Whole blood is a suspension of cells of different density in plasma. A centrifuge replaces weak gravity with a strong, controllable centrifugal field so the suspension separates in minutes instead of hours. This is exactly how a lab measures hematocrit (packed cell volume, PCV) — the fraction of blood volume occupied by red cells.
Relative centrifugal force:
RCF = 1.118×10⁻⁵ · r · N² (× g)
r = rotor radius (cm), N = rotor speed (RPM)
Stokes settling velocity of a particle in the spinning tube:
v = 2·a²·(ρ_cell − ρ_plasma)·g_eff / (9·η)
a = cell radius, η = plasma viscosity, g_eff = RCF·g
This 2D version computes that settling velocity directly and numerically from real radius, density-difference and viscosity values (verified with a standalone script: at 12,000 RPM / 10 cm radius / default parameters it works out to ≈27 mm/s for a red cell — a few km-of-column per hour), rather than the fixed 1×/0.42× speed ratio a purely visual build might use. Denser, larger red cells (ρ ≈ 1.10 g/mL) settle fastest and pack at the outer end of the tube first. Lighter white cells and platelets (ρ ≈ 1.065 g/mL, the buffy coat) settle more slowly and form a thin band on top of the red-cell column. Plasma (ρ ≈ 1.025 g/mL) stays on top. Because v ∝ g_eff ∝ N², doubling the rotor speed roughly quadruples the RCF and cuts spin time accordingly; because v ∝ a², a slightly larger red cell (macrocytosis) settles noticeably faster than a smaller one (microcytosis) at the same speed.
- Rotor speed — sets the RCF (×g) applied to the sample; higher speed settles cells faster and more completely.
- Patient RBC volume — the true hematocrit the sample was drawn with (anemia ≈ 20–35%, normal ≈ 38–50%, polycythemia ≈ 52–60%); this is the value the readout should converge to once separation finishes.
- Rotor radius — the physical swing-out radius of the rotor; RCF scales linearly with it, so a larger rotor reaches the same g-force at a lower RPM.
- Plasma viscosity — thicker plasma (as in dehydration or high plasma-protein states) drags on settling cells and slows separation; velocity scales as 1/η.
- Red-cell radius — a stand-in for red-cell size (macrocytic vs microcytic anemia); velocity scales as radius².
- Simulation speed — a time-lapse multiplier only; the relative order and final layer thicknesses follow the physics above.
- Measured hematocrit — computed live from the height of the packed red-cell column relative to the total column, exactly as a technician reads a spun capillary tube against a hematocrit chart.
Drag inside the rotor view (top panel) to rotate and tilt it — the four tube slots and the active sample tube swing into view from any angle, just like turning a real centrifuge rotor over in your hands. Real-world relevance: this same density separation underlies the microhematocrit test used to screen for anemia and polycythemia, and the buffy-coat layer it produces is where white cells are harvested for a manual differential count or flow cytometry.