Honorary CSI

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On nightshift in ER you get to know a lot of cops, and it’s helped me with a few tickets. I help them too, with a stitch here or an antibiotic there for an embarrassingly contracted disease they don’t want the Police Department to know about. I have even helped patch a bullet hole in a cop I was friends with, and I admire anyone who puts his life on the line for ordinary people. So now, when Detective Franklin asked me to help him out, I was glad to try.

Franklin had known me for years now, way back to when I had first started in the ER as a college student cleaning equipment to earn my tuition. He was always nice to me, even back then when I was on the bottom of the totem pole so to speak; and as I finished college and had gone onto medical school he had followed my progress.

            “Rob it’s the weirdest thing. The victim died of stab wounds. He also had a knife in his hand, but it was clearly not suicide. At the murder site, we have lots of blood spatter evidence, and the new CSI’s are working it out, but we are confused by something. One set of drops seem to have been manipulated somehow.  A series of drops seems to have been hollowed out, almost as if the perp just scraped out the center of each drop. We don’t know what to make of it.”

Our small city in southern New England had just hired some new CSI trainees to start a Crime Scene Department, and I knew our local guys did not want to call the State Lab for advice if they could avoid it.

            “What’s the angle of impact Frank?” The angle at which the blood droplets strike an object can be inverted and traced back to its source. It can also tell you if the object causing the spatter was moving or not.

He took out a Polaroid and showed a pic of the drops he had mentioned. They looked like little hollowed out red crowns, round on one side, hollow in the middle, and the ridges of the crown on the opposite side. I knew immediately the angle of impact was about 45 degrees, because if it was less than 45, the ‘crown shape’, would become more like an oval with an exclamation point over it. I knew this instinctively because my ER job for the first two years I had been in college was cleaning the resuscitation equipment, and most of what I cleaned was blood. I had observed enough bloody Codes to be able to categorize the spatter patterns in my mind. Later, I actually read a book for forensic investigators on blood pattern analysis on my summer break. It’s amazing how physics can predict the way a viscous fluid like blood will behave.

Here is a reproduction of the hollowed out blood drop Dective Franklin showed me. There were twenty two others almost just like it.

I knew I had seen something like this before. I had to sit back and close my eyes for a bit because it had been years since I had been concerned with blood drops.  My primary job was now stopping patients from bleeding, not cleaning it up. Then it came to me. I had seen it years ago while cleaning up a respirator. I  knew the answer and told Detective Franklin.

The answer is below, but as an exercise worthy of a premiere CSI, try to figure out what could make blood drops hollow.

ANSWER

The crime scene was a knife fight, and the one who got away had been stabbed in the chest. He coughed up frothy blood drops while running from the crime scene. The hollowed out drops were caused by air bubbles in  the blood drops! As the drops dried, the bubbles evaporated, and only their hollowed out imprint remained in the drops.  Franklin found the man in a nearby hospital  with a stab wound to the chest.

Below is a diagram explaining how the angle of impact determines the shape of a blood drop. For those of you who may be interested in the math, here is the equation:  Sine (impact angle) = W/ L     (where W = width of the drop, and L = length)

Here is the diagram :