Surface Tension
Definition
The cohesive force at a liquid's surface that lets a drop hold a spherical shape in flight. Whole blood sits around 58 dyn/cm at 37 degrees Celsius, close to water but not identical.
- Field
- Bloodstain pattern analysis
- Whole blood value
- About 58 dyn/cm at 37 degrees Celsius
- Water comparison
- Close to water but not identical
- Effect
- Lets a falling drop hold a spherical shape
Common questions
Why does the difference between blood's surface tension and water's matter for bloodstain analysis?+
Analysts sometimes use water in training exercises or preliminary testing because it behaves similarly, but the slightly different surface tension, along with blood's higher viscosity, means stain size, satellite spatter, and edge characteristics from water are not perfectly interchangeable with real blood. Casework interpretation relies on reference data collected using actual blood, not water substitutes.
How does surface tension influence the shape of a bloodstain on impact?+
Surface tension pulls the falling drop into a sphere during flight, and on striking a surface it governs how the drop resists breaking apart, contributing to the characteristic circular or scalloped-edge stain along with impact angle and surface texture. Lower surface tension, such as from added detergent, would produce more irregular spatter, which is why contamination is a documented concern in casework.
Related terms
- Area of Convergence
- The 2-D point on a surface where the projected trajectories of multiple impact stains intersect. It's the floor-projection of the impact source.
- Area of Origin
- The 3-D point in space where the blood-shedding event occurred, derived from the area of convergence plus the angle of impact for...
- Cast-Off
- Blood thrown off a swung object (a hammer, a knife, a bat). The trail records the arc of the swing and often...
- Non-Newtonian Fluid
- A fluid whose viscosity changes with the shear rate. Blood thins under fast shear (a fast swing of a weapon) and thickens...
- Parent Stain
- The primary drop that produces a drip pattern. The smaller surrounding spatters are the satellite spatter; together they tell you about drop...