Micro-manipulation of small particles by node position control of an ultrasonic standing wave

For the controlled positioning of small particles with ultrasound a standing wave in a fluid is used. The standing wave is implemented in a resonator, that consists of a fluid filled tube and two piezoelectric transducers on each end. A one-dimensional model of a piezo-device including the fluid-loading on one side and a backside support is introduced. This model allows the calculation of the transmitted wave as a function of the applied electric voltage and the incident wave. In addition, when an electrical impedance is connected to the piezo-device, the reflection coefficient can be varied in amplitude and phase, so that the parameters of the reflected wave can be controlled completely. The resonator itself, consisting of a piezo-device on each end and the fluid between, is included in the model. Several methods to shift the nodes of the standing wave in the resonator are investigated and the ability to position particles is discussed.     

Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.     

Non-destructive evaluation of elastic material properties in anisotropic circular cylindrical structures

In this paper, non-axisymmetric guided wave propagation in circular cylindrical, anisotropic structures is studied in a frequency range up to 1 MHz. The investigations are carried out with carbon fibre reinforced tubes. The aim is the experimental determination of their effective linear elastic material properties in a non-destructive way. Therefore, an analytical model of the dispersion equation is fitted to the experimentally detected dispersion curves by systematically adjusting the desired material properties. A total least square scheme accompanied by an outlier detection criterion is used for this optimization task. Since the raw data of the measured dispersion curves contain a lot of noise, these outliers have to be detected and excluded, to achieve accurate results. Good agreement is found between the measured curves and the analytically calculated curves based on the estimated parameters. This fact indicates a high accuracy of the determined material properties.     

Pulse laser acoustics for the characterization of inhomogeneities at interfaces of microstructures

Interfaces between neighbouring materials are often subjected to diffusion processes which cause layers having gradually varying mechanical properties--like densities, Young's moduli or shear moduli--perpendicular to the surface or interface. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. The simulated results are part of an on-going project in which material properties of MEMS devices are investigated by short pulse laser acoustic methods. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. For metals this leads to wavelengths of 10-20 nm and the corresponding frequencies amount to 0.3-0.6 THz. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multilayers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nm. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford backscattering spectroscopy (RBS), and glow discharge optical emission spectroscopy (GDOES). Short pulse laser acoustic methods and RBS have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.     

Theoretical and numerical calculations for the time-averaged acoustic force and torque acting on a rigid cylinder of arbitrary size in a low viscosity fluid

In this paper, theoretical calculations as well as numerical simulations are performed for the time-averaged acoustic force and torque on a rigid cylinder of arbitrary size in a fluid with low viscosity, i.e., the acoustic boundary layer is thin compared to the cylinder radius. An exact analytical solution and its approximation are proposed in the form of an infinite series including Bessel functions. These solutions can be evaluated easily by a mathematical software package such as mathematica and matlab. Three types of incident waves, plane traveling wave, plane standing wave, and dual orthogonal standing waves, are investigated in detail. It is found that for a small particle, the viscous effects for an incident standing wave may be neglected but those for an incident traveling wave are notable. A nonzero viscous torque is experienced by the rigid cylinder when subjected to dual orthogonal standing waves with a phase shift even when the cylinder is located at equilibrium positions without imposed acoustic forces. Furthermore, numerical simulations are carried out based on the FVM algorithm to verify the proposed theoretical formulas. The theoretical results and the numerical ones agree with each other very well in all the cases considered.     

Acoustic field radiated into a transversely isotropic solid from a small aperture spherical surface

The acoustic field modelling reported in this paper finds application in the design of a scanning probe tip for measuring the near-surface elastic properties of solids and surface structures at high frequencies and with high spatial resolution. The underlying concept is for a longitudinally polarized pulse to be launched from a spherically-shaped portion of the upper surface of the pyramidal or conical shaped tip, and focused towards the narrow lower end. The change in the reflectivity when the narrow end is brought into contact with a solid will provide a measure of the local frequency dependent compliance of that solid. The calculations assume the material from which the tip is fabricated to be transversely isotropic, with symmetry axis coinciding with the axis of the tip. The main issue addressed in this paper is the role of the curvature of the radiating surface and anisotropy of the medium in determining the focal length and focal spread of the radiated field. Two complementary approaches are taken, firstly the discretization of the equations of motion on an irregular mesh of around 3 × 10⁵ triangular elements and solution using the commercial FE package ABAQUS/Explicit, and secondly an analytical approach based on ray tracing and a Green’s function method exploiting the angular spectrum method and stationary phase approximation in its evaluation. Consistency is achieved between these approaches regarding the characteristics of the focal region. With the combination of the two approaches it is thus possible to model the wave field from low frequencies, where the FE method is computationally economical and able to handle complex geometries, to high frequencies, where advantage increasingly lies with ray tracing and the Green’s function method.     

