Improved Tracking Of Brownian Milk Globules

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Improved Tracking Of Brownian Milk Globules<br>My thought process in improving a Brownian motion dataset.<br>CasualPhysicsEnjoyer<br>Jun 10, 2026

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In my last post I tried to use particle tracking on other peoples' videos on Brownian motion. But the problem was that the footage was short and the current tracking software I tried was bad - the trajectories kept getting mixed up and duplicated. But I think this was a function of the quality of video.

So then I tried using my own microscope and milk with 40x magnification, which I outlined here. Attaching the camera to my mac meant I could record with better quality, with a higher frame rate.<br>Along with the improved footage, I tried writing better tracking code. This seems to have worked. There looked like less particle switching, and the dataset was longer. I managed to capture 5 minutes of video, so at least I got more content out of it. There are less duplicate tracks for the same particle too.<br>Here is a zoomed in example of tracking on my footage.

I'm not sure why there are some particles darker than others. Also, the milk that I used was homogenised and you can still see size variation.<br>Here is the whole view at once. The tracking is better, but there is drift from the force of the cover slip spreading the milk and causing a shear force. So I need to let it settle next time.

This is footage of the 100x magnified globule without immersion oil

The particle tracking didn't work so well for this one though.

From the 40x magnified tracking, here is a graph of the average displacement against √t. For Brownian motion |r| = √(4D)·√t (a relation I derived previously), so this should be a straight line through the origin . The drift-corrected points (orange) sit on the line up to ~1 s, then bend upward. You can see when the series is de-trended for the drift, it initally obeys a brownian motion relation but there is still residual drift!<br>I need to figure out why there is still residual drift. So my next experiment will be figuring how to get the milk globules to settle on the slide properly.

The code is in my github here.

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