Optical tweezers are a powerful tool in colloidal physics; they can be used to trap single colloids and drive them at constant velocity through a complex environment such as a viscoelastic fluid, a colloidal crystal or a glassy structure. The optical trap works as a spring so any displacement of the particles from the trap's center can be used to measured colloidal forces of the order of few pN (see left panel below). 

(Left) Force measurements using optical tweezer. (Right) Schematic of a colloidal particle optically driven through a crystals

We have measured the forces required to drive a colloidal probe through a crystalline lattice as a function of the driving angle, i.e. the orientation of the driving force with respect to the crystalline axis (see right panel above). Our results highlight that, while the underlying potential can be considered isotropic for small displacements, the discreetness of the lattice becomes important upon stronger deformations.

Forces measured when driving a colloidal particle through a crystal