Oscillatory ductile compaction dynamics in a cylinder.

Ductile compaction is common in many natural systems, but the temporal evolution of such systems is rarely studied. We observe surprising oscillations in the weight measured at the bottom of a self-compacting ensemble of ductile grains. The oscillations develop during the first ten hours of the experiment, and usually persist through the length of an experiment (one week). The weight oscillations are connected to the grain-wall contacts, and are directly correlated with the observed strain evolution and the dynamics of grain-wall contacts during the compaction. Here, we present the experimental results and characteristic time constants of the system, and discuss possible reasons for the measured weight oscillations.

Structure of plastically compacting granular packings.

In this paper we present results of structural studies of compacting experimental systems of ductile grains in two and three dimensions. The high precision of our two-dimensional experiments enables a detailed study of the evolution of coordination numbers and local crystalline arrangements as a function of the packing fraction. The structure in both dimensions deviates considerably from that of hard disks and spheres, although geometrically, crystalline arrangements dominate on a local scale (in two dimensions). In three dimensions, the evolution of the coordination number is compared to experimental packings of hard and ductile grains from the literature. This comparison shows that the evolution of coordination number with packing fraction is not unique for ductile systems in general, but must depend on rheology and grain size.

High-resolution measurements of pressure solution creep.

Two dilatometers with high precision and stability have been developed for measurement of indentation by pressure solution creep. The indentation of gold wires or glass cylinders into sodium chloride has been measured with down to 10 A accuracy and 6% precision. The indentation curves show a strong history dependence and the indentation rate decreases by three orders of magnitude over 400 h. The indentation mechanism is shown to be a pressure solution creep process in which material is dissolved at the indentor-sodium chloride contacts and transported to the free surface, where it precipitates in the proximity of the indentors. The indentation rates are not controlled by precipitation rates, the density of preexisting dislocations in the material, by change in the contact widths, or by ordinary plastic deformation. Small amplitude sinusoidal variations of temperature and normal stress are shown to have a large effect on the indentation rate. Moreover, sudden increase in normal stress from the indentor on the sodium chloride is shown to initiate an increased, time-dependent indentation rate. A model for pressure solution creep with time-dependent contact sizes explains the history dependence of the indentation data presented.

Universal scaling in transient creep.

We present experimental evidence that pressure solution creep does not establish a steady-state interface microstructure as previously thought. Conversely, pressure solution controlled strain and the characteristic length scale of interface microstructures grow as the cubic root of time. Transient creep with the same scaling is known in metallurgy (Andrade creep). The apparent universal scaling of pressure solution transient creep is explained using an analogy with spinodal dewetting.