Plasticity of a Colloidal Polycrystal under Cyclic Shear

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 non-trivial power law tails. The shearinduced dynamics initially slow down, similarly to the aging of the spontaneous dynamics in glassy materials, but eventually reach a steady state. Surprisingly, the cross-over 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. The mechanical properties of amorphous solids are a topic of intense research. Recent works focus on the irreversible (or plastic) rearrangements at the microscopic level [1][2][3][4][5][6][7], resulting from an applied deformation or stress, which are ultimately responsible for the macroscopic mechanical behavior. Research on amorphous solids is also relevant to crystalline materials [8]. On the one hand, simulations and experiments have revealed that particles in the grain boundaries (GBs) separating crystalline grains exhibit glassy dynamics [9,10]. On the other hand, polycrystals may be regarded as an amorphous assembly of crystalline grains separated by GBs. In fact, driven polycrystals display mechanical features similar to those of amorphous solids [11] and GB process has been shown to be at the origin of the plasticity of polycrystals in the limit of small grain sizes [11][12][13][14][15].A large number of numerical works have explored the microscopic dynamics induced in amorphous systems by a continuous shear, finding quite generally diffusive dynamics at the particle level [1][2][3], once the affine component of the displacement is removed, as also confirmed by experiments on sheared colloidal glasses [2,7,16]. By contrast, the effect of a cyclic shear has been less investigated, in spite of its relevance to the fatigue tests commonly adopted in material science. Furthermore, cyclic deformation tests allow one to unambiguously identify irreversible rearrangements (as opposed to the non-affine displacement measured in continuous shear, which may be reversible) and to follow the evolution, or aging, of the dynamics as the sample is kept under an oscillatory deformation.Similarly to the case of continuous shear, the microscopic dynamics in cyclically deformed amorphous solids have been found to be diffusive (or even subdiffusive), in simulations [18,19] as well as in experiments on colloids [2,20] or granular matter [21,22]. Aging effects have been reported for macroscopic quantities, e.g. pressure or compacity [23,24], or for the microscopic dynamics. In the latter case, however, aging has been probed over a few tens of cycles at most [22,25].In this Letter, we report on experiments probing the irreversible rearrangements induced in a colloidal polycrystal by thousands of sh...

Computing the linear viscoelastic properties of soft gels using an optimally windowed chirp protocol

“…Their highly adaptive and tunable rheological response is of interest for novel technologies and smart material design, but distinguishing the role of different microstructural features over different lengthscales and timescales in order to fully understand and control the wide relaxation spectrum of these soft materials is extremely difficult. Recent advancements in experimental techniques have enabled accurate and efficient determination of the rheological response of soft materials across a broad range of linear and non-linear deformations [8][9][10][11][12][13][14][15] and the combination of such approaches with imaging, ultrasound velocimetry or spectroscopy provides unique opportunities to bridge the gap between the macroscopic rheological behavior of a material and its micro-and even nano-scale structure/dynamics [16][17][18][19][20][21][22][23][24][25][26] . Nevertheless, constitutive models that capture the link between the microstructure and the mechanical response are still funa) mb1853@georgetown.edu b) bavand@mit.edu; *Contributed equally c) mgeri@mit.edu d) divoux@crpp-bordeaux.cnrs.fr e) ed610@georgetown.edu f) gareth@mit.edu damentally lacking, and this limits quantitative interpretation of the rheological measurements.…”

Irreversibility transition of colloidal polycrystals under cyclic deformation

“…The critical strain amplitude for irreversibility should correspond to the depinning stress for grain boundary growth20. Our results should be directly testable in colloidal particle experiments15161718.…”

“…Depending on the experimental conditions colloidal particles can be assembled into desired states, from perfectly ordered crystals13 to disordered amorphous glasses14. In between these two extremes lie colloidal polycrystals, where ordered crystalline regions are separated by extended grain boundaries formed by dislocation arrays15161718. The application of a cyclic shear to colloidal polycrystals allows to follow at the same time the dynamics of individual particles (“atoms”) and the large-scale response of the polycrystalline texture15161718.…”

“…The system we consider is similar to the experimental setup studied in ref. 18, where a colloidal crystal is doped with larger colloidal particles whose concentration is used to control the grain size. Our simulations show that for large enough strain amplitudes grain boundaries eventually disappear and the system orders, while at small amplitude the system settles after an initial transient to a state very close to the original metastable configuration and reversibility is maintained.…”

Probing shear-induced rearrangements in Fourier space. I. Dynamic light scattering

“…For measurements under shear, the home-made plane-plane shear cell described in 23 was used. The cell consists of two glass plates confining the sample in a gap ranging from e = 300 µm to e = 1500 µm.…”