Franz Grieser

University of Melbourne, Melbourne, Victoria, Australia

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Publications (355)717.25 Total impact

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    ABSTRACT: In complex fluids, soft or deformable components, such as drops, bubbles, or capsules, respond to their surrounds in a far more complicated manner than rigid particle dispersions. This creates challenges in the processing and the characterization of these systems for a diverse set of applications. These applications range from the purification of minerals or pharmaceuticals using solvent extraction processes, to the formulation of emulsions and foams in food and personal care products. Through a combination of novel experimental methods, mainly using Atomic Force Microscopy (AFM) to visualize the collisions between micro-drops or micro-bubbles on the nanoscale, coupled with theoretical models, we have been able to quantitatively link the dynamic coupling of shape changes with external forces that control their behavior for a range of systems involving drops and bubbles. This talk will focus on how oil-water and air-water deformable interfaces are mediated by the presence of highly concentrated surfactant systems or non-adsorbing poly-electrolytes. We observe significant differences between the structural forces between these deformable interfaces compared to the periodic oscillatory structural forces commonly observed between rigid interfaces. In addition, quantitative comparison between these system types indicates that the deformable nature of droplets allows them to act as far more sensitive probes than solid spheres. Furthermore, the responsive nature of soft surfaces can give rise to unexpected behavior not encountered in rigid systems including reversible aggregation/flocculation for emulsion droplets and, potentially, spatial ordering within concentrated emulsion phases. The impact of hydrodynamic drainage effects on the resulting force behavior as well as changes associated with the aspect ratio of the nano-colloid will also be discussed.
    14 AIChE Annual Meeting; 11/2014
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    ABSTRACT: The hydrophobic force describes the attraction between water-hating molecules (and surfaces) that draws them together, causing aggregation, phase separation, protein folding and many other inherent physical phenomena. Attempts have been made to isolate the range and magnitude of this interaction between extended surfaces for more than four decades, with wildly varying results. In this perspective, we critically analyse the application of common force-measuring techniques to the hydrophobic force conundrum. In doing so, we highlight possible interferences to these measurements and provide physical rationalisation where possible. By analysing the most recent measurements, new approaches to establishing the form of this force become apparent, and we suggest potential future directions to further refine our understanding of this vital, physical force.
    Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
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    ABSTRACT: The multibubble sonoluminescence (MBSL) signals generated by 3.5 ms pulses of 515 kHz ultrasound in air-saturated ethylene glycol, water and ethylene glycol/water mixtures were examined in the absence and presence of a range of solutes; including aliphatic alcohols of various chain lengths (C3 - C6), and ionic and zwitterionic surfactants. In general, the alcohols quenched the SL in most solvent mixtures and the surfactants enhanced the SL signal. However, in some solvent mixtures complex effects were observed in the presence of the solutes. The discussion presented rationalises the varied behaviour of the solutes on the MBSL observed in terms of their influence on inter- and intra- bubble effects experienced by bubbles in an ultrasound field.
    The Journal of Physical Chemistry B 12/2013; · 3.61 Impact Factor
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    ABSTRACT: The hydrophobic force is a fundamental interaction in nature, and is central to many processes ranging widely from dispersing oil spills to forming cell membranes. Yet, many studies by different groups over the past 30 years to directly measure the hydrophobic force have led to the observations of extra attractions that have varied in range from tens to hundreds of nanometers. Perhaps most surprisingly, the hydrophobic effect is often invoked to explain results using solid (mainly mica) surfaces in contact with water, but rarely in the study of drops and bubbles, arguable some of the most hydrophobic interfaces in nature. For solid surfaces many secondary phenomena, surface chemical heterogeneity and roughness, and the presence of nano-bubbles, have been uncovered without a clear picture of how to describe the hydrophobic effect on a microscopic scale. We employ molecular smooth and inert oil droplets in aqueous solutions to isolate the intrinsic hydrophobic attraction, showing it to be far a shorter range and stronger attraction than the conflicting results in the literature suggest. This is also more consistent with an expected molecular origin. We have measured this force directly by suppressing any extraneous forces leaving only a “hydrophobic force” between two drops in aqueous solution. We also introduce separately different mild repulsive forces of quantum electrodynamic, electrical and hydrodynamic origin between the drops, and in each case the same attraction is observed.
