Colloidal glasses show viscoelastic properties with increasing packing fraction, particularly close to the glass transition. Recent experiments close with self-propelled particles in quasi-2D suspensions of hard colloidal spheres near the glass transition show strong enhancement of rotational diffusion [1]. Such rotational enhancement was previously reported in a system of dilute polymers also exhibiting viscoelastic response [2]. In this talk, we provide an explanation behind the geneneric features underlying these seemingly different observations using a simple model based on rotational coupling [3]. This model allows us to also recognize the role of contact interactions in achieving reorientation and identify a simple criterion that leads to maximum enhancement. Interestingly, contact dynamics may not have the same time scale as the relaxation of the underlying glass former, which is important in understanding the micromechanical information that these probes provide. This realization also shifts attention away from the viscoelastic nature of the suspension, towards its inherrent complexity. We will provide a outlook in this direction and show that contrasting experiment and simulation reveals additional opportunties for future study.
[1] C. Lozano, J.R. Gomez-Solano, and C. Bechinger, Nat. Mat. 18, 1118-1123 (2019)
[2] J.R. Gomez-Solano, A. Blokhuis, and C. Bechinger, Phys. Rev. Lett. 16, 138301 (2016)
[3] C. Abaurrea-Velasco, C. Lozano, C. Bechinger, and J. de Graaf, Phys. Rev. Lett. 125, 258002 (2020)