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Written by David A. Thornton   

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The Amazing World of the Blood Drop

INSIDE THE TINIEST blood drop lies an amazing interaction of biology and physics. Cohesion, adhesion, viscosity, surface tension,


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gravity, and the human body come together to form something as apparently simple as a single drop of blood. This relationship affects the flight characteristics of the drop and the formation of a stain.

Over almost a century and a half, bloodstain pattern analysis has evolved from the pages of Sir Arthur Conan Doyle’s mythical Sherlock Holmes into a multi-disciplinary field of forensic science. Understanding the rudimentary physics involved in the creation of a blood droplet can help analysts better understand the story that a bloodstain may reveal.

Blood is a connective tissue that is critical to life. Blood carries nutrients and removes waste in support of cells. The fluid also helps control body temperature and combat infections. Plasma—made up of 55 percent water—is the largest component of blood. Carried in the plasma are the erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). Typically packed into a cubic milliliter of blood are 5 million red blood cells; 5,000 to 10,000 white blood cells; and 200,000 to 300,000 platelets. In the average person, eight to nine percent of the body mass is blood, but life at high altitude can add as much as 1.9 liters to a person’s system.

Fully appreciating the blood drop requires a basic understanding of the physics involved in its creation. The behavior of blood outside of the body and the distribution of stains depend on fluid dynamics and physical laws that make the interpretation of bloodstains possible. A key property—viscosity—is simply the thickness or thinness of a liquid. Blood is indeed thicker than water. In fact, blood is four times more viscous than water. Blood viscosity is determined by the ratio of hemoglobin to whole blood: the hematocrit value. The higher the hematocrit value of blood, the higher its viscosity.

Blood is a non-Newtonian fluid, like quicksand. These fluids react differently than water or other liquids. The viscosity of a non-Newtonian fluid can change, rather than remain constant like the viscosity of water. Fluids running inside a pipe meet resistance from the sides of the pipe. Following Newton’s Law of Viscosity, in which shear stress and shear strain are proportional, doubling the velocity of a liquid moving through a pipe (shear stress) causes the resistance to increase proportionally (shear strain). An example of this is squeezing the trigger on a spray bottle. Making the stream move two times faster requires the trigger to be pulled two times harder.

Blood does not behave in this way. Blood’s viscosity changes because of increasing or decreasing sizes of blood vessels. This characteristic allows circulation to flow easily through the small capillaries and the much larger arteries. Blood viscosity varies from person to person. Increased levels of hemoglobin will increase viscosity. Consequently, people at high altitudes have more viscous blood. Hematocrit levels, gender, nicotine use, cholesterol levels, and physical fitness can also cause variations from person to person.

Wipe or swipe patterns in which an object either spreads existing blood or transfers blood to other surfaces is common. A relatively small amount of blood can contaminate a crime scene because of the inherent adhesive qualities of blood. Adhesion and cohesion play a significant role in not only the stickiness of a liquid, but also its surface tension.

Adhesion refers to the attractive forces between unlike molecules, and cohesion is the attraction of like molecules. Liquid molecules at the surface do not have like molecules on all sides. Cohesion causes the surface molecules to bond more strongly with each other than with the air, creating a surface film or surface tension. The electrostatic attraction of molecules and the resulting surface tension is measured in force per unit of length called dynes. The surface tension of blood is 50 dynes at 20°C compared to water, which is 72.8 dynes at the same temperature (Raymond et al., 1996). Surface tension plays a significant role in the development of blood drops and stain development because it tries to hold the blood together in a spherical shape despite the external and internal forces that are exerted upon it.

Surface tension is responsible for many of blood’s flight characteristics and the resulting stain size. Warmer liquids have lower surface tensions and are better wetting agents because the weaker surface tension allows the drop to break and penetrate the pores and crevices of the receiving surface. Surface tension also creates capillary action or the attraction of a fluid to a solid surface and its own surface tension. A classic example is that of a hand placed in a pool of blood and slowly lifted. Adhesion causes the blood to rise with the hand until the surface tension is defeated by the mass of the blood and the pull of gravity.

Wettability, or the degree to which a liquid wets a surface, is related to surface tension. When analyzing patterns, the target surface must be considered. Depending on the characteristics of the receiving surface, the same amount of blood can create different sized stains. Dr. Herb MacDonnell uses the analogy of a waxed car to explain the idea (MacDonnell, 2009). A drop of water on a polished car hood will bead up and roll off the edge without wetting the hood. Dust on the surface of a car hood causes the water drop’s surface tension to break and leave more water on the hood.

