1. Introduction
Blood is a versatile type of
evidence in forensic science investigations and can provide important
information, such as the ‘who’, ‘what’, and ‘how’ as it relates to
criminal investigations. For example, blood can be collected and used
for DNA analysis, chemically analyzed to identify drugs and other
substances, and physically observed to conduct bloodstain pattern
analysis (BPA) [1,2]. Whole blood is composed of red blood cells
(RBCs), white blood cells (WBCs) – which contain DNA, and platelets,
all suspended in a liquid plasma [3]. Blood is a non-Newtonian and
shear-thinning fluid, meaning its viscosity is dependent on the amount
of applied shear stress, which causes blood to become more liquid-like
at higher shear rates [4]. Bloodstains observed at crime scenes
display a large variability in appearance. For example, their size,
shape, distribution and colour are largely dependent on the mechanism,
environmental and surface conditions of their deposition and observed
degradation state (more commonly referred to as time since deposition,
or TSD) [1,5].
The drying process of blood begins immediately after exiting the body
[6] and has been summarized in detail by Sobac and Brutin [7] to
occur in three phases; the pre-gelation phase (Phase 0), the gelation
phase (Phase 1) and the post-gelation phase (Phase 2). In Phase 0, RBCs
begin to migrate toward the edge of the bloodstain where a desiccation
line begins to form. In Phase 1, a compaction front forms at the edge of
the bloodstain and moves inwards toward the center. Simultaneously, the
bloodstain desiccates inward, while a dark red ‘donut’ can be seen; this
donut is highly concentrated with RBCs and moves from the center of the
bloodstain to the outside. The donut begins to desiccate, and the center
of the bloodstain turns a lighter red. By this point, the edge of the
bloodstain is almost fully desiccated, and the donut shape has become a
solid mass; cracks have begun to form at the edge of the bloodstain and
propagate inwards. By Phase 2, the center of the bloodstain begins to
dry and cracks begin to form, while the rim becomes fully desiccated.
The remainder of the bloodstain then becomes desiccated and no further
changes are observed. Laan et al. [7] and Benabdelhalim et al.
[6] found a similar drying process analyzing blood pools
(~4 mL); however, the first phase consisted of the
bloodstain coagulating, and increased colour changes were observed,
including the pool changing to a black colour as it desiccated.
Droplet desiccation is influenced by a variety of parameters such as
packed cell volume (PCV%, the packed cell volume percentage in a blood
sample), surface wettability, temperature, and relative humidity (RH)
[8]. Larkin et al. [9] investigated the effects of PCV% on the
drying process and corroborated the drying mechanism described by Sobac
et al. [10]. In their study, it was found that a decrease in PCV%
increased the effects of Marangoni flow due to surface tension
differences, but did not influence drying time [9]. RH also plays a
key role in the drying process and phase separation of larger
bloodstains [6]; as RH increases, the transfer of water between its
liquid and gaseous state is limited by the increased concentration of
water in the air [11]. This decreased evaporation rate leads to a
variation in plaque formation in the rim, which are sections of the rim
that separate from the surrounding bloodstain to produce islands of
dried blood [11]. Temperature differences also influence the drying
time and morphology of bloodstains. Ramsthaler et al. [8] observed
an increase in drying times of bloodstains from 30 min (24 °C) to
upwards of 120 min (15 °C), and Pal et al. [12] observed
sharper-edged rings in bloodstains deposited at greater temperatures (35
and 45°C) compared to 25°C.
Degrading bloodstains have
been imaged by techniques such as scanning electron microscopy (SEM)
[13], atomic force microscopy (AFM) [14,15], and hyperspectral
imaging [16,17]. Surface profilometry is a technique used to measure
and analyze the topographies of small surfaces. Optical profilometry
provides a contact-free measurement using optical sensors, providing
detailed topographical information without touching the bloodstain
[18]. As a non-destructive technique, optical profilometry has
emerged as a useful tool in forensic science analyses. Alcaraz-Fossoul
et al. [19] showed that optical profilometry could visualize latent
fingerprints without pre-treating them, and in aging studies,
fingerprints were not required to be re-developed before each collection
point. Heikkinen et al. [20] used white light interferometry to
identify similarities and differences between tool mark samples and
further identified firing pins via impression details that could not be
identified using 2D imaging. Hertaeg et al. [21] used laser confocal
microscopy to collect height profiles of bloodstains with varying
concentrations of RBCs suspended in three different solutions: plasma,
phosphate buffered saline (PBS), and bovine serum albumin (BSA). Their
results corroborated previous findings that higher RBC concentrations
increase the amount of RBC deposition at the edge of the bloodstain
[21]. From this, we asked whether optical profilometry, using full
scan and centre profiles, can also be useful in monitoring time-wise
changes in degrading bloodstains. We investigated the changes in
bloodstain surface characteristics such as surface height, surface
roughness, and number of cracks and pits for small drip bloodstains over
the course of four weeks. In addition, we evaluated the influence of
small differences in bloodstain volume on the extent of these changes.