Professor Grant Webber

Specific particle material properties such as conductivity, cohesion, and density have been neither directly nor thoroughly studied regarding particle behavior in an electrostatic field and the follow-on impact this has on the electrostatic formation of liquid marbles. In this method, an applied electric field drives the extraction of particles from a bed and their transport to a pendent, earthed water droplet. Herein, prior studies of electrostatic formation of particle-stabilized droplets and liquid marbles have been expanded to compare the impact of density using the spherical polystyrene (PS) latex and glass particles of similar shape and size. The addition of thin polymer shells to both samples, which increases the conductivity and cohesion, allows the interplay of these three properties to be examined systematically. Separation distances between the particle bed and the droplet from which particles can initially be extracted increase as the negative applied potential increases. Initial extraction distances of both core particles were found to be similar, ~1.5 mm at 2.0 kV applied potential, despite the greater density, and thus mass of the glass particles. It is demonstrated that this is a result of competitive interactions between particle density, conductivity, and cohesion; PS is less conductive and more cohesive than glass. Introducing a polypyrrole shell increases the separation distance for extraction to approximately 4 mm for PS core particles but has little impact on glass core particles, demonstrating that for particles with constant conductivity and cohesion reducing the density facilitates extraction. Modeling and quantification of extraction threshold forces for each particle type were undertaken, utilizing the measurement of a radially symmetric area of the particle bed from which particles were observed in the initial extraction stages. This measurement highlighted that it is significantly easier to extract PS compared to glass, with particles extracted from a region in the bed up to 5 times the width in the PS case. Particle density is hypothesized to not be the determining factor in the stabilization of the coated liquid droplets; therefore, the interplay of a multitude of physical properties must be considered when determining the suitability of particulate materials for this electrostatic method.

We report the manufacture of complex structures of silica, coal or sphalerite particles around a water droplet, driven by an electrostatic field. A particle bed was deposited on an electrically biased substrate and an earthed water drop brought close, such that the particles jumped to the drop. These structures' shape and internal composition were determined by a combination of the particles' wettability and electrical properties, and other attributes such as shape, size and density were also thought to play a role. Hydrophilic particles tend to be internalised by the drop, while hydrophobic ones tend to form a layer or shell on the surface. Thus, one example of these structures was a 'complex liquid marble', with a hydrophilic particle suspension core and a stabilising shell of hydrophobic particles.

Specific heat is a key thermal property in energy systems and is directly linked with heat storage and transfer. Concentrated solar thermal (CST) power plants which incorporate thermal storage show promise to deliver affordable, renewable and reliable electricity without harmful emissions. As compared to wind and photovoltaic installations, which are intermittent, CST systems (with thermal storage) represent a technology which can be dispatched to provide a good match with energy demand - or at least to maximize revenue. At present, molten salts represent the dominant thermal storage medium. However, the low specific heat value of molten salts severely limits their applicability. Increasing the specific heat of molten salts could allow for a sizable reduction in storage volume, or for more energy to be stored in the same volume. Recently, researchers have shown that the specific heat of fluids can be altered through the addition of nanoparticles, although the fundamental governing mechanisms for the observed changes in this property have not yet been agreed upon. This paper provides a comprehensive review of recent, albeit sometimes contrary, nanofluid specific heat studies. Since a critical review of the ongoing work in this field has not yet been compiled, this paper provides insights into the range and magnitude of specific heat changes. This review also discusses possible underlying enhancement mechanisms, the impacts of colloidal stability, and uses these to rationalize the diverse range of results seen in the literature. Through reviewing the salient literature, it is hoped that this paper will help to guide future efforts in controlling the specific heat of nanosalts.

© 2016 The Royal Society of Chemistry.The interactions of two oil droplets grown in the presence of swollen, lightly cross-linked cationic poly(tert-butylamino)ethyl methacrylate (PTBAEMA) microgels was monitored using a high-speed video camera. Three oils (n-dodecane, isopropyl myristate and sunflower oil) were investigated, each in the absence and presence of an oil-soluble cross-linker [tolylene 2,4-diisocyanate-terminated poly(propylene glycol), PPG-TDI]. Adsorption of the swollen microgel particles was confirmed by interfacial tension, interfacial elasticity and dilational viscosity measurements on single pendant oil droplets, and assessment of the oscillatory dynamics for coalescing droplet pairs. Like the analogous bulk emulsions, particle adsorption alone did not prevent coalescence of pairs of giant Pickering emulsion droplets. However, prior addition of surface-active PPG-TDI cross-linker to the oil phase results in the formation of highly stable microgel colloidosomes via reaction with the secondary amine groups on the PTBAEMA chains. Colloidosome stability depended on the age of the oil-water interface. This reflects a balance between the adsorption kinetics of the PPG-TDI cross-linker and the microgel particles, each of which must be present at the interface to form a stable colloidosome. Colloidosome formation was virtually instantaneous in n-dodecane, but took up to 120 s in the case of isopropyl myristate. The impact of an acid-induced latex-to-microgel transition on the interaction of giant colloidosomes (originally prepared at pH 10 using isopropyl myristate) was also studied. This acid challenge did not result in coalescence, which is consistent with a closely-related study (A. J. Morse et al., Langmuir, 2014, 30(42), 12509-12519). No evidence was observed for inter-colloidosome cross-linking, which was attributed to retention of an aqueous film between the adjacent pair of colloidosomes.

