Flow-induced dynamic surface tension effects at nanoscale

The aim of this study is to investigate flow-induced dynamic surface tension effects, similar to the well-known Marangoni phenomena, but solely generated by the nanoscale topography of the substrates. The flow-induced surface tension effects are examined on the basis of a sharp interface theory. It is demonstrated how nanoscale objects placed at the boundary of the flow domain result in the generation of substantial surface forces acting on the bulk flow

Influence of biomaterial nanotopography on the adhesive and elastic properties of Staphylococcus aureus cells

Despite the well-known beneficial effects of biomaterial nanopatterning on host tissue integration, the influence of controlled nanoscale topography on bacterial colonisation and infection remains unknown. Therefore, the aim of the present study was to determine the nanoscale effect of surface nanopatterning on biomaterial colonisation by S. aureus, utilising AFM nanomechanics and single-cell force spectroscopy (SCFS). Nanoindentation of S. aureus bound to planar (PL) and nanopatterned (SQ) polycarbonate (PC) surfaces suggested two distinct areas of mechanical properties, consistent with a central bacterial cell surrounded by a capsullar component. Nevertheless, no differences in elastic moduli were found between bacteria bound to PL and SQ, suggesting a minor role of nanopatterning in bacterial cell elasticity. Furthermore, SCFS demonstrated increased adhesion forces and work between S. aureus and SQ surfaces at 0 s and 1 s contact times. Although WLC modelling showed similarities in contour lengths for attachment to both surfaces, Poisson analysis suggests increased short-range forces for the S. aureus–SQ interactions. In the case of S. aureus–PL, long-range forces were found to not only be dominant but also repulsive in nature, which may help explain the reduced adhesion forces observed during AFM probing. In conclusion, although surface nanopatterning does not significantly influence the elasticity of attached bacterial cells, it was found to promote the early-adhesion of S. aureus cells to the biomaterial surface

Nanoscale spectroscopic origins of photoinduced tip-sample force in the midinfrared.

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip-sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light-matter coupling in optomechanical force-based spectronanoscopy

Switch on, switch off: stiction in nanoelectromechanical switches

We present a theoretical investigation of stiction in nanoscale electromechanical contact switches. We develop a mathematical model to describe the deflection of a cantilever beam in response to both electrostatic and van der Waals forces. Particular focus is given to the question of whether adhesive van der Waals forces cause the cantilever to remain in the ‘ON’ state even when the electrostatic forces are removed. In contrast to previous studies, our theory accounts for deflections with large slopes (i.e. geometrically nonlinear). We solve the resulting equations numerically to study how a cantilever beam adheres to a rigid electrode: transitions between free, ‘pinned’ and ‘clamped’ states are shown to be discontinuous and to exhibit significant hysteresis. Our findings are compared to previous results from linearized models and the implications for nanoelectromechanical cantilever switch design are discussed

Nanoparticle manipulation through inter-particle optical forces and torques

Recently, emerging from studies based on quantum electrodynamics, it has been shown possible to significantly modify the form and magnitude of inter-particle forces by the throughput of intense laser light. This paper identifies these laser-induced forces as being the result of coherent stimulated scattering by particle pairs. Such forces have the capacity to generate novel patterns of nanoscale response, entirely determined and controlled by the frequency, intensity, polarisation and other features of the laser input. Results are given, based on general calculations of the optical forces and torques operating between a pair of dielectric particles. It is subsequently shown, by further development of the analysis, that it is possible to address the case of a twisted (Laguerre-Gaussian) laser beam as the input radiation. Here, the results reveal additional and highly distinctive torques operating between pairs of nanoparticles. Significantly the results demonstrate that these laser-induced forces and torques can be either positive or negative according to conditions. As a consequence, new possibilities emerge for the optical control of nanoparticle ordering, clustering and trapping

Giant Transverse Optical Forces in Nanoscale Slot Waveguides of Hyperbolic Metamaterials

Here we demonstrate that giant transverse optical forces can be generated in nanoscale slot waveguides of hyperbolic metamaterials, with more than two orders of magnitude stronger compared to the force created in conventional silicon slot waveguides, due to the nanoscale optical field enhancement and the extreme optical energy compression within the air slot region. Both numerical simulation and analytical treatment are carried out to study the dependence of the optical forces on the waveguide geometries and the metamaterial permittivity tensors, including the attractive optical forces for the symmetric modes and the repulsive optical forces for the anti-symmetric modes. The significantly enhanced transverse optical forces result from the strong optical mode coupling strength between two metamaterial waveguides, which can be explained with an explicit relation derived from the coupled mode theory. Moreover, the calculation on realistic metal-dielectric multilayer structures indicates that the predicted giant optical forces are achievable in experiments, which will open the door for various optomechanical applications in nanoscale, such as optical nanoelectromechanical systems, optical sensors and actuators.Comment: 18 pages, 6 figure