    13 AIChE Annual Meeting; 11/2013
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    ABSTRACT: The hydrophobic attraction describes the well-known tendency for nonpolar molecules and surfaces to agglomerate in water, controlled by the reorganization of intervening water molecules to minimize disruption to their hydrogen bonding network. Measurements of the attraction between chemically hydrophobised solid surfaces have reported ranges varying from tens to hundreds of nanometers, all attributed to hydrophobic forces. Here, by studying the interaction between two hydrophobic oil drops in water under well-controlled conditions where all known surface forces are suppressed, we observe only a strong, short-ranged attraction with an exponential decay length of 0.30 ± 0.03 nm—comparable to molecular correlations of water molecules. This attraction is implicated in a range of fundamental phenomena from self-assembled monolayer formation to the action of membrane proteins and nonstick surface coatings.
    Journal of Physical Chemistry Letters 11/2013; 4(22):3872–3877. · 6.59 Impact Factor
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    ABSTRACT: The formation of the hydrated electron through the secondary reaction, H + OH- → H2O + eaq, has been examined in the sonolysis of argon-saturated aqueous solutions at an ultrasound frequency of 355 kHz. The detection of the hydrated electron was achieved by measuring its reaction with the one-electron acceptors Fe(CN) and methyl viologen. The results obtained indicate that hydrated electrons are produced predominately at the bubble/aqueous solution interface at comparatively high local concentrations, estimated to be > 1.5 x 10-3 M. The half-life of the hydrated electron under such conditions is estimated to be < 60 ns.
    The Journal of Physical Chemistry A 02/2013; · 2.77 Impact Factor
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    ABSTRACT: Understanding the mechanics and outcome of droplet and bubble collisions is central to a range of processes from emulsion stability to mineral flotation. The atomic force microscope has been shown to be sensitive and accurate in measuring the forces during such interactions; in combination with a suitable model framework, a powerful tool is obtained for understanding surface forces and droplet stability in dynamic systems. Here we demonstrate for the first time that this process is not limited to linear motion, and that accelerating, decelerating and cyclical droplet velocities can be used to explore the collisions between droplets and bubbles in ways that much more closely mimic real systems. In particular, the motion of droplets experiencing oscillating pumping pressures is explored, providing insight into fluid handling for microfluidics. By modelling a range of processes in which drops collide and deform, and sometimes coalesce, the validity of the theoretical model - which accounts for deformation, surface forces and dynamic lubrication - is demonstrated. Further, it is shown how this model can be used as a predictive tool to determine whether a given droplet collision will be stable or colaescent.
    Soft Matter 01/2013; 9:2426. · 4.15 Impact Factor
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    ABSTRACT: The effects of the water-soluble polymer polyvinylpyrrolidone (PVP) on the multi-bubble sonoluminescence (MBSL) intensity generated in aqueous solutions exposed to ultrasound at the two ultrasound frequencies of 20 kHz and 363 kHz, have been examined. In both cases the presence of PVP, at concentrations of up to 2 g/100 mL, was found to enhance the MBSL intensity emitted from the solutions. Based on the intensity behaviour of the SL observed from aqueous solutions containing PVP/surfactant and PVP/alcohol mixtures it is suggested that PVP enhances MBSL by increasing the number of active bubbles in the system by hindering bubble-bubble coalescence processes and probably also by changing the structure of the bubble "clouds" formed at the acoustic antinodes in solution. The influence of PVP on bubble-bubble coalescence rates was also measured to support the interpretation of the MBSL emission experiments.
    The Journal of Physical Chemistry B 10/2012; · 3.61 Impact Factor
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    ABSTRACT: The redox reactions of organic radicals, with Fe(CN) and methyl viologen, generated from the sonochemical decomposition of aliphatic alcohols in aqueous solutions, have been studied. The number of radicals produced was found to relate to the amount of adsorbed alcohol molecules (Gibbs surface excess) at the gas-aqueous solution interface for any bulk solution concentration of the alcohol. The majority of the radicals produced are from the thermal degradation of the alcohol molecules that have entered imploding cavitation bubbles. The maximum rate of reduction at 355 kHz, of Fe(CN) , was 2.6±0.3 μM min-1 whereas for methyl viologen it was 1.2 ±0.3 μM min-1 under the conditions used. The difference in the rates is attributed to the reaction of various pyrolytically produced organic radicals with the methyl viologen radical cation. The possible reactions occurring in the sonolysis of alcohol/water systems are discussed in detail.
