Rhodri Williams

A study of the tensile properties of liquids in confined spaces using an atomic force microscope

We report work in which an atomic force microscope (AFM) is used to stretch (and ultimately, to rupture) a thin film of liquid between a moving colloid sphere and a static plane surface. Under some circumstances, when the sphere and the surface are pulled apart sufficiently rapidly, an unexpected transient decrease in the sphere‐surface separation is recorded. The results of numerical simulations of cavitation bubble dynamics suggest that the growth of a cavitation bubble within a liquid may result in the development of sufficiently large negative pressures to account for this phenomenon. The results of separate experiments, which involve acoustic pulse propagation within metre‐long columns of liquid and high‐speed microphotography (using a novel optical system designed for this work), are used to show that the peak tensile forces recorded in the AFM experiments correspond to the development of tensile stresses that are commensurate with the fluids effective tensile strength (or ‘cavitation threshold’). The results of this study, which, to the best of our knowledge, is the first to apply the AFM in cavitation bubble dynamics work, provide evidence that, in the cavitation of liquids within confined spaces, the growth of a cavity may be more damaging than its subsequent collapse.

Gel-point studies in reacting systems by shear wave dispersion measurements

Abstract A study of shear wave dispersion in a gelling system shows that marked changes in dispersion occur due to the viscoelastic liquid to viscoelastic solid transition. A theoretical study suggests that an immediate pre-gel combination of liquid-like and solid-like properties might be detected by shear wave dispersion measurements. Experimental work on a system which exhibits critical gel behaviour gives a high frequency wave dispersion in good agreement with theoretical predictions. The work illustrates the interdependence of the development of the relaxation spectrum at low frequencies and the high frequency wave dispersion, and suggests that measurements of the latter might be used to determine the gel-point in some systems.

Rheology of Dilute Polymer Solutions and Engine Lubricants in High Deformation Rate Extensional Flows Produced by Bubble Collapse

We report a study of liquid jets formed by the collapse of bubbles under cavitation-generated pressure waves. Such jets involve an extensional flow which is characterized by high rates of extension, the latter being relevant to considerations of the flow of oils within dynamically loaded journal bearings. The technique reported here is found to be sensitive to the influence of extremely small concentrations of high molecular weight polymeric additive (xanthan gum). Commercial multigrade oils are also found to exhibit significantly larger resistance to extensional flow than their Newtonian counterparts and, insofar as the multigrade oils studied here are made viscoelastic by polymer additives, and possess significant levels of resistance to extension, the results provide evidence in support of a mitigating effect of viscoelasticity on cavitation, as mooted by Berker et al. [3].

Rheometrical aspects of the viscoelastic dispersion of shear waves in gel-like mechanical networks

A modified form of the Gross-Marvin ladder model is used to simulate gelation processes in which α, the viscoelastic stress relaxation exponent in the Gel Equation, can be varied in the range O<α<1. The model has been used to investigate the interdependence of the high- and low-frequency features of the evolving relaxation time spectra associated with the growth of discrete, mechanically self-similar nodal networks. Analysis of the growth of the networks in terms of their wavelengths reveals a fractal characteristic of the underlying gel microstructure. The results obtained may provide a viable rheometrical basis for examining the gel-like characteristics of pre-gel point viscoelastic liquid systems in terms of the Gel Equation but in the absence of a frequency independent loss tangent.

In-situ synthesis of magnetic iron-oxide nanoparticle-nanofibre composites using electrospinning

We demonstrate a facile, one-step process to form polymer scaffolds composed of magnetic iron oxide nanoparticles (MNPs) contained within electrospun nano- and micro-fibres of two biocompatible polymers, Poly(ethylene oxide) (PEO) and Poly(vinyl pyrrolidone) (PVP). This was achieved with both needle and free-surface electrospinning systems demonstrating the scalability of the composite fibre manufacture; a 228 fold increase in fibre fabrication was observed for the free-surface system. In all cases the nanoparticle-nanofibre composite scaffolds displayed morphological properties as good as or better than those previously described and fabricated using complex multi-stage techniques. Fibres produced had an average diameter (Needle-spun: 125±18nm (PEO) and 1.58±0.28μm (PVP); Free-surface electrospun: 155±31nm (PEO)) similar to that reported previously, were smooth with no bead defects. Nanoparticle-nanofibre composites were characterised using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), dynamic light scattering (DLS) (Nanoparticle average diameter ranging from 8±3nm to 27±5nm), XRD (Phase of iron oxide nanoparticles identified as magnetite) and nuclear magnetic resonance relaxation measurements (NMR) (T1/T2: 32.44 for PEO fibres containing MNPs) were used to verify the magnetic behaviour of MNPs. This study represents a significant step forward for production rates of magnetic nanoparticle-nanofibre composite scaffolds by the electrospinning technique.

