7 research outputs found
Memory of shear flow in soft jammed materials
Cessation of flow in simple yield stress fluids results in a complex stress
relaxation process that depends on the preceding flow conditions and leads to
finite residual stresses. To assess the microscopic origin of this phenomenon,
we combine experiments with largescale computer simulations, exploring the
behavior of jammed suspensions of soft repulsive particles. A spatio-temporal
analysis of microscopic particle motion and local particle configurations
reveals two contributions to stress relaxation. One is due to flow induced
accumulation of elastic stresses in domains of a given size, which effectively
sets the unbalanced stress configurations that trigger correlated dynamics upon
flow cessation. This scenario is supported by the observation that the range of
spatial correlations of quasi-ballistic displacements obtained upon flow
cessation almost exactly mirrors those obtained during flow. The second
contribution results from the particle packing that reorganize to minimize the
resistance to flow by decreasing the number of locally stiffer configurations.
Regaining rigidity upon flow cessation then effectively sets the magnitude of
the residual stress. Our findings highlight that flow in yield stress fluids
can be seen as a training process during which the material stores information
of the flowing state through the development of domains of correlated particle
displacements and the reorganization of particle packings optimized to sustain
the flow. This encoded memory can then be retrieved in flow cessation
experiments
Liquid-liquid critical point in supercooled silicon
A novel liquid-liquid phase transition has been proposed and investigated in
a wide variety of pure substances recently, including water, silica and
silicon. From computer simulations using the Stillinger-Weber classical
empirical potential, Sastry and Angell [1] demonstrated a first order
liquid-liquid transition in supercooled silicon, subsequently supported by
experimental and simulation studies. Here, we report evidence for a
liquid-liquid critical end point at negative pressures, from computer
simulations using the SW potential. Compressibilities exhibit a growing maximum
upon lowering temperature below 1500 K and isotherms exhibit density
discontinuities below 1120 K, at negative pressure. Below 1120 K, isotherms
obtained from constant volume-temperature simulations exhibit non-monotonic,
van der Waals-like behavior signaling a first order transition. We identify Tc
~ 1120 +/- 12 K, Pc -0.60 +/- 0.15 GPa as the critical temperature and pressure
for the liquid-liquid critical point. The structure of the liquid changes
dramatically upon decreasing the temperature and pressure. Diffusivities vary
over 4 orders of magnitude, and exhibit anomalous pressure dependence near the
critical point. A strong relationship between local geometry quantified by the
coordination number, and diffusivity, is seen, suggesting that atomic mobility
in both low and high density liquids can usefully be analyzed in terms of
defects in the tetrahedral network structure. We have constructed the phase
diagram of supercooled silicon. We identify the lines of compressibility,
density extrema (maxima and minima) and the spinodal which reveal the
interconnection between thermodynamic anomalies and the phase behaviour of the
system as suggested in previous works [2-9]Comment: (to be published in revised form); small corrections to previous
version; Nature Physics 201
Prestressed elasticity of amorphous solids
Prestress in amorphous solids bears the memory of their formation and plays a profound role in their mechanical properties. Here we develop a set of mathematical tools to investigate mechanical response of prestressed systems, using stress rather than strain as the fundamental variable. This theory allows microscopic prestress to vary for the same bond or contact configuration and is particularly convenient for nonconservative systems, such as granular packings and jammed suspensions, where there is no well-defined reference state, invalidating conventional elasticity. Using prestressed nonconservative triangular lattices and a computational model of amorphous solids, we show that drastically different mechanical responses can show up in amorphous materials at the same density, due to nonconservative interactions which evolve over time, or different preparation protocols. In both cases, the information is encoded in the prestress of the network and not visible at all from the configurations of the network in the case of nonconservative interactions