  We describe a method to track particles undergoing large displacements.
Starting with a list of particle positions sampled at different time points, we
assign particle identities by minimizing the sum across all particles of the
trace of the square of the strain tensor. This method of tracking corresponds
to minimizing the stored energy in an elastic solid or the dissipated energy in
a viscous fluid. Our energy-minimizing approach extends the advantages of
particle tracking to situations where particle imaging velocimetry and digital
imaging correlation are typically required. This approach is much more reliable
than the standard squared-displacement minimizing approach for
spatially-correlated displacements that are larger than the typical
interparticle spacing. Thus, it is suitable for particles embedded in a
material undergoing large deformations. On the other hand, squared-displacement
minimization is more effective for particles undergoing uncorrelated random
motion. In the Supplement, we include a flexible MATLAB particle tracker that
implements either approach. This implementation returns an estimation of the
strain tensor for each particle, in addition to its identification.
