Nanoparticle Tracking Analysis in 2D: Camera-Plane Brownian Motion

Sample & Medium

Mean vesicle diameter 120 nm

Polydispersity (size spread) 0.25

Temperature 298 K

Medium viscosity 0.89 cP

Drag the view to pan, scroll to zoom the imaging field.

Diffusion (Stokes–Einstein, 2D camera plane)

D theoretical

0 µm²/s

D measured (2D MSD)

0 µm²/s

Size from tracking

— nm

Track time

0.0 s

Live Classification

Exosomes (30–150 nm)0%

Microvesicles (150–1000 nm)0%

Apoptotic bodies (>1000 nm)0%

How it works

This is the 2D companion to the 3D NTA simulator, computed independently rather than a flattened render of the 3D scene. A real NTA camera never sees the full 3D trajectory of a vesicle — it only ever records the particle's position projected onto the 2D imaging plane, frame by frame. This simulator models exactly that: every vesicle diffuses with the same 3D Stokes–Einstein diffusion coefficient as the physical particle, but only its (x, y) camera-plane position is tracked and drawn.

Stokes–Einstein (unchanged, still a 3D relation):  D = k·T / (3·π·η·d)
2D projected mean-square displacement:  <r²(t)> = 4·D·t   (not 6·D·t — that's the 3D relation)
Inverted for size:  d = k·T / (3·π·η·D_measured)

The factor of 4 instead of 6 is not an approximation: a 3D isotropic random walk has each axis independently accumulating variance 2·D·t, so the full 3D displacement (x,y,z) sums to 6·D·t while a 2D camera that only resolves x and y sums to 4·D·t. This was checked numerically — simulating a full 3D random walk and measuring only its (x,y) projection converges on 4·D·t, matching the closed-form result and confirming the 2D inversion formula used here (and by real NTA instruments, which are inherently 2D-projection devices).

Mean diameter / polydispersity — regenerate the population from a log-normal size distribution and restart tracking, exactly like loading a fresh sample.
Temperature / viscosity — change D live for every vesicle already in view (e.g. plasma is more viscous than water and slows diffusion measurably).
D measured — the ensemble 2D mean-square displacement accumulated since the last reset, divided by 4t, exactly how a real NTA instrument (NanoSight, ZetaView) estimates D from tracked camera trajectories.
Size from tracking — D measured inverted back through Stokes–Einstein into a hydrodynamic diameter; it should converge toward the mean diameter slider as tracking time grows.
MSD-vs-time strip — plots the running ensemble MSD (dots) against the theoretical 4·D·t line (dashed), a genuinely 2D-native diagnostic with no equivalent in the 3D orbiting view.
Size histogram strip — the population's live log-normal size distribution, colour-coded by MISEV class.

The MISEV guidelines use exactly this kind of size distribution, together with protein markers, to separate exosomes (30–150 nm, from multivesicular body fusion) from larger microvesicles (plasma-membrane budding) and apoptotic bodies — the classification panel bins the current population the same way.

220 particles trackedImaging plane: 24×24 µm

Exosome Microvesicle Apoptotic body