Publications by authors named "Dirk G Aarts"

61 Publications

Topology of Orientational Defects in Confined Smectic Liquid Crystals.

Phys Rev Lett 2021 Nov;127(19):198001

Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany.

We propose a general formalism to characterize orientational frustration of smectic liquid crystals in confinement by interpreting the emerging networks of grain boundaries as objects with a topological charge. In a formal idealization, this charge is distributed in pointlike units of quarter-integer magnitude, which we identify with tetratic disclinations located at the end points and nodes. This coexisting nematic and tetratic order is analyzed with the help of extensive Monte Carlo simulations for a broad range of two-dimensional confining geometries as well as colloidal experiments, showing how the observed defect networks can be universally reconstructed from simple building blocks. We further find that the curvature of the confining wall determines the anchoring behavior of grain boundaries, such that the number of nodes in the emerging networks and the location of their end points can be tuned by changing the number and smoothness of corners, respectively.
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http://dx.doi.org/10.1103/PhysRevLett.127.198001DOI Listing
November 2021

Detection of magnetic field effects by confocal microscopy.

Chem Sci 2020 Jul 22;11(30):7772-7781. Epub 2020 Jul 22.

Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory Oxford OX1 3QR UK

Certain pairs of paramagnetic species generated under conservation of total spin angular momentum are known to undergo magnetosensitive processes. Two prominent examples of systems exhibiting these so-called magnetic field effects (MFEs) are photogenerated radical pairs created from either singlet or triplet molecular precursors, and pairs of triplet states generated by singlet fission. Here, we showcase confocal microscopy as a powerful technique for the investigation of such phenomena. We first characterise the instrument by studying the field-sensitive chemistry of two systems in solution: radical pairs formed in a cryptochrome protein and the flavin mononucleotide/hen egg-white lysozyme model system. We then extend these studies to single crystals. Firstly, we report temporally and spatially resolved MFEs in flavin-doped lysozyme single crystals. Anisotropic magnetic field effects are then reported in tetracene single crystals. Finally, we discuss the future applications of confocal microscopy for the study of magnetosensitive processes with a particular focus on the cryptochrome-based chemical compass believed to lie at the heart of animal magnetoreception.
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http://dx.doi.org/10.1039/d0sc01986kDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8163210PMC
July 2020

Characterization of MIPS in a suspension of repulsive active Brownian particles through dynamical features.

J Chem Phys 2021 Apr;154(16):164901

Departamento de Estructura de la Materia, Física Térmica y Electrónica, Universidad Complutense de Madrid, 28040 Madrid, Spain.

We study a two-dimensional system composed by Active Brownian Particles (ABPs), focusing on the onset of Motility Induced Phase Separation (MIPS), by means of molecular dynamics simulations. For a pure hard-disk system with no translational diffusion, the phase diagram would be completely determined by their density and Péclet number. In our model, two additional effects are present: translational noise and the overlap of particles; we study the effects of both in the phase space. As we show, the second effect can be mitigated if we use, instead of the standard Weeks-Chandler-Andersen potential, a stiffer potential: the pseudo-hard sphere potential. Moreover, in determining the boundary of our phase space, we explore different approaches to detect MIPS and conclude that observing dynamical features, via the non-Gaussian parameter, is more efficient than observing structural ones, such as through the local density distribution function. We also demonstrate that the Vogel-Fulcher equation successfully reproduces the decay of the diffusion as a function of density, with the exception of very high densities. Thus, in this regard, the ABP system behaves similar to a fragile glass.
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http://dx.doi.org/10.1063/5.0040141DOI Listing
April 2021

Particle-resolved topological defects of smectic colloidal liquid crystals in extreme confinement.

Nat Commun 2021 Jan 27;12(1):623. Epub 2021 Jan 27.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, UK.

