Measuring viscoelastic properties using compliant systems

An analysis of a novel indentation model has been implemented to obtain master curves describing the optimal experimental parameters necessary to achieve the highest possible accuracy in the determination of viscoelastic properties of soft materials. The indentation model is a rigid indenter driven by a compliant measurement system, such as an atomic force microscope or optical tweezers, into a viscoelastic half space. The viscoelastic material is described as a multiple relaxation Prony series. The results have been extended via an application of a viscoelastic equivalence principle to other physical models such as poroelasticity. Optimisation of the indentation parameters has been conducted over many orders of magnitude of the velocity, viscoelastic moduli, spring stiffness, relaxation times and the duration of indentation resulting in a characteristic master curve. It is shown that using sub-optimal conditions gives the appearance of a more elastic material than is actually the case. For a two term Prony series the ideal ramp duration was found to be approximately one eighth of the relaxation. Also the ideal ramp duration for a three term Prony series was determined and shown to guarantee distinct relaxation times under specific conditions

Selecting suitable image dimensions for scanning probe microscopy

The use of scanning probe microscopy to acquire topographical information from surfaces with nanoscale features is now a common occurrence in scientific and engineering research. Image sizes can be orders of magnitude greater than the height of the features being analysed, and there is often a trade-off between image quality and acquisition time. This work investigates a commonly encountered problem in nanometrology - how to choose a scan size which is representative of the entire sample. The topographies of a variety of samples are investigated, including metals, polymers, and thin films

Closed-Form Expressions for Contact Angle Hysteresis: Capillary Bridges between Parallel Platens

A closed form expression capable of predicting the evolution of the shape of liquid capillary bridges and the resultant force between parallel platens is derived. Such a scenario occurs within many micro-mechanical structures and devices, for example, in micro-squeeze flow rheometers used to ascertain the rheological properties of pico- to nano-litre volumes of complex fluids, which is an important task for the analysis of biological liquids and during the combinatorial polymer synthesis of healthcare and personal products. These liquid bridges exhibit capillary forces that can perturb the desired rheological forces, and perhaps more significantly, determine the geometry of the experiment. The liquid bridge has a curved profile characterised by a contact angleat the three-phase interface, as compared to the simple cylindrical geometry assumed during the rheological analysis. During rheometry, the geometry of the bridge will change in a complex nonlinear fashion, an issue compounded by the contact angle undergoing hysteresis. Owing to the small volumes involved, ascertaining the bridge geometry visually during experiment is very difficult. Similarly, the governing equations for the bridge geometry are highly nonlinear, precluding an exact analytical solution, hence requiring a substantial numerical solution. Here, an expression for the bridge geometry and capillary forces based on the toroidal approximation has been developed that allows the solution to be determined several orders of magnitude faster using simpler techniques than numerical or experimental methods. This expression has been applied to squeeze-flow rheometry to show how the theory proposed here is consistent with the assumptions used within rheometry. The validity of the theory has been shown through comparison with the exact numerical solution of the governing equations. The numerical solution for the shape of liquid bridges between parallel platens is provided here for the first time and is based on existing work of liquid bridges between spheres

The stability and degradation of PECVD fluoropolymer nanofilms

Fluoropolymer films are frequently used in microfabrication and for producing hydrophobic and low-k dielectric layers in various applications. As the reliability of functional coatings is becoming a more pressing issue in industry, it is necessary to determine the physical stability and degradation properties of this important class of films. To this end, a study has been undertaken to ascertain the aging characteristics of fluoropolymer films under various environmental conditions that such a film may experience during its use. In particular, fluorocarbon films formed by plasma-enhanced chemical vapour deposition (PECVD) using octafluorocyclobutane, or c-C4F8, as a precursor gas have been exposed to abrasive wear, elevated temperatures, ultraviolet radiation, as well as oxygen plasma and SF6 plasma, the latter being commonly used in conjunction with these films in ion etching processes. The results show that sub-micron thick fluoropolymer films exhibit a significant amount of elastic recovery during nanoscratch tests, minimising the impact of wear. The films exhibit stability when exposed to 365 nm UV light in air, but not 254 nm light in air, which generated significant decreases in thickness. Exposure to temperatures up to 175 °C did not generate loss of material, whereas temperatures higher than 175 °C did. Etching rates upon exposure to oxygen and SF6 plasmas were also measured

A dynamic model of the jump-to phenomenon during AFM analysis

The measurement of the physical properties of surfaces on the nanoscale is a long-standing problem, and the atomic force microscope (AFM) has enabled the investigation of surface energies and mechanical properties over a range of length scales. The ability to measure these properties for softer materials presents a challenge when interpreting data obtained from such measurements, in particular because of the dynamics of the compliant AFM microcantilever. This work attempts to better understand the interaction between an AFM tip and samples of varying elastic modulus, in the presence of attractive van der Waals forces. A theoretical model is presented in which the dynamics of the approach of an atomic force microscope cantilever tip toward a surface, prior to and during the van der Waals-induced jump-to phenomenon, are included. The cantilever mechanics incorporates the motion of the air through which the cantilever moves, the acceleration, inertia, and torque of the cantilever, and the squeezing of the fluid between the cantilever tip and the surface, leading to elastohydrodynamic lubrication and deformation of the substrate. Simulations of the cantilever approach are compared to measurements performed using an atomic force microscope, and the effect of cantilever drive velocity is investigated. Cantilevers presenting (1) spherical colloid probe tips and (2) pyramidal tips are employed, and substrates exhibiting Young’s moduli of 3 MPa, 500 MPa, and 75 GPa are measured. The analysis presented could be extended to enhance understanding of dynamic phenomena in micro/nanoelectromechanical systems such as resonators and microrheometers, particularly those which contain soft materials and also where surface interactions are important

