Jamming Criticality of Near-Crystals

We report on the critical properties of minimaly-polydisperse crystals, hexagonal in 2d and face-centered cubic in 3 dimensions, at the isostatic jamming point. The force and gap distributions display power-law tails for small values. The vibrational density of states (VDOS) is flat. The scaling behavior of forces of extended floppy modes and the VDOS are universal and in agreement with an infinite-dimensional mean-field theory and maximally amorphous packings down to 2 dimensions. The distributions of gaps and forces of localized floppy modes of near-crystals appear non-universal. A small fraction of normal modes exhibit partial localization at low frequency. The majority of normal modes is delocalized exhibiting a characteristic inverse participation ratio scaling with frequency. The packing fraction and order at jamming decay linearly and quadratically respectively with polydispersity down to the maximally amorphous state.Comment: main text 5 pages, 7 figures. Supplementary material included in the en

Determination of the universality class of crystal plasticity

Although scaling phenomena have long been documented in crystalline plasticity, the universality class has been difficult to identify due to the rarity of avalanche events, which require large system sizes and long times in order to accurately measure scaling exponents and functions. Here we present comprehensive simulations of two-dimensional dislocation dynamics under shear, using finite-size scaling to extract scaling exponents and the avalanche profile scaling function from time-resolved measurements of slip-avalanches. Our results provide compelling evidence that both the static and dynamic universality classes are consistent with the mean-field interface depinning model.Comment: 6 pages, 4 figures. Figure 4 inset has been corrected as compared to the EPL publication. We thank Michael Zaiser for bringing its incorrect caption to our attention. The correction leaves all results unaffecte

Statistics of Dislocation Slip Avalanches in Nanosized Single Crystals Show Tuned Critical Behavior Predicted by a Simple Mean Field Model

We show that slowly sheared metallic nanocrystals deform via discrete strain bursts (slips), whose size distributions follow power laws with stress-dependent cutoffs. We show for the first time that plasticity reflects tuned criticality, by collapsing the stress-dependent slip-size distributions onto a predicted scaling function. Both power-law exponents and scaling function agree with mean-field theory predictions. Our study of 7 materials and 2 crystal structures, at various deformation rates, stresses, and crystal sizes down to 75 nm, attests to the universal characteristics of plasticity

Universal Quake Statistics: From Compressed Nanocrystals to Earthquakes

Slowly-compressed single crystals, bulk metallic glasses (BMGs), rocks, granular materials, and the earth all deform via intermittent slips or “quakes”. We find that although these systems span 12 decades in length scale, they all show the same scaling behavior for their slip size distributions and other statistical properties. Remarkably, the size distributions follow the same power law multiplied with the same exponential cutoff. The cutoff grows with applied force for materials spanning length scales from nanometers to kilometers. The tuneability of the cutoff with stress reflects “tuned critical” behavior, rather than self-organized criticality (SOC), which would imply stress-independence. A simple mean field model for avalanches of slipping weak spots explains the agreement across scales. It predicts the observed slip-size distributions and the observed stress-dependent cutoff function. The results enable extrapolations from one scale to another, and from one force to another, across different materials and structures, from nanocrystals to earthquakes

Studies towards the development of label-free AC impedimetric immunosensors for healthcare and food quality control

This thesis describes work focused towards the fabrication and characterisation of immunosensor platforms for the label-free detection of analytes of importance in the health and food industries. Due to their low unit cost and ease of fabrication, the immunosensor market has significantly increased recently, resulting in a constant demand for new immunosensor applications. Within this thesis, therefore, a novel fabrication protocol is reported towards the production of immunosensor platforms for the detection of the antibiotic, ciprofloxacin, the stroke and multiple sclerosis marker, Myelin Basic Protein (MBP) and the ovarian cancer marker CA-125. Initial investigations were aimed towards the electrochemical characterization of the available electrode substrates at the onset of this research project namely, gold sputter coated, screen-printed gold and carbon electrodes. They showed that only carbon electrodes provide sufficiently reproducible results and thus these electrodes have been employed for immunosensor fabrication. Due to the advantages of microelectrodes over planar electrodes, attempts to fabricate microelectrode arrays were also made via the ultrasonic ablation of passivated electrode assemblies. For the site-specific immobilisation of antibodies on polymer modified surfaces biotin-neutravidin affinity technologies were used. The fabricated immunosensors were then interrogated upon exposure to antigen solutions utilising the technique of electrochemical impedance spectroscopy (EIS). Changes in the obtained impedance spectra were used to plot calibration profiles for the detection of ciprofloxacin in buffer and in milk. Similar profiles have been plotted for Myelin Basic Protein (MBP) and the ovarian cancer marker CA-125

Plasticity as a depinning phase transition

Crystalline materials deform in an intermittent way with slip-avalanches that are power-law distributed. In this work we study plasticity as a pinning-depinning phase transition employing a discrete dislocation dynamics (DDD) model and a phase eld crystal (PFC) model in two dimensions. Below a critical ( ow) stress, the dislocations are pinned/jammed within their glide plane due to long-range elastic interactions and the material exhibits plastic response. Above this critical stress the dislocations are mobile (the depinned/ unjammed phase) and the material constantly ows. We employ discrete dislocation dynamics to resolve the temporal pro les of slip-avalanches and extract the nite-size scaling properties of the slip-avalanche statistics, going beyond gross aggregate statistics of slip avalanche sizes. We provide a comprehensive set of scaling exponents, including the depinning exponent . Our work establishes that the dynamics of plasticity, in the absence of hardening, is consistent with the mean eld interface depinning universality class, even though there is no quenched disorder. We also use dislocation dynamics and scaling arguments in two dimensions to show that the critical stress grows with the square root of the dislocation density (Taylor's relation). Consequently, dislocations jam at any density, in contrast to granular materials, which only jam above a critical density. Finally, we utilize a phase eld crystal model to extract the size, energy and duration distributions of the avalanches and show that they exhibit power law behavior in agreement with the mean eld interface depinning universality class