898 research outputs found
Collodial particles at a range of fluid-fluid particles
The study of solid particles residing at fluid-fluid interfaces has become an established area in surface and colloid science recently experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes like anti-foam formulations, crude oil emulsions, aerated foodstuffs and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also and the area is now multi-disciplinary involving research in the fundamental science and potential applications. In this Feature Article, a flavour of some of this interest is given based on recent work from our own group and includes the behaviour of particles at oil-water, air-water, oil-oil, air-oil and water-water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilised emulsions, particle-stabilised aqueous and oil foams, dry liquids, liquid marbles and powdered emulsions
A lithium depletion boundary age of 21 Myr for the Beta Pictoris moving group
Optical spectroscopy is used to confirm membership for 8 low-mass candidates
in the young Beta Pic moving group (BPMG) via their radial velocities,
chromospheric activity and kinematic parallaxes. We searched for the presence
of the Li I 6708A resonance feature and combined the results with literature
measurements of other BPMG members to find the age-dependent lithium depletion
boundary (LDB) -- the luminosity at which Li remains unburned in a coeval
group. The LDB age of the BPMG is 21 +/- 4 Myr and insensitive to the choice of
low-mass evolutionary models. This age is more precise, likely to be more
accurate, and much older than that commonly assumed for the BPMG. As a result,
substellar and planetary companions of BPMG members will be more massive than
previously thought.Comment: Accepted for MNRAS Letter
Tm3+/Ho3+ codoped tellurite fiber laser
Continuous-wave and Q-switched lasing from a Tm 3+ /Ho 3+ codoped tellurite fiber is reported. An Yb 3+ /Er 3+ -doped silica fiber laser operating at 1.6μm was used as an in-band pump source, exciting the Tm 3+ ions into the F 4 3 level. Energy is then nonradiatively transferred to the upper laser level, the I 7 5 state of Ho 3+ . The laser transition is from the I 7 5 level to the I 8 5 level, and the resulting emission is at 2.1μm . For continuous wave operation, the slope efficiency was 62% and the threshold 0.1W ; the maximum output demonstrated was 0.16W . Mechanical Q switching resulted in a pulse of 0.65μJ energy and 160ns duration at a repetition rate of 19.4kHz
Particles at oil–air surfaces : powdered oil, liquid oil marbles, and oil foam
The type of material stabilized by four kinds of fluorinated particles (sericite and bentonite platelet clays and spherical zinc oxide) in air–oil mixtures has been investigated. It depends on the particle wettability and the degree of shear. Upon vigorous agitation, oil dispersions are formed in all the oils containing relatively large bentonite particles and in oils of relatively low surface tension (γla < 26 mN m⁻¹) like dodecane, 20 cS silicone, and cyclomethicone containing the other fluorinated particles. Particle-stabilized oil foams were obtained in oils having γla > 26 mN m⁻¹ where the advancing air–oil–solid contact angle θ lies between ca. 90° and 120°. Gentle shaking, however, gives oil-in-air liquid marbles with all the oil–particle systems except for cases where θ is <60°. For oils of tension >24 mN m⁻¹ with omniphobic zinc oxide and sericite particles for which advancing θ ≥ 90°, dry oil powders consisting of oil drops in air which do not leak oil could be made upon gentle agitation up to a critical oil:particle ratio (COPR). Above the COPR, catastrophic phase inversion of the dry oil powders to air-in-oil foams was observed. When sheared on a substrate, the dry oil powders containing at least 60 wt % of oil release the encapsulated oil, making these materials attractive formulations in the cosmetic and food industries
A revised age greater than 50 Myr for the young cluster IC 4665
IC 4665 is one of only a dozen young open clusters with a ``lithium depletion
boundary" (LDB) age. Using an astrometrically and spectroscopically filtered
sample of cluster members, we show that both the positions of its low mass
stars in Gaia absolute colour-magnitude diagrams and the lithium depletion seen
among its K- and early M-stars are discordant with the reported LDB age of (32
+4/-5) Myr. Re-analysis of archival spectra suggests that the LDB of IC 4665
has not been detected and that the published LDB age should be interpreted as a
lower limit. Empirical comparisons with similar datasets from other young
clusters with better-established LDB ages indicate that IC 4665 is bracketed in
age by the clusters IC 2602 and IC 2391 at (55 +/- 3) Myr.Comment: For publication in MNRAS, accepted versio
Saddle-splay modulus of a particle-laden fluid interface
The scaled-particle theory equation of state for the two-dimensional
hard-disk fluid on a curved surface is proposed and used to determine the
saddle-splay modulus of a particle-laden fluid interface. The resulting
contribution to saddle-splay modulus, which is caused by thermal motion of the
adsorbed particles, is comparable in magnitude with the saddle-splay modulus of
a simple fluid interface.Comment: 10 pages, 2 figure
Arresting bubble coarsening: A two-bubble experiment to investigate grain growth in presence of surface elasticity
Many two-phase materials suffer from grain-growth due to the energy cost
which is associated with the interface that separates both phases. While our
understanding of the driving forces and the dynamics of grain growth in
different materials is well advanced by now, current research efforts address
