2d diffusion simulator with particle track option

Simulates Brownian diffusion with the option to show tracks of two particles.

This is a modified version of an earlier program by Maxim Vedenyov (https://www.mathworks.com/matlabcentral/fileexchange/34103-2d-diffusion-simulation--gui). The code has been modified to optionally show the tracks of two particles whose motion is being simulated.