Particle tracking using IDL -- John C. Crocker and Eric R. Weeks (this page written primarily by Brenton Hoffman)
Home | Download software | Tutorial | Extra software

Click here to return to microrheology summary page


The purpose of this program is to take the two point correlation tensor data generated by and turn it into distinct mean squared displacements. These are analogous to normal mean square displacement but are generated from correlated particle motions. In a hypothetical system where tracer particles are trapped in pores in an other wise homogeneous material, the distinct mean square displacement could be thought of as the msd the particle "should have" if the particles were probing bulk mechanical properties. This code assumes that the material being probe has a Poisson ratio of 0.5.


The code requires a two point correlation tensor from, either as a filename or IDL variable and the particle separation distances to be used as determined by the spatial behavior of the two point correlation tensor (see tutorial for



IDL> md=msdd(m2,2,8,a=0.5,tmax=1,/lfit)

where m2 is a two point correlation tensor, the minimum and maximum particle separation distances are 2 and 8 microns respectively, the assumed particle size is 0.5 microns, a maximum time of 1 second, and a linear fitting of the data will be used.


The program returns a data structure which is (6,number of time points) where the
           (0,*) column is the time points, in seconds
           (1,*) column is the msdd  generated from Drr component, in um^2
           (2,*) column is the msdd generated from Dtt component, in um^2
           (3:4,*) is the standard error in the msdd from the Drr,Dtt components, in um^2
           (5,*) column contains N's appropriate for weighted averages 

Note: errors may be greater than expected due to correlation effects in the data. Also, error estimates and N weighting are only approximate when using 'lfit'.

This program was written by John, and this webpage was written by Brenton Hoffman, a student working with John (with some editing to html-ize it by Eric).

Click here to return to microrheology summary page

Contact us