01 September 2015

John Spavin

Engineering Insight: Whodunnit?

  • Blood spatter patterns and fluid dynamics
  • A hammer impacting a pool of blood sends droplets radiating out from the point of impact to create a pattern of stains on the wall behind. Image: Mark Jermy.
  • Stills from a high-speed video camera show a droplet of blood splashing on the experimenter’s hand. Photos: Mark Jermy.

Engineers aren’t usually the people you’d think of in the context of crime scene investigations. Yet they have much to offer – especially when blood is involved.

Forensically examining the circumstances surrounding battered, bruised or dead people might not sound like the average job. For many Institute of Environmental Science and Research (ESR) scientists and police, attending violent crime scenes is all in a day’s work. Backing their attempts to find out what happened is Associate Professor of Mechanical Engineering at the University of Canterbury, Mark Jermy. He doesn’t attend the crime scenes, or testify in a court room. Instead, he researches and teaches the engineering and science that helps determine the story of how bloodstains at crime scenes are formed.

Forensics generally interest people. Consider the popularity of the many crime thrillers that screen each week on television where the bloodstained victims litter the sets and simple plots produce instant results, all neatly wrapped into a commercial hour. Dr Jermy’s research goes way beyond that popularised Hollywood view bringing science and engineering, computer modelling and fluid dynamics to bear in pursuit of finding out what happened to victims of violence. With Dr Michael Taylor, a Senior Forensic Scientist at ESR who instigated the forensic research programme and course at Canterbury, he uses that experience to train police officers and forensic scientists so they understand the evidence bloodstains leave at the site
of a serious assault or homicide.

The science can also help unravel the type of weapon, the time of the event, the amount of force used and, importantly, the location of victim and assailant.

Fluid mechanics provide key information

Along with ESR and researchers at Canterbury, Auckland and Otago universities, Dr Jermy’s work aims to advance the forensic study of bloodstains. This involves using his expertise on fluid mechanics, a field that, until recently, wasn’t widely applied to the subject. He’s a long way from Cranfield University in England, which specialises, among other technical fields, in automotive research. There, Dr Jermy was studying the fluid mechanics of fuel sprays; measuring droplet sizes and velocities as they were injected into Jaguar’s engines.

He moved to Canterbury in 2005, and out of the blue, Dr Taylor called and asked him how he would feel researching the behaviour of blood droplets. It was something he had never considered, but having accepted the work, he wasn’t fazed at the change from studying liquid fuels to bloodstains. “As I later found out, Michael has played a lead role internationally in getting more scientific research into bloodstain analysis” he says.

Pig’s blood is the liquid of choice when forensic scientists study what happens when someone is stabbed, shot or bludgeoned in a violent assaults or murder. Researchers dilute the pig’s blood slightly to adjust its viscosity to that of human blood.

The crime scene

Blood, scientifically and forensically speaking, doesn’t just deposit itself at a crime scene in a standardised flow and drip: it spatters, splashes, drips and oozes across walls and floors and concrete. It seeps into clothing and gets absorbed differently by, say, a polyester shirt or denim jeans. Understanding the patterns of drips and stains – fluid mechanics – can give insight into why droplets fell where they did, and may help seal a conviction or absolve an innocent. It’s all about understanding how human blood behaves under violent forces. Human blood, emotion aside, is a fluid and it behaves as one, just like petrol.

“The mechanism by which the fluid is broken up into droplets is a little bit different but the same principles apply,” Dr Jermy says.

The essential piece of equipment in this work is a high-speed video camera. Blood, in this case, moves so fast under the influence of a blow that the naked eye can’t follow it and hope to recognise a pattern. To slow the fluid’s movement on video, the camera exposes its path at up to 20 thousand frames per second; about 800 times the normal frame rate.

Dr Jermy, with Drs Patrick Geoghegan and Mathieu Sellier of the University of Canterbury and other colleagues, also studies how blood seeps and spreads on different materials. He says this is because the one thing that almost everyone brings to a crime scene is clothes. There are also the fabric coverings on furniture, curtains and carpets.

Along with other forensics fields, there has been renewed interest and effort in analysing bloodstains in the past few years. This followed a United States Academy of Science report in 2009, entitled Strengthening Forensic Science in the United States: A Way Forward. The report found the state of scientific knowledge in a number of forensic disciplines wanting and called for better certification methods, better training and for forensic laboratories to require accreditation. Its pursuit of improved forensics and more specialised research has been echoed internationally, and has resulted in many countries making greater efforts to advance the forensic examination of crime. Dr Jermy’s fluid dynamics expertise is a manifestation of that general desire for improvement.

Training crime scene personnel

The sound implementation of scientific principles and training applied to crime scenes is vital for any police force. They use this knowledge to investigate crimes and take the accused to court with confidence that the case may be proven beyond reasonable doubt.

When he trains those in the front line who examine crime scenes, Dr Jermy has them for a week. It’s not long but it’s intensive. Participants include forensic scientists and scene of crime officers from all over the world. They receive a crash course in fluid mechanics and how stains form.

Most of Dr Jermy’s work involves teaching undergraduates and supervising PhD and Masters students. He says taking on a room full of business-like scene of crime officers to try to impart some fairly complex science to them has tested his teaching abilities: “We’re teaching people often with little prior knowledge of fluid mechanics ... and we have to teach them a lot of difficult concepts within a week.”

On the plus side, these courses are full of extremely motivated and very dedicated people. “They come willing to work – very hard,” he says.

“What has impressed me about this discipline is that they are constantly revising their procedures and practices, to try to get more certainty.”

Hard work is routine for scene of crime officers. The TV shows portray the ability of forensic success in crime solving in a “very optimistic” fashion, Dr Jermy says.

“TV portrays that you can get a lot of information from very small samples or very small clues and that’s not really true.”

Just part of the puzzle

He says his specialty of fluid dynamics is a small part of a big puzzle, the clues to which may include information gleaned from DNA, examination of fibres, eyewitness accounts and the like. While the seductive attraction of television crime shows may have oversimplified the task of the forensic scene examiner, they have certainly roused the interest of the public; maybe too much.

“They have also raised the expectation of jurors, and in some cases judges, beyond what’s reasonable,” Dr Jermy says. Instead, the aim of his work is to extract forensic information with more certainty and to be able to say when reliable information is not present.

When considering his future, Dr Jermy says he has years more work to complete to study fully how blood behaves under trauma. “We’re a long way from working it out,” he says.