Improving the gas barrier,mechanical and thermal properties of poly(vinyl alcohol) with molybdenum disulfide nanosheets

New multifunctional materials with both high structural and gas barrier performances are important for a range of applications. Herein we present a one‐step mechanochemical process to prepare molybdenum disulfide (MoS₂) nanosheets with hydroxy functional groups that can simultaneously improve mechanical strength, thermal conductivity, and gas permittivity of a polymer composite. By homogeneously incorporating these functionalized MoS₂ nanosheets at low loading of less than 1 vol %, a poly(vinyl alcohol) (PVA) polymer exhibits elongation at break of 154%, toughness of 82 MJ/m³, and in‐plane thermal conductivity of 2.31 W/m K. Furthermore, this composite exhibits significant gas barrier performance, reducing the permeability of helium by 95%. Under fire condition, the MoS₂ nanosheets form thermally stable char, thus enhancing the material's resistance to fire. Hydrogen bonding has been identified as the main interaction mechanism between the nanofillers and the polymer matrix. The present results suggest that the PVA composite reinforced with 2D layered nanomaterial offers great potentials in packaging and fire retardant applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 406–414     

The use of acoustic radiation forces to position particles within fluid droplets

Handling of micrometer sizes particles, such as biological cells or coated beads, plays a relevant role in the field of life science. A number of devices have been presented in the last years, in which acoustic forces generated by coupling the vibration of a solid structure excited by a piezoelectric transducer to the particle suspension are used to collect particles in lines or position them in clumps on a grid. Following the trend of lab-on-a-chip devices, efforts have been made to shrink the size of such systems, aiming at less reagent consumption and shorter reaction times. The majority of these systems consist of closed fluid filled volumes, typically channels. Here the use of an open fluid volume, a droplet, is examined. By exciting resonances into the droplet positioned on a surface, particles can be gathered into a line, two parallel lines or, as the frequency of excitation is increased, into more complex patterns. Such a concentration process will have useful applications in improved detection sensitivity of low concentration particulate solutions.     

Finite element modeling of a microparticle manipulator

The contactless movement of microparticles and cells to known locations within a fluid volume is of interest in the fields of microtechnology and life sciences. A device which can position such inhomogeneities suspended in a fluid at multiple locations is described and modeled. The device consists of a thin fluid layer contained in a channel etched into a silicon wafer. Waves are excited by a macro-piezoelectric plate with electrodes on the top and bottom surfaces and, as a result, waves propagate into the adjacent fluid. The result is a pressure field throughout the fluidic volume. When an inhomogeneity in a fluid is exposed to an ultrasonic field the acoustic radiation force results; this is found by integrating the pressure over the surface of the particle, retaining second order terms, and taking the time average. Thus, due to the presence of a pressure field in the fluid in which the particles are suspended, a force field is created. The particles are then collected at the locations of the force potential minima. In the device described here, the force field is used to position particles into lines. The locations of the particles are predicted by using a finite element model of the system. The experimental and modeling results, presented here, are in good agreement.     

On-line atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry

A new technique is presented for the coupling of atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) mass spectrometry with liquid delivery systems. Mass measurements of polymers and peptides are demonstrated using a co-dissolved matrix, e.g. alpha-cyano-4-hydroxycinnamic acid (HCCA). Improvements in terms of sensitivity are achieved by optimizing the shape und control of the exit capillary and by using a laser (355 nm) at a 1 kHz repetition rate. Two calibration experiments promise a good applicability of the presented coupling method for quantitative measurements. The limit of detection achieved so far is 500 nM for peptides in methanol solution containing 25 mM HCCA.