliptical stains have been successfully modelled as a function of the perpendicular component of impact velocity vn using a modification of the energy conservation approach through the inclusion of a single, empirical constant characteristic of the surface itself. By further development of this theory, the characteristic dynamical length parameter which governs the spreading has been modified to be dependent on angle thus enabling the length of elliptical stains to be predicted independently for the first time. For all these surfaces, experiment shows that the impact angles, calculated using the conventional trigonometric formula, are in deficit by around 1° at high vn increasing to around 5° at lower vn. By modelling this deficit independently using the theoretical expressions for stain width and length, this variation with vn has been successfully explained. This work has demonstrated some of the fundamental systematic sources of error in using the conventional formula for interpreting elliptical stains and established some of the basic theory on which to develop the interpretation of casework stains on surfaces, in the future.