We report the first study of the influence of drop and particle size on the electrostatic manufacture and subsequent stability of liquid marbles. It is clear from this study that the ¿rules¿ for electrostatic formation of liquid marbles are quite different for those for conventional direct-contact manufacture. Formation of liquid marbles was observed when an earthed water drop of volume 3¿7¿µL was brought into proximity with a bed of highly-charged polystyrene particles of diameter 22¿153¿µm. Under appropriate conditions the particles jumped to and coated the drop, producing a particle-liquid aggregate that dropped to the bed surface in the form of either a stable liquid marble or a particle-stabilised sessile drop. The subsequent evolution of the physical dimensions of the metastable aggregate was measured as the liquid drained into the bed, and its stability assessed. Formation of stable liquid marbles appeared to occur more easily for smaller drops and larger particles, and some of these considerably exceeded the conventionally-understood limit for the ratio of particle to drop size of stable liquid marbles.

The conformation of a hydrophobic, weak cationic poly(2-diisopropylamino)ethyl methacrylate (PDPA) brush was studied using neutron reflectometry as a function of aqueous solution pH, ionic strength, and anion identity. In pH 4, 10 mM potassium nitrate the brush is highly charged, resulting in an extended, dilute conformation; at pH 9 the uncharged brush collapses to a single, dense layer. The brush response to added salt at constant pH (4.5) for varying concentrations of the potassium salts of acetate, nitrate, and thiocyanate revealed ion-specific conformations of the brush. At low ionic strength (0.1 mM) the brush was collapsed, independent of salt identity, while at higher ionic strengths (up to 500 mM) the conformation was dependent on counterion identity. The brush exhibited extended conformations in the presence of kosmotropic acetate counterions, while collapsed conformations were retained in the presence of strongly chaotropic thiocyanate counterions. The brush showed a richer set of behaviors in the solutions containing the weakly chaotropic nitrate anion, being similar to acetate (swollen) at intermediate concentrations but similar to thiocyanate (collapsed) at high salt concentrations. Numerical self-consistent field (nSCF) simulations indicate that the response of the brush to pH changes is dominated by the hydrophobicity of the polymer at pH values near the pKa. Furthermore, the simulations reveal that the addition of a single Flory-Huggins interaction parameter analogous to the hydrophilicity of the counterion is sufficient to replicate the observed specific anion response of a hydrophobic weak polyelectrolyte brush.

Atomic force microscopy (AFM), density functional theory (DFT) calculations, and contact angle measurements have been used to investigate the liquid-highly ordered pyrolytic graphite (HOPG) electrode interface for three deep eutectic solvents (DESs) as a function of applied potential. The DESs examined are 1:2 mixtures of choline chloride and urea (ChCl:urea), choline chloride and ethylene glycol (ChCl:ethylene glycol), and choline chloride and glycerol (ChCl:glycerol). DFT calculations reveal that in all cases the molecular component is excluded from the graphite interface at all potentials, while chloride and choline are attracted into the Stern layer at positive and negative potentials, respectively. AFM force curves confirm these trends and also show that the first near surface liquid layer in contact with the Stern layer is rich in the molecular component. The extent of near surface layering increases with potential and the hydrogen bonding capacity of the molecular component. The variation in the macroscopic contact angle with potential is consistent with changes in the Stern layer composition.

The effect of anion identity and temperature on the internal nanostructure of poly(N-isopropylacrylamide) brushes were investigated using neutron reflectometry (NR), atomic force microscopy (AFM), and quartz crystal microbalance with dissipation monitoring (QCM-D). NR and QCM-D measurements showed that addition of strongly kosmotropic acetate anions shifted the lower critical solution temperature (LCST) to lower temperatures relative to pure D2O/H2O, while strongly chaotropic thiocyanate anions shifted the LCST to higher temperatures. Polymer density profiles derived from NR showed direct evidence of vertical phase separation at temperatures around the LCST in all conditions. Results indicate that the density profiles were not simple modulations of structures observed in D2O to higher or lower temperatures, with both anion identity and ionic strength found to influence the qualitative features of the profiles. In particular, the presence of thiocyanate broadened the LCST transition which is attributed to the ability of the thiocyanate anion to electrosterically stabilize the brush above its LCST. Complementary AFM data showed that the acetate ion induced collapsed structures while a broader transition is observed in the presence of thiocyanate.