    The Journal of Physical Chemistry A 07/2012; · 2.77 Impact Factor
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    ABSTRACT: A comparison between the temperatures within imploding acoustic cavitation bubbles and the extent of sonoluminescence (SL) quenching by C(1)-C(5) aliphatic alcohols in 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO(4)], a well known imidazolium based room temperature ionic liquid (RTIL)), has been made at an ultrasound frequency of 213kHz. The temperatures obtained ranged from 3500±200K, in neat [EMIM][EtSO(4)], to about 3200±200K in RTIL-alcohol containing solutions. It was also found that the SL intensity decreased with increasing concentration (up to 1M) of the alcohols to a greater extent compared with the relative changes to the bubble temperatures. Both the extent of the reduction in the bubble temperatures and the SL quenching were much smaller than those obtained in comparable aqueous solutions containing aliphatic alcohols. Possible reasons for the differences in the observed trends between water/alcohol and [EMIM][EtSO(4)]/alcohol systems under sonication at 213kHz are discussed.
    Ultrasonics Sonochemistry 06/2012; 20(1):47-51. · 3.52 Impact Factor
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    ABSTRACT: The use of atomic force microscopy to measure and understand the interactions between deformable colloids - particularly bubbles and drops - has grown to prominence over the last decade. Insight into surface and structural forces, hydrodynamic drainage and coalescence events has been obtained, aiding in the understanding of emulsions, foams and other soft matter systems. This article provides information on experimental techniques and considerations unique to performing such measurements. The theoretical modelling frameworks which have proven crucial to quantitative analysis are presented briefly, along with a summary of the most significant results from drop and bubble AFM measurements. The advantages and limitations of such measurements are noted in the context of other experimental force measurement techniques.
    Journal of Colloid and Interface Science 04/2012; 371(1):1-14. · 3.55 Impact Factor
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    ABSTRACT: A systematic study of collisions between surfactant-free organic drops in aqueous electrolyte solutions reveals the threshold at which continuum models provide a complete description of thin-film interactions. For collision velocities above ~1 μm/s, continuum models of hydrodynamics and surface forces provide a complete description of the interaction, despite the absence of surfactant. This includes accurate prediction of coalescence at high salt concentration (500 mM). In electrolyte solutions at intermediate salt concentration (50 mM), drop-drop collisions at lower velocity (<1 μm) or extended time of forced drop-drop interaction exhibit a strong pull-off force of systematically varying magnitude. The observations have implications on the effects of ion-specificity and time-dependence in drop-drop interactions where kinetic stability is marginal.
    Langmuir 03/2012; 28(9):4259-66. · 4.38 Impact Factor
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    ABSTRACT: Polymeric stabilizers are used in a broad range of processes and products, from pharmaceuticals and engine lubricants to formulated foods and shampoos. In rigid particulate systems, the stabilization mechanism is attributed to the repulsive force that arises from the compression of the polymer coating or "steric brush" on the interacting particles. This mechanism has dictated polymer design and selection for more than thirty years. Here we show, through direct measurement of the repulsive interactions between immobilized drops with adsorbed polymers layers in aqueous electrolyte solutions, that the interaction is a result of both steric stabilization and drop deformation. Drops driven together at slow collision speeds, where hydrodynamic drainage effects are negligible, show a strong dependence on drop deformation instead of brush compression. When drops are driven together at higher collision speeds where hydrodynamic drainage affects the interaction force, simple continuum modeling suggests that the film drainage is sensitive to flow through the polymer brush. These data suggest, for drop sizes where drop deformation is appreciable, that the stability of emulsion drops is less sensitive to the molecular weight or size of the adsorbed polymer layer than for rigid particulate systems.
