Material characterisation of nanowires with intrinsic stress.

When fabricating nanowires (NWs) in a doubly-clamped beam configuration it is possible for a residual axial stress to be generated. Here, we show that material characterisation of metal and semiconductor NWs subjected to residual axial stress can be problematic. Benchmark measurements of the Young's modulus of NWs are performed by sectioning a doubly-clamped NW into two cantilevered wires, eliminating residual axial stress. Use of models for doubly-clamped beams that incorporate the effects of residual stress are found to lead to ambiguity in the extracted Young's modulus as a function of displacement fit range, even for NWs with no residual stress. This is due to coupling of bending and axial stress effects at small displacements, and the limited displacement range of force curves prior to fracture or plastic deformation. This study highlights the importance of fabricating metal and semiconductor NWs that exhibit little or no residual axial stress for materials characterisation.

Modelling apical columnar epithelium mechanics from circumferential contractile fibres.

Simple columnar epithelia are formed by individual epithelial cells connecting together to form single cell high sheets. They are a main component of many important body tissues and are heavily involved in both normal and cancerous cell activities. Prior experimental observations have identified a series of contractile fibres around the circumference of a cross section located in the upper (apical) region of each cell. While other potential mechanisms have been identified in both the experimental and theoretical literature, these circumferential fibres are considered to be the most likely mechanism controlling movement of this cross section. Here, we investigated the impact of circumferential contractile fibres on movement of the cross section by creating an alternate model where movement is driven from circumferential contractile fibres, without any other potential mechanisms. In this model, we utilised a circumferential contractile fibre representation based on investigations into the movement of contractile fibres as an individual system, treated circumferential fibres as a series of units, and matched our model simulation to experimental geometries. By testing against laser ablation datasets sourced from existing literature, we found that circumferential fibres can reproduce the majority of cross-sectional movements. We also investigated model predictions related to various aspects of cross-sectional movement, providing insights into epithelium mechanics and demonstrating the usefulness of our modelling approach.

Nanomechanical torsional resonators for frequency-shift infrared thermal sensing.

We investigate use of nanomechanical torsional resonators for frequency-shift-based infrared (IR) thermal sensing. Nanoscale torsion rods, ~1 µm long and 50-100 nm in diameter, provide both extraordinary thermal isolation and excellent angular displacement and torque sensitivities, of order ~10(-7) rad·Hz(-1/2) and ~10(-22) (N·m) Hz(-1/2), respectively. Furthermore, these nanorods act as linear torsional springs, yielding a maximum angular displacement of 3.6° and a dynamic range of over 100 dB; this exceeds the performance of flexural modes by as much as 5 orders of magnitude. These attributes lead to superior noise performance for torsional-mode sensing. We demonstrate the operational principles of torsional-mode IR detection, attaining an uncooled noise equivalent temperature difference (NETD) of 390 mK. By modeling the fundamental noise processes, we project that further reduction of device size can significantly improve thermal responsivity; a room-temperature NETD below 10 mK appears feasible.

Nonlinear mode-coupling in nanomechanical systems.

Understanding and controlling nonlinear coupling between vibrational modes is critical for the development of advanced nanomechanical devices; it has important implications for applications ranging from quantitative sensing to fundamental research. However, achieving accurate experimental characterization of nonlinearities in nanomechanical systems (NEMS) is problematic. Currently employed detection and actuation schemes themselves tend to be highly nonlinear, and this unrelated nonlinear response has been inadvertently convolved into many previous measurements. In this Letter we describe an experimental protocol and a highly linear transduction scheme, specifically designed for NEMS, that enables accurate, in situ characterization of device nonlinearities. By comparing predictions from Euler-Bernoulli theory for the intra- and intermodal nonlinearities of a doubly clamped beam, we assess the validity of our approach and find excellent agreement.

Stress-induced variations in the stiffness of micro- and nanocantilever beams.

The effect of surface stress on the stiffness of cantilever beams remains an outstanding problem in the physical sciences. While numerous experimental studies report significant stiffness change due to surface stress, theoretical predictions are unable to rigorously and quantitatively reconcile these observations. In this Letter, we present the first controlled measurements of stress-induced change in cantilever stiffness with commensurate theoretical quantification. Simultaneous measurements are also performed on equivalent clamped-clamped beams. All experimental results are quantitatively and accurately predicted using elasticity theory. We also present conclusive experimental evidence for invalidity of the long-standing and unphysical axial force model, which has been widely applied to interpret measurements using cantilever beams. Our findings will be of value in the development of micro- and nanoscale resonant mechanical sensors.