Lab 3, Part A: Brownian motion


For this week, you will

  1. Familiarize yourself with our inverted microscopes.
  2. See Brownion motion of a small object (a micron-size bead)
  3. Record a short video of Brownian motion.
  4. Manually track one bead and plot its motion.

We will employ these on a larger scale (with automated tracking) in the next two weeks, to generate orders of magnitude better statistics on the Brownian motion of different beads under different conditions.

Materials and Process
  1. Set up a microscope, and connect to its video camera on the lab PC using the AmScope program.
  2. Find the beads on a slide at the highest magnification.
  3. Take 30 frames of video.
    1. Remember to write down the frame rate.
  4. Load video into Fiji.
  5. Manually track a bead for 30 frames using Fiji.
    1. Zoom in to high magnification while tracking.
    2. Obviously you will need to choose a single bead that remains in focus for all 30 frames.
  6. Export your tracking data and calculate the displacements { x-x(0), y-y(0) } and squared displacements. Plot your results as a function of time.
Lab Report

Grab a copy of this blank lab report to be filled out: lab-3a-blank

You will write a very short lab report. We simply want you to go through once the process of acquiring video on the microscope, exporting to ImageJ, tracking, and plotting. In the next two weeks you will repeat / automate this process to get much more data.

Include a plot of the x- and y- displacements (in pixels, unless you have performed a distance calibration) as a function of time, and also a plot of the x-, y- and r- squared displacements. That is, for your tracked bead, plot the quantities:





Eventually, once we have enough data to make meaningful averages, we expect to find that


However, with the limited data available at this point the average behavior may not be obvious. We will get more data in the next two weeks to refine our measurement.

Questions to be answered:

Does it look like the average displacement is zero over time, and that the average displacement squared is growing linearly in time? An opinion of a sentence or two is all we’re looking for here.