Green-function Method for Nonlinear Interactions of Elastic Waves

In the linear wave propagation regime, an analytical mesh-free Green-function decomposition has been shown as a viable alternative to FDTD and FEM. However, its expansion into nonlinear regimes has remained elusive due to the inherent linear properties of the Green-function approach. This work presents a novel frequency-domain Green function method to describe and model nonlinear wave interactions in isotropic hyperelastic media. As an example of the capabilities of the method, we detail the generation of sum frequency waves when initial quasimonochromatic waves are emitted in a fluid by finite sources. The method is supported by both numerical and experimental results using immersion ultrasonic techniques

Non-Classical Second-Order Nonlinear Elastic Wave Interactions

We report a novel ultrasonic measurement technique based on non-classical nonlinear evanescent field interactions. We demonstrate significant enhancement in sensitivity of contactless measurements at interfaces, with the potential to detect material degradation, such as fatigue and ageing, which is currently not possible using linear ultrasonic

Breaking the Symmetry of Momentum Conservation Using Evanescent Acoustic Fields

Although the conservation of momentum is a fundamental law in physics, its constraints are not fulfilled for wave propagation at material boundaries, where incident waves give rise to evanescent field distributions. While nonlinear susceptibility tensor terms can provide solutions in the optical regime, this framework cannot be applied directly to acoustic waves. Now, by considering a complete representation of wave interactions and scattering at boundaries, we are able to show a generic formalism of sum-frequency mixing for the whole scattering field including all evanescent waves. This general case was studied analytically and verified both numerically and experimentally for ultrasonic waves, showing that considering evanescent waves leads to an anomalous nonlinear interaction which enhances sum-frequency generation. This new interpretation not only provides a deeper understanding of the momentum conservation laws in acoustics but also promises translation of this new understanding into optics and photonics, to enhance nonlinear interactions

Micromirror Angle Dependence with Etchant Choice on <100> Silicon Via Wet Etching

In creating mirrored silicon structures for micro-optics, the smoothness of the surface and etch rate are crucial parameters. We demonstrate a method of creating both 45° and 90° etch-planes from monocrystalline silicon for use as retro-reflective sidewalls in a microfluidic device. The technique uses the same photolithographic pattern orientation, but with two different etchants. Etching on direction in Si(100) with potassium hydroxide (KOH) gives vertical surfaces (where e.g. the high surface tension influences etching of crystallographic silicon planes), whilst tetramethylammonium hydroxide (TMAH) gives 45° sidewalls. We illustrate the use of these fabricated structures by creating arrays of micromirrors that enable an optical beam to be reflected parallel back and forth from 45° and -45° tilted vertical structures. This device has potential uses in optofluidic spectroscopic applications, where there is a need to increase the effective pathlength of a beam through a sample whilst keeping the device as small as possible

Micromirror Angle Dependence with Etchant Choice on <100> Silicon Via Wet Etching

In creating mirrored silicon structures for micro-optics, the smoothness of the surface and etch rate are crucial parameters. We demonstrate a method of creating both 45° and 90° etch-planes from monocrystalline silicon for use as retro-reflective sidewalls in a microfluidic device. The technique uses the same photolithographic pattern orientation, but with two different etchants. Etching on &lt;;100&gt; direction in Si(100) with potassium hydroxide (KOH) gives vertical surfaces (where e.g. the high surface tension influences etching of crystallographic silicon planes), whilst tetramethylammonium hydroxide (TMAH) gives 45° sidewalls. We illustrate the use of these fabricated structures by creating arrays of micromirrors that enable an optical beam to be reflected parallel back and forth from 45° and -45° tilted vertical structures. This device has potential uses in optofluidic spectroscopic applications, where there is a need to increase the effective pathlength of a beam through a sample whilst keeping the device as small as possible

Branched hybridization chain reaction—using highly dimensional DNA nanostructures for label-free, reagent-less, multiplexed molecular diagnostics