Confined samples of liquid crystals are characterized by a variety of topological defects and can be exposed to external constraints such as extreme confinements with nontrivial topology. Here we explore the intrinsic structure of smectic colloidal layers dictated by the interplay between entropy and an imposed external topology. Considering an annular confinement as a basic example, a plethora of competing states is found with nontrivial defect structures ranging from laminar states to multiple smectic domains and arrays of edge dislocations, which we refer to as Shubnikov states in formal analogy to the characteristic of type-II superconductors. Our particle-resolved results, gained by a combination of real-space microscopy of thermal colloidal rods and fundamental-measure-based density functional theory of hard anisotropic bodies, agree on a quantitative level.
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http://dx.doi.org/10.1038/s41467-020-20842-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7840983PMC
January 2021

Stabilisation of hollow colloidal TiO particles by partial coating with evenly distributed lobes.

Soft Matter 2021 Feb;17(6):1480-1486

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK.

Photo-catalytically active crystalline TiO2 has attracted special attention due to its relevance for renewable energy and is typically obtained by the calcination of amorphous TiO2. However, stabilising hollow colloidal TiO2 particles against aggregation during calcination without compromising their photocatalytic activity poses two conflicting demands: to be stable their surface needs to be coated, while efficient photocatalysis requires an exposed TiO2 surface. Here, this incompatibility is resolved by partially coating TiO2 shells with evenly distributed 3-trimethoxysilyl propyl methacrylate (TPM) lobes. These lobes act both as steric barriers and surface charge enhancers that efficiently stabilise the TiO2 shells against aggregation during calcination. The morphology of the TPM lobes and their coverage, and the associated particle stability during the calcination-induced TiO2 crystallization, can be controlled by the pH and the contact angle between TPM and TiO2. The crystal structure and the grain size of the coated TiO2 shells are controlled by varying the calcination temperature, which allows tuning their photocatalytic activity. Finally, the durable photocatalytic activity over many usage cycles of the coated TiO2 compared to uncoated shells is demonstrated in a simple way by measuring the photo-degradation of a fluorescent dye. Our approach offers a general strategy for stabilising colloidal materials, without compromising access to their active surfaces.
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http://dx.doi.org/10.1039/d0sm02100hDOI Listing
February 2021

Shaping colloidal bananas to reveal biaxial, splay-bend nematic, and smectic phases.

Science 2020 08;369(6506):950-955

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK.

Understanding the impact of curvature on the self-assembly of elongated microscopic building blocks, such as molecules and proteins, is key to engineering functional materials with predesigned structure. We develop model "banana-shaped" colloidal particles with tunable dimensions and curvature, whose structure and dynamics are accessible at the particle level. By heating initially straight rods made of SU-8 photoresist, we induce a controllable shape deformation that causes the rods to buckle into banana-shaped particles. We elucidate the phase behavior of differently curved colloidal bananas using confocal microscopy. Although highly curved bananas only form isotropic phases, less curved bananas exhibit very rich phase behavior, including biaxial nematic phases, polar and antipolar smectic-like phases, and even the long-predicted, elusive splay-bend nematic phase.
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http://dx.doi.org/10.1126/science.abb4536DOI Listing
August 2020

Phage liquid crystalline droplets form occlusive sheaths that encapsulate and protect infectious rod-shaped bacteria.

Proc Natl Acad Sci U S A 2020 03 18;117(9):4724-4731. Epub 2020 Feb 18.

Sir William Dunn School of Pathology, University of Oxford, OX1 3RE Oxford, United Kingdom;

The opportunistic pathogen is a major cause of antibiotic-tolerant infections in humans. evades antibiotics in bacterial biofilms by up-regulating expression of a symbiotic filamentous inoviral prophage, Pf4. We investigated the mechanism of phage-mediated antibiotic tolerance using biochemical reconstitution combined with structural biology and high-resolution cellular imaging. We resolved electron cryomicroscopy atomic structures of Pf4 with and without its linear single-stranded DNA genome, and studied Pf4 assembly into liquid crystalline droplets using optical microscopy and electron cryotomography. By biochemically replicating conditions necessary for antibiotic protection, we found that phage liquid crystalline droplets form phase-separated occlusive compartments around rod-shaped bacteria leading to increased bacterial survival. Encapsulation by these compartments was observed even when inanimate colloidal rods were used to mimic rod-shaped bacteria, suggesting that shape and size complementarity profoundly influences the process. Filamentous inoviruses are pervasive across prokaryotes, and in particular, several Gram-negative bacterial pathogens including , and harbor these prophages. We propose that biophysical occlusion mediated by secreted filamentous molecules such as Pf4 may be a general strategy of bacterial survival in harsh environments.
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http://dx.doi.org/10.1073/pnas.1917726117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7060675PMC
March 2020

Colloidal Liquid Crystals Confined to Synthetic Tactoids.

