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The Fluid Dynamics of Droplet Impact
on Inclined Surfaces with Application
to Forensic Blood Spatter Analysis

G. Paul Neitzel and Marc Smith


Crime scene reconstruction is the process of determining the events involved with a crime using the physical evidence present at the scene along with scientific methodology and deductive reasoning. Various techniques are used as part of this reconstruction process; including DNA evaluation, fingerprint identification, and the focus of this project, blood spatter analysis. In blood spatter analysis, the residual bloodstains left at the scene of a crime are used to determine the point of origin of a particular bloodletting event and the type of trauma that led to the bloodstain (e.g., gunshot, blunt force, sharp object, etc.). The determination of these factors assists crime-scene analysts in their efforts to establish the sequence of events that occurred during the crime.

Figure 1: Typical bloodstain patterns.
(Top) Near normal impact showing spines and satellite drops. (Bottom) Oblique impact to the right showing length and width measurements of the elliptical portion of the bloodstain.

Two typical bloodstains are shown in Figure 1. It is known that the size and shape of a bloodstain are caused by some of the physical conditions of the blood droplet at the point and time of impact and the roughness of the impact surface. Stain size is quantitatively described as the area of the stain's circular or elliptical shape, whereas shape is quantified by the stain's aspect ratio as well as the number of spines and satellite drops around the stain contour. A spine is a disruption in the smooth periphery of a stain whereas a satellite drop is a small drop that has completely separated from the main droplet and has landed away from the main bloodstain, i.e., a small splash.

Within this general framework of using the bloodstain size and shape to determine the blood droplet impact conditions, a number of different models and tools are used in the field. The use of specific correlations, strategies, software, etc. is dependent on the resources available to an examiner, which in turn are impacted by where and for whom the examiner works. As an example, the simplest technique is called stringing. This method starts by using the aspect ratio of a bloodstain Ar to determine the impact angle a of the blood droplet using the relation

sin(a)= W / L = Ar

where the impact angle a is measured from the plane of the impact surface and the width W and the length L of the bloodstain are indicated in the bottom image in Figure 1. Balthazard, Piedelivre, Desoille and DeRobert proposed this relationship in 1939 based on their experimental work, and it has become the predominant correlation in forensics analysis. Using this relationship, a ray is typically drawn in a straight line from the stain in the direction of the angle a. Repeating this process for a large number of individual droplet stains in a blood spatter pattern, the individual rays may converge in a nexus, or region of convergence. The point of origin of the bloodletting event is presumed to be this nexus. Additionally, under this method the speed of the droplet is determined by the size of the stain and the assumption that the volume of the blood droplet was "normal", where a “normal” droplet is defined as one containing 0.05 mL of blood. This technique assumes that all stains are created by droplets of equal volume.

Recently, more refined methods have been developed in an effort to increase the accuracy of the results. Ballistics software has allowed for a more rigorous application of physics in determining droplet flight paths. Using these tools, gravity, drag, and other forces acting on a droplet in flight may be taken into account. Correlations have also been developed that relate the number of spines to droplet impact speed. With these developments, the assumption of a droplet of "normal" volume is no longer required. Although these methods represent significant improvements over previous methods, they are still limited. A correlation is only valid for impacts within a certain range of impact conditions and for impacts on certain materials. Surface properties such as roughness, stiffness, and porosity affect spreading behavior and can either inhibit or promote spine and satellite formation. Thus, a more complete examination of the physics of droplet impacts may improve current blood stain examination techniques and lead to less ambiguity in the events surrounding crime-scene blood spatter analysis.

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The Biological Evidence Preservation Handbook:
Best Practices for Evidence Handlers

Technical Working Group on Biological Evidence Preservation

The Biological Evidence Preservation Handbook offers guidance for individuals involved in the collection, examination, tracking, packaging, storing, and disposition of biological evidence. This may include crime scene technicians, law enforcement officers, healthcare professionals, forensic scientists, forensic laboratory managers, evidence supervisors, property managers, storage facility personnel, lawyers, testifying experts, court staff members, and anyone else who may come in contact with biological evidence. While many of the recommendations relate to the physical storage, preservation, and tracking of evidence at the storage facility, this handbook also covers the transfer of the material between the storage facility and other locations and discusses how the evidence should be handled at these other locations.

This report is divided into five main sections that detail issues and make recommendations related to biological evidence storage, tracking, preservation, and disposition. A glossary, which provides standard definitions of the technical terms used in this report, follows these sections.

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New CSI and Forensic Job Announcements

The most comprehensive listing of Crime Scene Investigation and Forensic
employment opportunities on the internet! We typically have over 200 current listings!

