2 Nov 2017

The physics of blood spatter

From Our Changing World, 9:06 pm on 2 November 2017

Engineer Mark Jermy, at the University of Canterbury, specialises in fluid dynamics, which means he’s interested in the way gases and liquids move.

He’s worked on industrial applications, such as the way fuel and other sorts of liquids behave in a spray.

Then one day a forensic expert knocked on his door, and asked Mark if he could take his knowledge of physics and apply it to sprays and droplets of blood at a crime scene.

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Blood spatter is a common piece of evidence found at crime scenes

Blood spatter is a common piece of evidence found at crime scenes. Photo: CC BY 2.0 tanakawho/Flickr

The forensic scientist was Michael Taylor, an expert in blood spatter and blood stains at ESR. He started working with Mark to develop training for forensics staff attending crime scenes so they could better analyse evidence and present it in court.

“Droplet impact has been studied [industrially] for years. It’s really important for cooling, like if you’re spraying water over something to cool it down,” says Mark. He says the printing industry, for example, has done a lot of research into how to generate ink droplets of different sizes and what happens when they hit the surface of a piece of paper.

Marks says that when a drop of fluid hits a surface it is roughly spherical, or ball-shaped. How far and how evenly that droplet spreads - whether it makes a nice circular pattern or whether it splashes into fingers or smaller droplets – depends on its speed, size, as well as the density and surface tension of the fluid. Blood has a higher surface tension than water, and is also more viscous.

Mark says the viscosity of blood varies between individuals – the blood of a person with anaemia is thinner than the blood of someone with a high red blood cell count. The viscosity of a single blood drop can also change during a splashing event, as the red blood cells disperse .

Mark Jermy

Mark Jermy Photo: UCNZ

Forensics investigators at a crime scene have to calculate where the blood drops originated from, and Mark says it is much more difficult to solve a reverse physics problem like this.

“If you know that a blood drop hit the wall at this point, well, there’s many different paths that can lead it there,” says Mark. “Calculating backwards is much more difficult, and falls into a class of problems known as inverse problems.”

Investigators get around this problem by tracing the trajectory of many droplets back to where they cross, which is the probable source.

It is more difficult when there is more than one source of blood splatter, and another challenge is to differentiate between blood caused by an impact wound, and that produced by someone shouting, coughing or breathing out blood.

ESR is currently carrying out research in this area using pig’s blood or synthetic blood as an alternative, and Mark says they discovered early on that pig blood varies much more between individuals than human blood does.

Another contribution Mark and his students have made in this area is to design machines that produce blood splatter, as well as come up with more realistic alternatives to body part such as brains that can be used in experiments