Sci Rep 2019 12 31;9(1):20391. Epub 2019 Dec 31.

AMOLF, Department of Living Matter, Amsterdam, 1098XG, The Netherlands.

When a liquid crystal forming particles are confined to a spatial volume with dimensions comparable to that of their own size, they face a complex trade-off between their global tendency to align and the local constraints imposed by the boundary conditions. This interplay may lead to a non-trivial orientational patterns that strongly depend on the geometry of the confining volume. This novel regime of liquid crystalline behavior can be probed with colloidal particles that are macro-aggregates of biomolecules. Here we study director fields of filamentous fd-viruses in quasi-2D lens-shaped chambers that mimic the shape of tactoids, the nematic droplets that form during isotropic-nematic phase separation. By varying the size and aspect ratio of the chambers we force these particles into confinements that vary from circular to extremely spindle-like shapes and observe the director field using fluorescence microscopy. In the resulting phase diagram, next to configurations predicted earlier for 3D tactoids, we find a number of novel configurations. Using Monte Carlo Simulations, we show that these novel states are metastable, yet long-lived. Their multiplicity can be explained by the co-existence of multiple dynamic relaxation pathways leading to the final stable states.
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http://dx.doi.org/10.1038/s41598-019-56729-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6938498PMC
December 2019

Interrupted Motility Induced Phase Separation in Aligning Active Colloids.

Phys Rev Lett 2019 Aug;123(9):098001

Gulliver UMR CNRS 7083, ESPCI Paris, PSL Research University, 10 rue Vauquelin, 75005 Paris, France.

Switching on high activity in a relatively dense system of active Janus colloids, we observe fast clustering, followed by cluster aggregation towards full phase separation. The phase separation process is however interrupted when large enough clusters start breaking apart. Following the cluster size distribution as a function of time, we identify three successive dynamical regimes. Tracking both the particle positions and orientations, we characterize the structural ordering and alignment in the growing clusters and thereby unveil the mechanisms at play in these regimes. In particular, we identify how alignment between the neighboring particles is responsible for the interruption of the full phase separation. Our large scale quantification of the phase separation kinetics in active colloids points towards the new physics observed when both alignment and short-range repulsions are present.
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http://dx.doi.org/10.1103/PhysRevLett.123.098001DOI Listing
August 2019

Model-Free Measurement of the Pair Potential in Colloidal Fluids Using Optical Microscopy.

Phys Rev Lett 2019 Aug;123(9):098002

Department of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom.

We report a straightforward, model-free approach for measuring pair potentials from particle-coordinate data, based on enforcing consistency between the pair distribution function measured separately by the distance-histogram and test-particle insertion routes. We demonstrate the method's accuracy and versatility in simulations of simple fluids, before applying it to an experimental system composed of superparamagnetic colloidal particles. The method will enable experimental investigations into many-body interactions and allow for effective coarse graining of interactions from simulations.
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http://dx.doi.org/10.1103/PhysRevLett.123.098002DOI Listing
August 2019

Dynamics of individual Brownian rods in a microchannel flow.

Soft Matter 2019 Jul;15(29):5810-5814

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, OX1 3QZ, UK.

We study the orientational dynamics of heavy silica microrods flowing through a microfluidic channel. Comparing experiments and Brownian dynamics simulations we identify different particle orbits, in particular in-plane tumbling behavior, which cannot be explained by classical Jeffery theory, and we relate this behavior to the rotational diffusion of the rods. By constructing the full, three-dimensional, orientation distribution, we describe the rod trajectories and quantify the persistence of Jeffery orbits using temporal correlation functions of the Jeffery constant. We find that our colloidal rods lose memory of their initial configuration in about a second, corresponding to half a Jeffery period.
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http://dx.doi.org/10.1039/c9sm00903eDOI Listing
July 2019

Synthesis of Colloidal SU-8 Polymer Rods Using Sonication.

