Abstract
We use confocal microscopy and time-resolved light scattering to investigate plasticity in a colloidal polycrystal, following the evolution of the network of grain boundaries as the sample is submitted to thousands of shear deformation cycles. The grain boundary motion is found to be ballistic, with a velocity distribution function exhibiting nontrivial power law tails. The shear-induced dynamics initially slow down, similarly to the aging of the spontaneous dynamics in glassy materials, but eventually reach a steady state. Surprisingly, the crossover time between the initial aging regime and the steady state decreases with increasing probed length scale, hinting at a hierarchical organization of the grain boundary dynamics.
Highlights
We use confocal microscopy and time-resolved light scattering to investigate plasticity in a colloidal polycrystal, following the evolution of the network of grain boundaries as the sample is submitted to thousands of shear deformation cycles
The polycrystal is transparent to visible light; microscopy and scattering experiments probe the network of grain boundaries (GBs) where the NP accumulate
A confocal microscopy image of a sample doped with fluorescent polystyrene NPs (2a = 36 nm, volume fraction φ = 0.05%) is shown in Fig. 1(b), where the network of GBs is clearly visible
Summary
The shear cell is composed of two parallel microscope slides, which are sand-blasted (rms roughness 1 μm) to prevent slipping, except for a small window of diameter ≈ 2 mm to probe optically the sample. The motor speed during the displacement is 0.05 mm s−1 For both light scattering and microscopy, we measure the thickness of the sample chamber by microscopy, by measuring eobj, the vertical displacement of the microscope objective when focusing the upper and lower plates, respectively, using a 20× air objective. We measure eobj at several locations separated by 1 cm, finding no difference to within the measurement uncertainty This implies that the maximum deviation from parallelism over 1 cm is smaller than the measurement uncertainty (≤ 8 μm), corresponding to less than 5×10−4e (respectively, 1.3×10−3e) over the region sampled by light scattering (respectively, microscopy)
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