Cohesion and surface tension hold a mass of blood together to form a drop. Surface tension strives to keep the drop as small as possible. In flight, equilibrium of surface tension, hydrostatic pressures, and aerodynamic pressures balance to keep the blood in a drop. The formation of a drop is more complex than it may at first appear. As a drop of blood begins to form from a dependent position, such as venous blood from a motionless fingertip, the forces of gravity and surface tension cancel each other out. Gravity pulls down while surface tension pulls up. Eventually, the weight of the accumulating blood is enough to break the surface tension and a drop falls.

Despite the differences of physical properties of water and blood, the study of raindrops can provide insight into the behavior of blood in flight. Oscillations in larger raindrops have been observed since 1879. Blood’s viscosity reduces the drop’s wobble to less than that of a raindrop.

Although no general theory for the cause of the oscillations has been developed, it is believed that aerodynamic vortexes along the drop’s leading edges create the wobble. Thinking of a drop as a water balloon with the balloon representing the surface tension, the vortexes create a wave action and internal circulation within the drop. As the movement works to deform the droplet, surface tension forces the drop back into the smallest size. How does a drop’s deformation from a sphere to an unpredictable shape affect the interpretation of the stain? That is a key question.

Research has demonstrated that drops less than 1 mm in diameter maintain a spheroid shape, and drops 5 mm or larger fluctuate considerably. Because surface tension strives to hold the blood to the smallest size possible, an oscillating drop will always want to return to a sphere-like shape. Damping is the tendency to reduce the internal circulation of fluid inside the drop, consequently reducing the wobble. Blood being more viscous than water, the damping time in a blood drop is four times faster than that in a raindrop (Bevel & Gardner, 1997). Although blood drops do oscillate, they do so less dramatically than a raindrop.

Although it is described as a typical drop, MacDonnell’s 4.5mm, .0505 ml drop is in reality only typical for free-falling drops. In “medium velocity” stains, 4-5 mm stains produce about 2 mm drops. Being more than half the size of a typical drop, oscillations are less pronounced. Ross Gardner discovered that within .5 seconds of flight, oscillations were minimized. For small stains, such as those created from gunshots, the drop remained spheroid in flight (Bevel & Gardner, 1997).

Tom Bevel and Gardner concluded that oscillations did not significantly affect angle-of-impact calculations because blood drops created in crimes are typically less than 2 mm in diameter. As a result, the oscillations damp quickly and are almost spheres on impact with the target surfaces (Bevel & Gardner, 1997).

Angle-of-impact calculations are based on the assumption that the drops are spheres on impact. The droplet size does affect the distance that it travels. At the same velocity and flight path, larger drops will travel farther than smaller ones.

Other researchers are more cautious when considering the effect of oscillation. For bloodstains approximately 2 mm in diameter, drops within one meter of a surface will oscillate enough to cause some distortion to the impact-ing drop and consequently affect angle-of-impact calculations. Their recommendation is to be wary of stains believed to have originated within one meter and to use several stains to calculate angle of impact.

Analysts understanding the ballistics and the stresses affecting the flight of blood can better testify to their conclusions in court. Surface tension—the result of several properties of liquids (e.g. adhesion, cohesion, and viscosity) plays a major role in the formation, flight, and impact of a blood droplet. Certainly, interpreting bloodstains and appreciating their evidentiary value requires greater understanding than the simple calculation of impact angles. Understanding how things work is as important as making things work. For bloodstain pattern analysts, this begins with the medium of blood.

About the Author

David A. Thornton is a crime-scene investigation training consultant with Thornton Consulting & Investigation in Thornton, Colorado. He has 17 years of law-enforcement experience, is a professional educator and law-enforcement trainer, and is a Certified Senior Crime Scene Analyst with the International Association for Identification. He can be reached at: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

Works Cited

Bevel, Tom and Ross M. Gardner. Bloodstain Pattern Analysis: With an Introduction to Crime Scene Reconstruction. Boca Raton, FL: CRC Press, 1997.

MacDonnell, Herbert Leon. “Cohesion, Wettability, and Blood Drops that Land on a Smooth, Hard Surface,” International Association of Bloodstain Pattern Analysts News, 2009: 25(3), 4-7.

Raymond, M. A., E. R. Smith, and J. Liesegang. “The Physical Properties of Blood-Forensic Considerations,” Science & Justice, 1996: 36(3) 153-160.

 
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