Adv Mater 2019 Apr 14;31(17):e1807514. Epub 2019 Mar 14.

Department of Chemistry Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK.

The bulk synthesis of fluorescent colloidal SU-8 polymer rods with tunable dimensions is described. The colloidal SU-8 rods are prepared by shearing an emulsion of SU-8 polymer droplets and then exposing the resulting non-Brownian rods to ultrasonic waves, which breaks them into colloidal rods with typical lengths of 3.5-10 µm and diameters of 0.4-1 µm. The rods are stable in both aqueous and apolar solvents, and by varying the composition of apolar solvent mixtures both the difference in refractive index and mass density between particles and solvent can be independently controlled. Consequently, these colloidal SU-8 rods can be used in both 3D confocal microscopy and optical trapping experiments while carefully tuning the effect of gravity. This is demonstrated by using confocal microscopy to image the liquid crystalline phases and the isotropic-nematic interface formed by the colloidal SU-8 rods and by optically trapping single rods in water. Finally, the simultaneous confocal imaging and optical manipulation of multiple SU-8 rods in the isotropic phase is shown.
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http://dx.doi.org/10.1002/adma.201807514DOI Listing
April 2019

A new family of urea-based low molecular-weight organogelators for environmental remediation: the influence of structure.

Soft Matter 2018 Nov;14(43):8821-8827

Division of Biomedical Engineering, School of Engineering, University of Glasgow, Rankine Building, Glasgow G12 8LT, UK.

Gelation processes grant access to a wealth of soft materials with tailorable properties, in applications as diverse as environmental remediation, biomedicine and electronics. Several classes of self-assembling gelators have been studied and employ non-covalent bonds to direct assembly, but recently attention has come to focus on how the overall shape of the gelator molecule impacts its gelation. Here we study a new sub-family of low molecular weight organogelators and explore how steric rearrangement influences their gelation. The gels produced are characterised with X-ray diffraction and small-angle neutron scattering (SANS) to probe their ex situ and in situ gelation mechanisms. The best examples were then tested for environmental remediation applications, gelling petrol and oils in the presence of water and salts.
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http://dx.doi.org/10.1039/c8sm01682hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6256360PMC
November 2018

Bulk synthesis of silver-head colloidal rodlike micromotors.

Soft Matter 2018 Sep;14(35):7119-7125

Institute for Advanced Study, Shenzhen University, Nanhai Avenue 3688, Nanshan District, Shenzhen, 518060, China.

Colloidal particles with asymmetric catalytic activities are emerging micro/nanomotors that harvest chemical energy for propulsion in fluids. It is of general interest to produce such particles with high performance, in large quantity and at low cost. In this paper, we present a facile bulk method to synthesize silver-head colloidal silica rods. These particles self-propel towards their active sites by reacting with hydrogen peroxide, and the velocity is tuned via the fuel concentration. We show that these motors are highly efficient; compared to the currently available chemical-phoretic micro/nanomotors they show similar performance of self-propulsion at fuel concentrations that are two orders of magnitude smaller.
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http://dx.doi.org/10.1039/c8sm00832aDOI Listing
September 2018

Communication: Contact values of pair distribution functions in colloidal hard disks by test-particle insertion.

J Chem Phys 2018 Jun;148(24):241102

Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom.

We apply Henderson's method for measuring the cavity distribution function y(r) [J. Henderson, Mol. Phys. 48, 389 (1983)] to obtain the pair distribution function at contact, g(σ). In contrast to the conventional distance-histogram method, no approximate extrapolation to contact is required. The resulting equation of state from experiments and simulations of hard disks agrees well with the scaled particle theory prediction up to high fluid packing fractions. We also provide the first experimental measurement of y(r) inside the hard core, which will allow for a more complete comparison with theory. The method's flexibility is further illustrated by measuring the partial pair distribution functions of binary hard-disk mixtures in simulation. The equation for the contact values can be used to derive familiar results from statistical geometry.
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http://dx.doi.org/10.1063/1.5038668DOI Listing
June 2018

Dislocation-controlled formation and kinetics of grain boundary loops in two-dimensional crystals.