The adhesive properties of viscoelastic liquid films

The adhesive characteristics of thin films (0.2 - 2 µm) of linear PDMS liquids with a wide range of molecular weights have been measured using an AFM with a colloid probe (diameters 5 and 12 µm) for different separation velocities. The data were consistent with a residual film in the contact region having a thickness of ~ 6 nm following an extended dwell time before separation of the probe. It was possible to estimate the maximum adhesive force as a function of the Capillary number, Ca, by applying existing theoretical models based on capillary interactions and viscous flow except at large values of Ca in the case of viscoelastic fluids, for which it was necessary to develop a nonlinear viscoelastic model. The compliance of the AFM colloid beam was an important factor in governing the retraction velocity of the probe and therefore the value of the adhesive force but the inertia of the beam and viscoelastic stress overshoot effects were not significant in the range of separation velocities investigated

Beta particle energy spectra shift due to self-attenuation effects in environmental sources

In order to predict and control the environmental and health impacts of ionising radiation in environmental sources such as groundwater, it is necessary to identify the radionuclides present. Beta-emitting radionuclides are frequently identified by measuring their characteristic energy spectra. The present work shows that self-attenuation effects from volume sources result in a geometry-dependent shift in the characteristic spectra which needs to be taken into account in order to correctly identify the radionuclides present. These effects are shown to be compounded due to the subsequent shift in the photon spectra produced by the detector, in this case an inorganic solid scintillator (CaF2:Eu) monitored using a Silicon Photomultiplier (SiPM). Using tritiated water as an environmentally relevant, and notoriously difficult to monitor case study, analytical predictions for the shift in the energy spectra as a function of depth of source have been derived. These predictions have been validated using Geant4 simulations and experimental results measured using bespoke instrumentation

Development of an optimized converter layer for silicon carbide based neutron sensor for the detection of fissionable materials

Here, we describe the early stage design, construction and testing of a miniature silicon carbide diode neutron sensing instrument. It is intended that a more mature version of this instrument will be used as part of a robotic manipulator to investigate various parts of the stricken Fukushima nuclear power plant. Here, three such silicon carbide based proto-type sensors have been created, two of which have differing thicknesses of boron-10 deposited on, with the final one left bare. The thicknesses and materials chosen have been informed via Monte Carlo software (MCNP 6.2) which was also used to assess the suitability of two other potential converter materials – Lithium-6 and gadolinium-157. The work goes on to describe the design, construction and testing of the prototype device at two sites around the UK. The project is part of a UK/Japanese collaboration between Lancaster University and Kyoto University and is supported by an EPSRC grant via the UK Japan Civil nuclear research program

Transient bioimpedance monitoring of mechanotransduction in artificial tissue during indentation

Mechanotransduction is of fundamental importance in cell physiology, facilitating sensing in touch and hearing as well as tissue development and wound healing. This study used an impedance sensor to monitor the effective resistance and permittivity of artificial tissues, alginate hydrogel with encapsulated fibroblasts, which were kept viable through the use of a bespoke microfluidic system. The observed transient impedance responses upon the application of identical compressive normal loads differed between acellular hydrogels and hydrogels in which fibroblasts were encapsulated. These differences resulted from changes in the conductivity and permeability of the hydrogel due to the presence of the encapsulated fibroblasts, and transient changes in ion concentrations due to mechanotransduction effects

Design and Optimisation of a Three Layers Thermal Neutron, Fast Neutron and Gamma-Ray Imaging System

The design and configuration of a multi-layered imaging system with the ability to detect thermal neutrons, fast neutrons and gamma rays has been developed and its efficacy demonstrated. The work presented here numerically determines the systems efficiency and spatial resolution, using 252Cf and 137Cs as a case study. The novelty of this detection system lies in the use of small form factor detectors in a three-layer design, which utilises neutron elastic scattering and Compton scattering simultaneously. The current configuration consists of 10 mm thick natural lithium glass (GS10) scintillator integrated with a 20 mm thick plastic scintillator (EJ-204) in the first layer, a 15 mm thick lithium glass (GS10) scintillator in the second and a 30 mm thick CsI(Tl) scintillator forming the final layer. Each of these layers is backed with an 8 x 8 silicon photomultiplier diode (SiPM) array. The overall size of the imaging system is 27 mm x 27 mm x 135 mm. MCNPv6.1 and Geant4-10.04 were alternatively used to optimise the overall configuration and to investigate detection modalities. Results show promising performance with high precision source localisation and characterization abilities. Measurements were virtually obtained of two gamma-ray sources within steel enclosures at angles of 15o, 30o and 50o separation in order to test spatial resolution ability of the system. With the current active size of the system and the 8x8 SiPM configuration, the results estimate the spatial resolution to be close to 30o. The ability of the system to characterise and identify sources based on the type and energy of the radiation emitted, has been investigated and results show that for all radiation types the system can identify the source energy within the energy range of typical reported sources in literature