Matthew S. Barrow

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.

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].

STUDIES OF THE FORMATION OF MICROPOROUS POLYMER FILMS IN "BREATH FIGURE" CONDENSATION PROCESSES

We report studies of the formation of ordered microporous polymer films by the evaporation of polymer solutions following exposure to a humid atmosphere. High-speed microphotographic (HSMP) studies of the formation process reveal that near the surface of the polymer solution, vapor condensation produces near monodisperse water droplets which crystallize to form a close-packed array. Following the evaporation of the solvent, characterization of the solid by Atomic Force Microscopy, confocal microscopy and white light interferometry reveals that the surface of the polymer film features extensive regions of hexagonally close-packed microscopic pores, whose spatial arrangement replicates that of the initial droplet monolayer. Defects recorded by HSMP in the packing of the colloidal monolayer of liquid droplets formed above the surface of the polymer solution are found to correspond to those transferred into the eventual solid film, providing the first direct evidence of the structure templating role of the droplet monolayer.

Studies of cavitation and ice nucleation in ‘doubly-metastable’ water: time-lapse photography and neutron diffraction

High-speed photographic studies and neutron diffraction measurements have been made of water under tension in a Berthelot tube. Liquid water was cooled below the normal ice-nucleation temperature and was in a doubly-metastable state prior to a collapse of the liquid state. This transition was accompanied by an exothermic heat release corresponding with the rapid production of a solid phase nucleated by cavitation. Photographic techniques have been used to observe the phase transition over short time scales in which a solidification front is observed to propagate through the sample. Significantly, other images at a shorter time interval reveal the prior formation of cavitation bubbles at the beginning of the process. The ice-nucleation process is explained in terms of a mechanism involving hydrodynamically-induced changes in tension in supercooled water in the near vicinity of an expanding cavitation bubble. Previous explanations have attributed the nucleation of the solid phase to the production of high positive pressures. Corresponding results are presented which show the initial neutron diffraction pattern after ice-nucleation. The observed pattern does not exhibit the usual crystalline pattern of hexagonal ice [I(h)] that is formed under ambient conditions, but indicates the presence of other ice forms. The composite features can be attributed to a mixture of amorphous ice, ice-I(h)/I(c) and the high-pressure form, ice-III, and the diffraction pattern continues to evolve over a time period of about an hour.

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.

Chapter 9 – Application of Atomic Force Microscopy for the Study of Tensile and Microrheological Properties of Fluids

Publisher Summary This chapter explains the application of atomic force microscopy (AFM) for the study of tensile and microrheological properties of fluids. AFM is one of the most successful techniques for the characterization of surfaces and is routinely used to describe structural details with nanoscale resolution. The ability of AFM to differentiate between local mechanical properties is well known. The atomic force microscope is clearly a powerful tool for the investigation of forces which govern the mechanics of processes occurring at or below the microscale, and the exceptional ability of atomic force microscopy to determine forces associated with the microscope deformation and flow of fluids is discussed. An understanding of the rheology of complex fluids is of fundamental importance in many practical engineering and biomedical applications. The rheological behavior of thin liquid films is an important aspect of lubrication and printing—processes that often involve mesoscale thickness films undergoing rapid deformation between separating surfaces. The nanorheological properties of polymeric liquids can be obtained by adapting techniques such as an surface force apparatus (SFA) or an AFM to act in a dynamic mode. However, the results of AFM studies are often more difficult to interpret than those derived from SFA experiments due to uncertainties about the zero separation distance, the influence of probe asperities, and torsional deflections.

High Deformation Rate Uniaxial Elongation Flow of Mobile Fluids in Cavitation Bubble Collapse Experiments

We report experimental work involving the rapid uniaxial elongation of jets of mobile (i.e. low shear viscosity) liquids formed by the collapse of a gas bubble under a cavitation-generated shockwave. The results of this work establish that the jets experience a significant degree of extensional deformation, at high rates of extension (typically > 1000 s−1 ) which are relevant to considerations of journal bearing lubrication. The results of experiments conducted on samples of commercial multigrade motor lubricants in the 15W40 category indicate a substantially increased resistance to extensional flow when compared with the behaviour of their Newtonian counterparts. Interestingly, the technique is able to distinguish between the rheological properties of lubricants of identical category (15W40) which are formulated for use in different engines (petrol engine and diesel engine, respectively). The lubricant formulated for the latter application is found to exhibit the highest resistance to extensional flow.Copyright