    Langmuir 02/2012; 28(10):4599-604. · 4.38 Impact Factor
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    ABSTRACT: Compound drops arise from the contact of three immiscible fluids and can assume various geometric forms based on the interfacial chemistry of the phases involved. Here we present a study of a new class of compound drops that is sessile on a solid surface. The possible geometries are demonstrated experimentally with appropriate fluid combinations and accounted for with a quantitative theoretical description. Although such systems are broadly controlled by relative interfacial energies, subtleties such as the van der Waals force and effects of micro-gravity, despite drop sizes being well below the capillary length, come into play in determining the equilibrium state that is achieved. The drying of a compound sessile drop was measured experimentally, and the process revealed a novel transition between different characteristic configurations of compound sessile drops. Such drops may prove to be useful as the first step towards development of functional surfaces in applications such as soft optics, photonics and surface encapsulation.
    Soft Matter 01/2012; 8:11042. · 4.15 Impact Factor
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    ABSTRACT: Acoustically generated sonoluminescence (SL) and sonochemiluminescence (SCL) have been used to produce molecular hydrogen in a photocatalytic process using Ru(bipy)32+ as a photosensitizer, methyl viologen as an electron relay agent, and colloidal platinum as the catalyst. SL and SCL and various radicals were generated in water using 355 kHz ultrasound. The efficacy of the photocatalytic hydrogen production was to some extent limited by sonochemical side reactions, however, the in situ photon generation was sufficiently intense for pronounced H2 generation above the background sonochemical processes.
    The Journal of Physical Chemistry C. 12/2011; 116(1):1056–1060.
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    ABSTRACT: Our research has focused on the development of innovative methods using Atomic Force Microscopy (AFM) to quantitatively study the dynamic forces between drops, bubbles, particles on the nano-scale. The dynamic interactions on the nano-scale between these objects mediate or control behaviour on the macroscopic scale in complex fluids (e.g. emulsions, foams and particle suspensions). This talk will focus on two aspects of the complexities involved in measuring the collision and coalescence of micro-drops or micro-bubbles using AFM. First, we present a method to directly measure the interfacial or surface tension of the micro-drops or micro-bubbles used in the AFM force measurements. Quantitative theories to analyze these force data require the interfacial tension, obtained from macroscopic measurements. We present a novel method using a nano-needle probe to measure the interfacial tension of the drops in the AFM liquid cell directly1. Second, we present a method to directly measure the inter-droplet separation. AFM methods lack an independent measure of the absolute separation distance. For rigid surface this can be inferred from the data, but for deformable surface a model or fitting procedure is required. For the first time, we use in situ confocal fluorescence microscopy to directly image the position and separation of oil droplets in an atomic force microscopy experiment2. Comparison with predicted force vs. separation behavior to describe the interplay of force and deformation showed excellent agreement with continuum hydrodynamic lubrication theory in aqueous films less than 30 nm thick. The combination of force measurement and 3D visualization of geometric separation and surface deformation is applicable to interactions between other deformable bodies. References 1. Uddin, M. H.; Tan, S. Y.; Dagastine, R. R., Novel Characterization of Microdrops and Microbubbles in Emulsions and Foams Using Atomic Force Microscopy. Langmuir 27, (6), 2011, 2536-2544. 2. Tabor, R. F.; Lockie, H.; Mair, D.; Manica, R.; Chan, D. Y. C.; Grieser, F.; Dagastine, R. R., Combined AFM-Confocal Microscopy of Oil Droplets: Absolute Separations and Forces in Nanofilms. The Journal of Physical Chemistry Letters 2011, 961-965.
    2011 AIChE Annual Meeting; 10/2011
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    ABSTRACT: Bubbles represent one of the simplest and most pervasive entities in the world around us. Despite their prevalence in nature and in numerous industrial processes, the behaviour of bubbles is still incompletely understood. This is mainly due to the difficulty in developing sufficiently accurate and reproducible experimental techniques for their examination. Here we report direct observations of controlled collisions between two micro-bubbles in water to elucidate the effect of gas type and solution pH on their stability against coalescence, using pure CO2, air, nitrogen and argon. Bubble coalescence appears to be strongly linked to the presence of CO2: whereas inert gases behave as predicted by charge repulsion from adsorbed ions, pure CO2 bubbles are considerably more stable than predicted. Significantly, the trace concentration of CO2 in air strongly biases its behaviour away from that of the inert gases. The stability of CO2 bubbles cannot be explained by charge alone, suggesting an additional mechanism. Reference Tabor, RF, Chan, DYC, Grieser, F Dagastine, RR Angewandte Chemie 2011, 123, 3516-3518.