the question of how this process may be slowed down, or, ideally, arrested. We
use a model system of two bubbles to explore how the presence of a finite
surface elasticity may interfere with the coarsening process and the final
grain size distribution. Combining experiments and modelling in the analysis of
the evolution of two bubbles, we show that clear relationships can be predicted
between the surface tension, the surface elasticity and the initial/final size
ratio of the bubbles. We rationalise these relationships by the introduction of
a modified Gibbs criterion. Besides their general interest, the present results
have direct implications for our understanding of foam stability
Emulsification in binary liquids containing colloidal particles: a structure-factor analysis
We present a quantitative confocal-microscopy study of the transient and
final microstructure of particle-stabilised emulsions formed via demixing in a
binary liquid. To this end, we have developed an image-analysis method that
relies on structure factors obtained from discrete Fourier transforms of
individual frames in confocal image sequences. Radially averaging the squared
modulus of these Fourier transforms before peak fitting allows extraction of
dominant length scales over the entire temperature range of the quench. Our
procedure even yields information just after droplet nucleation, when the
(fluorescence) contrast between the two separating phases is scarcely
discernable in the images. We find that our emulsions are stabilised on
experimental time scales by interfacial particles and that they are likely to
have bimodal droplet-size distributions. We attribute the latter to coalescence
together with creaming being the main coarsening mechanism during the late
stages of emulsification and we support this claim with (direct)
confocal-microscopy observations. In addition, our results imply that the
observed droplets emerge from particle-promoted nucleation, possibly followed
by a free-growth regime. Finally, we argue that creaming strongly affects
droplet growth during the early stages of emulsification. Future investigations
could clarify the link between quench conditions and resulting microstructure,
paving the way for tailor-made particle-stabilised emulsions from binary
liquids.Comment: http://iopscience.iop.org/0953-8984/22/45/455102
Preparation of double emulsions using hybrid polymer/silica particles: New pickering emulsifiers with adjustable surface wettability
A facile route for the preparation of water-in-oil-in-water (w/o/w) double emulsions is described for three model oils, namely, n-dodecane, isopropyl myristate, and isononyl isononanoate, using fumed silica particles coated with poly(ethylene imine) (PEI). The surface wettability of such hybrid PEI/silica particles can be systematically adjusted by (i) increasing the adsorbed amount of PEI and (ii) addition of 1-undecanal to the oil phase prior to homogenization. In the absence of this long-chain aldehyde, PEI/silica hybrid particles (PEI/silica mass ratio = 0.50) produce o/w Pickering emulsions in all cases. In the presence of 1-undecanal, this reagent reacts with the primary and secondary amine groups on the PEI chains via Schiff base chemistry, which can render the PEI/silica hybrid particles sufficiently hydrophobic to stabilize w/o Pickering emulsions at 20 °C. Gas chromatography, 1H NMR and X-ray photoelectron spectroscopy provide compelling experimental evidence for this in situ surface reaction, while a significant increase in the water contact angle indicates markedly greater hydrophobic character for the PEI/silica hybrid particles. However, when PEI/silica hybrid particles are prepared using a relatively low adsorbed amount of PEI (PEI/silica mass ratio = 0.075) only o/w Pickering emulsions are obtained, since the extent of surface modification achieved using this Schiff base chemistry is insufficient. Fluorescence microscopy and laser diffraction studies confirm that highly stable w/o/w double emulsions can be achieved for all three model oils. This is achieved by first homogenizing the relatively hydrophobic PEI/silica hybrid particles (PEI/silica mass ratio = 0.50) with an oil containing 3% 1-undecanal to form an initial w/o emulsion, followed by further homogenization using an aqueous dispersion of relatively hydrophilic PEI/silica particles (PEI/silica mass ratio = 0.075). Dye release from the internal aqueous cores into the aqueous continuous phase was monitored by visible absorption spectroscopy. These studies indicate immediate loss of 12-18% dye during the high speed homogenization that is required for double emulsion formation, but no further dye release is observed at 20 °C for at least 15 days thereafter
Faraday instability on viscous ferrofluids in a horizontal magnetic field: Oblique rolls of arbitrary orientation
A linear stability analysis of the free surface of a horizontally unbounded
ferrofluid layer of arbitrary depth subjected to vertical vibrations and a
horizontal magnetic field is performed. A nonmonotonic dependence of the
stability threshold on the magnetic field is found at high frequencies of the
vibrations. The reasons of the decrease of the critical acceleration amplitude
caused by a horizontal magnetic field are discussed. It is revealed that the
magnetic field can be used to select the first unstable pattern of Faraday
waves. In particular, a rhombic pattern as a superposition of two different
oblique rolls can occur. A scaling law is presented which maps all data into
one graph for the tested range of viscosities, frequencies, magnetic fields and
layer thicknesses.Comment: 8 pages, 6 figures, RevTex
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