The specific and multiplexed detection of DNA underpins many analytical methods, including the detection of microorganisms that are important in the medical, veterinary, and environmental sciences. To achieve such measurements generally requires enzyme-mediated amplification of the low concentrations of the target nucleic acid sequences present, together with the precise control of temperature, as well as the use of enzyme-compatible reagents. This inevitably leads to compromises between analytical performance and the complexity of the assay. The hybridization chain reaction (HCR) provides an attractive alternative, as a route to enzyme-free DNA amplification. To date, the linear nucleic acid products, produced during amplification, have not enabled the development of efficient multiplexing strategies, nor the use of label-free analysis. Here, we show that by designing new DNA nanoconstructs, we are able, for the first time, to increase the molecular dimensionality of HCR products, creating highly branched amplification products, which can be readily detected on label-free sensors. To show that this new, branching HCR system offers a route for enzyme-free, label-free DNA detection, we demonstrate the multiplexed detection of a target sequence (as the initiator) in whole blood. In the future, this technology will enable rapid point-of-care multiplexed clinical analysis or in-the-field environmental monitoring

Holographic Microscopy with Acoustic Modulation for Detection of Nano-Sized Particles and Pathogens in Solution

We present a method for the detection of nanoparticles in solution using an acoustically actuated holographic microscope. This type of microscopy can be used for high-throughput biosensing applications, e.g., detection of viruses in a liquid

Holographic detection of nanoparticles using acoustically actuated nanolenses

The optical detection of nanoparticles, including viruses and bacteria, underpins many of the biological, physical and engineering sciences. However, due to their low inherent scattering, detection of these particles remains challenging, requiring complex instrumentation involving extensive sample preparation methods, especially when sensing is performed in liquid media. Here we present an easy-to-use, high-throughput, label-free and cost-effective method for detecting nanoparticles in low volumes of liquids (25 nL) on a disposable chip, using an acoustically actuated lens-free holographic system. By creating an ultrasonic standing wave in the liquid sample, placed on a low-cost glass chip, we cause deformations in a thin liquid layer (850 nm) containing the target nanoparticles (≥140 nm), resulting in the creation of localized lens-like liquid menisci. We also show that the same acoustic waves, used to create the nanolenses, can mitigate against non-specific, adventitious nanoparticle binding, without the need for complex surface chemistries acting as blocking agents

Computational Image Analysis of Guided Acoustic Waves Enables Rheological Assessment of Sub-nanoliter Volumes

We present a method for the computational image analysis of high frequency guided sound waves based upon the measurement of optical interference fringes, produced at the air interface of a thin film of liquid. These acoustic actuations induce an affine deformation of the liquid, creating a lensing effect that can be readily observed using a simple imaging system. We exploit this effect to measure and analyze the spatiotemporal behavior of the thin liquid film as the acoustic wave interacts with it. We also show that, by investigating the dynamics of the relaxation processes of these deformations when actuation ceases, we are able to determine the liquid’s viscosity using just a lens-free imaging system and a simple disposable biochip. Contrary to all other acoustic-based techniques in rheology, our measurements do not require monitoring of the wave parameters to obtain quantitative values for fluid viscosities, for sample volumes as low as 200 pL. We envisage that the proposed methods could enable high throughput, chip-based, reagent-free rheological studies within very small samples

Lipopeptides as dimerization inhibitors of HIV-1 protease

In AIDS therapy, attempts have been made to inhibit the virus-encoded enzymes, e.g, HIV-1 protease, using active site-directed inhibitors. This approach is questionable, however, due to virus mutations and the high toxicity of the drugs, An alternative method to inhibit the dimeric HIV protease is the targeting of the interface region of the protease subunits in order to prevent subunit dimerization and enzyme activity, This approach should be less prone to inactivation by mutation, A list of improved 'dimerization inhibitors' of HIV-1 protease is presented. The main structural features are a short `interface' peptide segment, including non-natural amino acids, and an aliphatic N-terminal blocking group. The high inhibitory power of some of the lipopeptides {[}e.g, palmitoyl-Tyr-Glu-Leu-OH, palmitoyl-Tyr-Glu-(L-thyronine)-OH, palmitoyl-Tyr-Glu-(L-biphenyl-alanine)-OH] with low nanomolar K-i values in the enzyme test suggests that mimetics with good bio-availability can be derived for AIDS therapy