Complementary interaction force measurements between an atomic force microscope (AFM) tip or colloid probe and a weak polybasic brush have been shown to yield a number of fundamental characteristics of the brush and its response to the presence of specific anions in aqueous solution. Stretching of the poly(2-diisopropylamino)ethyl methacrylate (PDPA) chains physisorbed to the AFM tip and modeling the resultant force curves allowed the persistence and contour lengths, molecular weight, and thus grafting density of the brush to be determined. In kosmotropic acetate, high osmotic forces associated with the swollen PDPA brush repelled the colloid probe during both approach and retraction. For mildly chaotropic nitrate the behavior was similar, but at high ionic strength and during retraction, the interaction was strongly adhesive partly because of decreased brush solvation. For strongly chaotropic thiocyanate, the interaction was adhesive over the entire concentration range studied. Here, physical contact between the poorly solvated brush and the colloid resulted in an attractive force.

Colloid probe friction force microscopy (FFM) has been used to study the lubricity of propylammonium nitrate (PAN) mixed with n-alkanols confined between sliding silica and mica surfaces. Mixtures of PAN with butanol, hexanol, octanol and dodecanol were investigated for various n-alkanol volume fractions to elucidate the effect of n-alkanol hydrocarbon chain length and concentration on shear forces. For all n-alkanols friction decreases with n-alkanol vol%. The trends in friction reduction with n-alkanol vol% do not correlate with changes in the bulk phase viscosity or the near surface nanostructure, and colloid probe atomic force microscope (AFM) fluid dynamic measurements showed that none of the mixtures shear thin. Thus, the reduction in friction is attributed to the n-alkanol disrupting solvophobic interactions between boundary layer propylammonium ions adsorbed to the mica and near surface liquid layers. The lowest friction is obtained for pure dodecanol, which is attributed to the dodecanol forming a robust boundary layer. Friction for the other pure n-alkanols is higher because the lateral attractions between adsorbed n-alkanols are too weak to facilitate the formation of a strong boundary layer, commensurate with the decreased hydrocarbon chain length.

Steady shear viscosity measurements have been performed on 100. kDa poly(ethylene oxide) (PEO) dissolved in the protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) and in water. The zero shear viscosity in all three solvents increases with polymer concentration, falling into three concentration regimes corresponding to dilute, semi-dilute and network solutions. Huggins plots reveal three distinct solvent conditions: good (water), good-theta (EAN) and theta (PAN). However, differences in the transition concentrations, power law behaviour of the viscosities, and relaxation times arising from shear thinning in the two ILs can be directly related to the effects of solvent nanostructure. © 2014 Elsevier Inc.

This study was concerned with the interaction between a gaseous dispersion of fine particles travelling in the horizontal direction and discrete drops of water falling vertically through the dispersion. A simple analytical model of the particle-drop collision was developed to describe the particle recovery by the drops as a function of the water flux, covering two extremes of relative velocity between the particles and drops. The Discrete Element Method was used to validate the analytical model. Further validation of the model and insights were obtained through experimental studies. The physical process of wetting was observed to be important in influencing the tendency of particles to become engulfed by the drops of water, or to either adhere to the drops or by-pass the drops altogether. Hydrophilic particles were readily engulfed while hydrophobic particles, at best, adhered to the surface of the drop, or failed to attach. Moreover, the recovery of the hydrophilic silica particles was significantly higher than the recovery of hydrophobic coal particles, with the selectivity ratio approximately 1.5. Spherical ballotini particles were the most sensitive, with a notable increase in recovery when cleaned, and evidence of increased recovery with increasing particle size. The recovery of irregular shaped silica flour particles, however, was largely independent of the particle size. A similar result was observed for irregular coal particles, though the recoveries were all lower than relatively more hydrophilic ballotini or silica flour. Crown Copyright © 2014.