Rheology of multigrade engine oils in high deformation rate extensional flows

Abstract An experimental study is reported of liquid jets formed by the collapse of bubbles under cavitation-generated pressure waves. The results reveal that jets formed from samples of commercial multigrade engine oils experience an extensional flow which is characterized by high rates of extension, the latter being relevant to considerations of the flow of oils within dynamically loaded journal bearings. Contrary to previous indications in the literature, commercial multigrade oils are found to exhibit significantly larger resistance to extensional flow than their Newtonian counterparts and, insofar as the multigrade oils studied here are made viscoelastic by polymer additives and evidently possess significant levels of resistance to extension, the results provide evidence in support of a mitigating effect of viscoelasticity on a cavitation damage related mechanism, as mooted by Berker, Bouldin, Kleis and Van Arnsdale.

A Study of Cavitation Phenomena Using an Atomic Force Microscope

We report work in which an Atomic Force Microscope (AFM) is used to deform a liquid film between a colloid sphere and a plane surface. Under some circumstances, when the sphere and the surface are pulled apart rapidly, an unexpected transient decrease of the sphere-surface separation is recorded. Numerical simulations of cavitation bubble dynamics are used to explain how the growth of a cavitation bubble may result in the development of sufficiently large negative pressures to account for this phenomenon. The results of this study provide evidence in support of a mechanism mooted by Israelachvilli and co-workers that in the cavitation of liquids within confined spaces, the growth of a cavity may be more damaging than its subsequent collapse.

Measurements of the Cavitation Threshold of Liquids Under Dynamic Stressing by Pulses of Tension

This paper addresses discrepancies between the results of different measurements of the effective tensile strength F(F c ) of liquids, in experiments in which a pulse of tension (or ’negative pressure’) is created by the reflection of a pressure pulse at a suitable boundary. We show that a key feature of the pressure records previously reported in experiments such as the ‘Bullet-Piston’ (B-P) pulse-reflection apparatus [1] may have been misinterpreted. The first complete account of such pressure records is reported here. We also report a new method of estimating F e in a modified B-P apparatus and the results obtained indicate that samples of degassed, deionised water can sustain tensions which are an order-of-magnitude greater than previously reported in B-P work. Results are also reported for work involving samples of Newtonian silicone oils, for which the dependence of F c on shear viscosity, μ, found in this work confirms that of an earlier study although the absolute values of F c are found to be considerably greater than previously reported.

Modified Bautista–Manero (MBM) modelling for hyperbolic contraction–expansion flows

In this study, modelling of network-structured material flow is considered through a rounded-corner, hyperbolic 4:1:4 contraction–expansion geometry, under axisymmetric configuration. Three representative constitutive models are adopted to represent networked behaviour and to investigate the flow of wormlike micellar fluids in this context. This includes the modified Bautista–Manero (MBM) model (for base thixotropic properties), some newly proposed micellar models (NM_τp and NM_T; for advanced thixotropic modelling), and the EPPT model (for contrast against non-thixotropic properties). In this configuration, emphasis is placed upon interpretation of flow behaviour for these constitutive models, against their response in simple rheometrical flows. To best determine the factors that contribute to epd prediction, current findings have also been contrasted against those reported earlier in López-Aguilar et al. (J Non-Newtonian Fluid Mech 204:7–21, 2014), for the counterpart abrupt rounded-corner, axisymmetric 4:1:4 contraction–expansion flow.

Measurements of the Effective Tensile Strength of Newtonian and Non-Newtonian Fluids Under Dynamic Stressing

The ‘Bullet-Piston’ (B-P) dynamic stressing technique has been used to study the tensile strength, Fc , of Newtonian and non-Newtonian fluids. A comparative study of the effects of viscosity on Fc for silicone oils (Newtonian) and aqueous polyacrylamide (PAA) solutions (non-Newtonian) was made. Both systems exhibited a power law dependency of the form Fc ∝ µn , where µ is the viscosity and n is the power index. For the Newtonian system, n was found to be 0.09 in close agreement with the findings of Couzens and Trevena (1974) for silicone oils whilst a value of 0.16 was found for the PAA solutions.Copyright