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Crime Scene Technician I
Salt Lake City Police Department, Salt Lake City, Utah, USA

Final Filing Date: January 21, 2018
Responsible for the collection, preservation, and analysis of evidence at crime scenes, traffic accidents, and other police related incidents. Documents crime scenes by means of sketches/diagrams and using a variety of photographic techniques and equipment. Writes crime scene reports and maintains photographic files on all evidence collected. Locates, develops, photographs, and lifts latent fingerprints at crime scenes and from evidence collected and brought to the Lab for chemical processing.
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Crime Scene Specialist I
Little Rock Police Department, Little Rock, Arkansas, USA

Final Filing Date: January 22, 2018
To conduct crime scene searches to gather physical evidence to include latent fingerprints, hair, fibers, blood, projectiles, shell casings and tool and tire marks to assist in the arrest and conviction of individuals responsible for the crimes.
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Crime Scene Evidence Technician II
Palm Beach Police Department, Palm Beach, Florida, USA

Final Filing Date: January 22, 2018
Position is responsible for the collection and preservation of evidence in criminal investigations and performing identification services to the public. It is also responsible for responding to crime scenes for processing of evidence related to crimes.
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Forensic Scientist Trainee - Toxicology
Las Vegas Metropolitan Police Department, Las Vegas, Nevada, USA

Final Filing Date: January 24, 2018
Incumbents receive supervised training in the scientific laboratory examination of physical evidence and in the scientific interpretation of test results.
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Forensic Scientist I, II or III - Latent Prints
Kansas Bureau of Investigation, Topeka, Kansas, USA

Final Filing Date: January 30, 2018
The Kansas Bureau of Investigation is accepting applications to fill a Forensic Scientist position in the Latent Print Section in Topeka, KS. This position will be filled based on the candidate's successful level of experience.
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Medicolegal Death Investigator
Arapahoe County Coroner's Office, Littleton, Colorado, USA

Final Filing Date: Continuous
The Coroner's office at Arapahoe County Government has a vacancy for a Medicolegal Death Investigator. This critical position on performs a variety of forensic analytical duties in the investigation of deaths in order to determine the cause and manner of death.
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CSI and Forensics in the News

CSI effect is a myth: forensic detective series do not make criminals better at crime
"We can now dispel certain of the myths that have been coursing through the media and other publications for the past 20 years because we are able to state with relative certainty that people who watch CSI are no better at covering their tracks than other people," said Dr. Andreas Baranowski.
The Telegraph - Sarah Knapton - January 5, 2018

Lakewood Police use drone technology to increase efficiency
The city's policies state drone technology can only be used for specific operations including: Crime scene investigation: document, measure, locate and assist in investigation of a crime scene. Traffic collision investigation: document, measure, locate and assist in collision investigations.
The Suburban Times - January 4, 2018

Think You Could Convict A Criminal Using Virtual Reality? You Might Soon.
Virtual Reality isn't anything new but its use in the law courts system isn't mainstream...yet. Seeing VR in the courts is set to 'rapidly' change in the next few years according to FBI agents and VR specialists. Not a good thing for criminals for sure but not without issue either.
Forbes - Paul Armstrong - January 3, 2018

Lancashire students get a feel for crimes, scenes and investigations
Hidden away inside a shabby row of terraced houses are the scenes of multiple brutal crimes. But these crime scenes are not all they appear, and the staged scenes are in fact used to teach students from across the world cutting-edge forensic science techniques - and has directly benefited the force keeping the Fylde coast safe.
The Gazatte - Daria Neklesa - January 3, 2018

Elkhart police hope to have 3-D crime scene technology soon
The Elkhart Police Department is looking at the system known as the faro laser scanner. It can document a crash or crime scene to map out, measure and preserve exactly how it looked at that moment. It takes a three dimensional image and photographs as well.
WSBT 22 - Ed Ernstes - January 3, 2018

California Supreme Court seems split on DNA collections voters approved
The California Supreme Court appeared to be closely divided Wednesday on whether a voter-approved state law requiring police to take DNA samples of anyone arrested for a felony violates the right of privacy.
SF Gate - Bob Egelko - January 3, 2018

Formerly disgraced DC crime lab eliminates rape kit backlog
Three years after D.C.'s DNA lab lost its accreditation, the city is touting a total turnaround. The Department of Forensic Science is back up to speed and unlike other cities its size, D.C. has no rape kit backlog, officials say.
WTOP - Megan Cloherty - January 2, 2018

Douglas County builds new state-of-the-art evidence building
The Douglas County Sheriff's Office has opened their own state-of-the-art evidence and armor building after years of renting out an inadequate space. One-way-key evidence lockers, car lifts for evidence searches and storage for firearms and SWAT equipment all sit inside a high security building that the county now owns.
iFIBER One News - Madison Meyer - January 2, 2018

Top SC Cop Says State Needs New $54 Million Crime Lab
Each year, thousands of cases flow through South Carolina's top law enforcement agency. Chief of SLED, Mark Keel says the agency is now out of space to process evidence causing a delay in hundreds of case across the state. It's something Solicitor Kevin Brackett has seen first hand.
WCNC - Billie Jean Shaw - January 2, 2018

Columbia city council set to discuss crime scene investigator position
Columbia's city council is expected to discuss removing a police administration position to make room for a new crime scene investigator at Tuesday night's meeting.
KMIZ 17 - Barry Mangold - January , 2018

Forensic Sciences labs backlogged
There is a backlog of nearly 29,700 drug samples - nearly a year's worth submitted by hundreds of law enforcement agencies across the state - waiting for testing at Alabama Department of Forensic Sciences labs. There are other types of tests that have waited more than 90 days, too: 769 forensic biology tests waiting to match blood, saliva or semen to a suspected criminal or crime victim. 882 firearm and toolmark tests. 83 forensic pathology tests needed to determine causes or manners of death.
Times Daily - Mary Sell - January 1, 2018

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