The thermal conductivities of nine protic ionic liquids (ILs) have been investigated between 293 and 340 K. Within this range, the thermal conductivities are between 0.18 and 0.30 W·m-1·K-1. These values are higher than those typically associated with oils and aprotic ILs, but lower than those of strongly hydrogen bonding solvents like water. Weak linear decreases in thermal conductivity with temperature are noted, with the exception of ethanolammonium nitrate (EtAN) where the thermal conductivity increases with temperature. The dependence of thermal conductivity on IL type is analyzed with use of the Bahe-Varela pseudolattice theory. This theory treats the bulk IL as an array of ordered domains with intervening domains of uncorrelated structure which enable and provide barriers to heat propagation (respectively) via allowed vibrational modes. For the protic ILs investigated, thermal conductivity depends strongly on the IL cation alkyl chain length. This is because the cation alkyl chain controls the dimensions of the IL bulk nanostructure, which consists of charged (ordered domains) and uncharged regions (disordered domains). As the cation alkyl chain controls the dimensions of the disordered domains, it thus limits the thermal conductivity. To test the generality of this interpretation, the thermal conductivities of propylammonium nitrate (PAN) and PAN-octanol mixtures were examined; water selectively swells the PAN charged domain, while octanol swells the uncharged regions. Up to a certain concentration, adding water increases thermal conduction and octanol decreases it, as expected. However, at high solute concentrations the IL nanostructure is broken. When additional solvent is added above this concentration the rate of change in thermal conductivity is greatly reduced. This is because, in the absence of nanostructure, the added solvent only serves to dilute the salt solution.

Density functional tight binding (DFTB), which is ~100-1000 times faster than full density functional theory (DFT), has been used to simulate the structure and properties of protic ionic liquid (IL) ions, clusters of ions and the bulk liquid. Proton affinities for a wide range of IL cations and anions determined using DFTB generally reproduce G3B3 values to within 5-10 kcal/mol. The structures and thermodynamic stabilities of n-alkyl ammonium nitrate clusters (up to 450 quantum chemical atoms) predicted with DFTB are in excellent agreement with those determined using DFT. The IL bulk structure simulated using DFTB with periodic boundary conditions is in excellent agreement with published neutron diffraction data.

Silica particle suspensions of 10 wt % have been investigated in the protic ionic liquids (ILs) ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), propylammonium nitrate (PAN), and dimethylethylammonium formate (DMEAF). Static and dynamic light scattering reveal that single particles coexist in dynamic equilibrium with flocculated networks at room temperature. These types of systems are classified as weakly flocculated and are quite rare. As weakly flocculated systems generally exist only within a narrow range of conditions, the effect of temperature was probed. When temperature is increased, the thermal motion of suspended particles increases, favoring dispersion, but in ILs suspensions, heating reduces the stabilizing effect of the interfacial structure of the IL. When subjected to a small increase in temperature, particle suspensions in ILs become unstable, indicated by the absence of a peak corresponding to single particles in the light scattering data. For EAN and DMEAF, further increasing temperatures above 40 C returns the systems to a weakly flocculated state in which thermal energy is sufficient to break particles away from aggregates. Weakly flocculated suspensions in EAN and EtAN settle more rapidly than predicted by the Stokes equation, as the particles spend a significant portion of time in large, rapidly settling flocs. Surprisingly, suspensions in PAN and DMEAF settle slower than predicted. Oscillatory rheology indicates that these suspensions are viscoelastic, due to a persistent, long-range structure in the suspension that slows settling. In aggregated systems, settling is very rapid. © 2014 American Chemical Society.

Transport of dry solid particles to a liquid is relevant to a number of emerging applications, including 'liquid marbles'. We report experiments where the transport of dry particles to a pendent water droplet is driven by an external electric field. Both hydrophilic and hydrophobic materials (silica, PMMA) were studied. For silica particles (hydrophilic, poorly conductive), a critical applied voltage initiated transfer, in the form of a rapid 'avalanche' of a large number of particles. The particle-loaded drop then detached, producing a metastable spherical agglomerate. Pure PMMA particles did not display this 'avalanche' behaviour, and when added to silica particles, appeared to cause aggregation and change the nature of the transfer mechanism. This paper is largely devoted to the avalanche process, in which deformation of the drop and radial compaction of the particle bed due to the electric field are thought to have played a central role. Since no direct contact is required between the bed and the drop, we hope to produce liquid marble-type aggregates with layered structures incorporating hydrophilic particles, which has not previously been possible.

Colloidal suspension behaviour is strongly influenced by the particle - particle interaction forces operative in the system. The stability of aqueous suspensions can be manipulated though the addition of electrolyte and changes in the pH. Such adjustments affect the net surface charge of the particles and/or the thickness of the diffuse electrical double layer (edl) surrounding the particles. If the charge or the double layer is sufficiently reduced, the ubiquitous attractive van der Waals interaction forces can dominate and the dispersion is destabilized. A Quartz Crystal Microbalance with dissipation monitoring (QCM-D) is used to measure the degree of destabilisation in a particle suspension. Mono-dispersed silica suspensions were prepared in several different chemical environments, with the conditions chosen from examination of the zeta potential and sedimentation data. The study shows that by varying the suspension stability, the resonating frequency of the 5MHz AT-cut quartz silica crystal, significantly deviates as the amplitude of oscillation is increased for a coagulated suspension, whilst remaining unchanged for a dispersed suspension.