    2011 AIChE Annual Meeting; 10/2011
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    ABSTRACT: Polymeric stabilisers are common in many processed products ranging from pharmaceuticals and engine lubricants to formulated foods and shampoos1,2. In particulate systems, steric stabilization provides a repulsive force that arises from the compression of the polymer brush, which screens the influence of attractive surface forces, such as the van der Waals force. This mechanism has dictated polymers design and selection for more than thirty years, and yet the mechanisms behind steric stabilization in emulsions systems may be fundamentally different or even counter intuitive compared to particulate systems. For the interactions between two drops, we show that the interplay between drop deformation, hydrodynamic drainage and steric forces combine to make their behaviour and emulsion stability much less sensitive to polymer architecture and yet a far more complex interaction than in rigid systems. We present direct force measurements between two oil droplets coated with an amphiphilic tri-block co-polymer as a function of collision speed using Atomic Force Microscopy (AFM). At slow collision speeds we show the contributions to the observed forces show little dependence on the steric force law and are far more dependent on the drop deformation. At higher collision speeds, the observed force is sensitive to the amount of drainage of liquid through the brush. Simple constitutive models for both force2 and drainage3 behaviour of the steric layer have been incorporated into an existing physical model4 to describe the interplay between droplet deformation, surface forces and hydrodynamic drainage to quantitatively analyze the AFM force measurements. These results demonstrate the importance of accurately understanding the role of deformation when involving steric stabilization in soft matter systems. 1. Israelachvili, J. N., Intermolecular and surface forces, 2 ed. (Academic press limited, San Diego, (1991). 2. De Gennes, P. G., Polymers At An Interface - A Simplified View. Adv. Colloid Interface Sci. 27 (3-4), 189 (1987). 3. Klein, J., Shear, friction, and lubrication forces between polymer-bearing surfaces. Annual Review Of Materials Science 26, 581 (1996). 4. Dagastine, R. R. et al., Dynamic Forces Between Two Deformable Oil Droplets in Water. Science 313 (5784), 210 (2006).
    2011 AIChE Annual Meeting; 10/2011
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    ABSTRACT: In this study, it was found that a hybrid technique, sonophotocatalysis, is able to degrade a parent organic pollutant (methyl orange) as well as its by-products. The analysis of products formed during the whole degradation has demonstrated that the pH or the selection of oxidation process (sonolysis/photocatalysis/sonophotocatalysis) is able to control the degradation pathway. It was shown in the by-products analysis that the solution pH can alter the amount of each product generated during the sonophotocatalytic degradation. It was revealed that the different degradation rates of methyl orange and its products result from the solution pH and the nature of the organic products. Furthermore, a comparison of the data obtained from the oxidation processes on the degradation of the reaction intermediates identified the advantages of the combined system. It is concluded that sonophotocatalysis is capable of yielding a more complete and faster mineralization of organic pollutants than the individual processes. However, as in the degradation of the parent compound, the overall mineralization is lower than an additive effect (negative synergistic effect).
    Ultrasonics Sonochemistry 09/2011; 18(5):974-80. · 3.52 Impact Factor
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    ABSTRACT: Bubble coalescence behavior in aqueous electrolyte (MgSO(4), NaCl, KCl, HCl, H(2)SO(4)) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the "critical transition concentration" that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.
    Langmuir 08/2011; 27(19):12025-32. · 4.38 Impact Factor

Publication Stats

2k Citations
717.25 Total Impact Points

Institutions

  • 1984–2014
    • University of Melbourne
      • • The Particulate Fluids Processing Centre (PFPC)
      • • Department of Chemical and Biomolecular Engineering
      • • School of Chemistry
      Melbourne, Victoria, Australia
  • 1984–2011
    • Victoria University Melbourne
      Melbourne, Victoria, Australia
  • 2006
    • Indiana University Northwest
      • Department of Chemistry
      Gary, Indiana, United States
  • 2002–2005
    • University of Bath
      • Department of Chemistry
      Bath, ENG, United Kingdom
  • 2003
    • University of Washington Seattle
      • Applied Physics Laboratory
      Seattle, WA, United States
  • 1999
    • RMIT University
      Melbourne, Victoria, Australia
  • 1977–1980
    • University of Notre Dame
      • Department of Chemistry and Biochemistry
      South Bend, Indiana, United States