Non-Newtonian Fluid
Definition
A fluid whose viscosity changes with the shear rate. Blood thins under fast shear (a fast swing of a weapon) and thickens at rest, which is why cast-off stains have a characteristic tear shape.
- Defining property
- Viscosity changes with shear rate
- Blood under fast shear
- Thins (shear-thinning behaviour)
- Blood at rest
- Thickens
- Bloodstain effect
- Produces the tapering tail seen in cast-off stains
Common questions
Why does this property matter for interpreting cast-off stains?+
Because blood thins as it accelerates off a swung weapon and then behaves differently once airborne and decelerating, the resulting droplets elongate into a tapered, tail-first shape rather than forming uniform round drops, which analysts use to read the direction and number of swings.
Is every biological stain fluid non-Newtonian in the same way?+
No, other body fluids and diluted or degraded blood can behave closer to Newtonian, so shear-rate effects are most pronounced in fresh, relatively undiluted blood moving at the velocities typical of a swung object.
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...
- Parent Stain
- The primary drop that produces a drip pattern. The smaller surrounding spatters are the satellite spatter; together they tell you about drop...
- Surface Tension
- The cohesive force at a liquid's surface that lets a drop hold a spherical shape in flight. Whole blood sits around 58...