Proc Natl Acad Sci U S A 2018 07 18;115(27):6922-6927. Epub 2018 Jun 18.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom

The formation and kinetics of grain boundaries are closely related to the topological constraints imposed on their complex dislocation structure. Loop-shaped grain boundaries are unique structures to establish such a link because their overall topological "charge" is zero due to their null net Burgers vector. Here, we observe that a local rotational deformation of a 2D colloidal crystal with an optical vortex results in a grain boundary loop only if the product of its radius and misorientation exceeds a critical value. Above this value, the deformation is plastic and the grain boundary loop spontaneously shrinks at a rate that solely depends on this product, while otherwise, the deformation is elastically restored. We show that this elastic-to-plastic crossover is a direct consequence of the unique dislocation structure of grain boundary loops. At the critical value, the loop is structurally equivalent to the so-called "flower defect" and the shrinkage rate diverges. Our results thus reveal a general limit on the formation of grain boundary loops in 2D crystals and elucidate the central role of defects in both the onset of plasticity and the kinetics of grain boundaries.
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http://dx.doi.org/10.1073/pnas.1804352115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6142269PMC
July 2018

Superparamagnetic nickel colloidal nanocrystal clusters with antibacterial activity and bacteria binding ability.

Nat Nanotechnol 2018 06 2;13(6):478-482. Epub 2018 Apr 2.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, UK.

Recent progress in synthetic nanotechnology and the ancient use of metals in food preservation and the antibacterial treatment of wounds have prompted the development of nanometallic materials for antimicrobial applications. However, the materials designed so far do not simultaneously display antimicrobial activity and the capability of binding and capturing bacteria and spores. Here, we develop a one-step pyrolysis procedure to synthesize monodisperse superparamagnetic nickel colloidal nanocrystal clusters (SNCNCs), which show both antibacterial activity and the ability to bind Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria, as well as bacterial spores. The SNCNCs are formed from a rapid burst of nickel nanoparticles, which self-assemble slowly into clusters. The clusters can magnetically extract 99.99% of bacteria and spores and provide a promising approach for the removal of microbes, including hard-to-treat microorganisms. We believe that our work illustrates the exciting opportunities that nanotechnology offers for alternative antimicrobial strategies and other applications in microbiology.
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http://dx.doi.org/10.1038/s41565-018-0108-0DOI Listing
June 2018

Bond-orientational order and Frank's constant in two-dimensional colloidal hard spheres.

J Phys Condens Matter 2018 03;30(10):104003

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom. Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom.

Recently, the full phase behaviour of 2D colloidal hard spheres was experimentally established, and found to involve a first order liquid to hexatic transition and a continuous hexatic to crystal transition (Thorneywork et al 2017 Phys. Rev. Lett. 118 158001). Here, we expand upon this work by considering the behaviour of the bond-orientational correlation time and Frank's constant in the region of these phase transitions. We also consider the excess entropy, as calculated from the radial distribution functions, for a wide range of area fractions covering the liquid, hexatic and crystal phases. In all cases, the behaviour of these quantities further corroborates the previously reported melting scenario of 2D colloidal hard spheres.
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http://dx.doi.org/10.1088/1361-648X/aaab31DOI Listing
March 2018

Structural disorder, filament growth and self-poisoning in short rods confined onto a flat wall.

Soft Matter 2017 Nov;13(46):8678-8683

Institute for Advanced Study, Shenzhen University, Nanshan District, Shenzhen, Guangzhou, 518060, China.

Confocal microscopy was used to directly observe the structural coarsening of the first layer of short colloidal rods sedimented onto a flat wall. Based on an image analysis algorithm we devised, quantitative information on the location, orientation and length of each particle can be extracted with high precision. At high density the system undergoes structural arrest, and becomes trapped in a disordered state of randomly arranged filaments that are composed of side-by-side aligned rods. The frustration of structural order is signalled by a new peak that emerges in the radial distribution function. Configuration analysis shows that the peak is primarily due to pairs of particles that are arranged in a "T" shape, a configuration that is compatible with neither crystallization nor filament growth. Our results point to a self-poisoning mechanism for the frustration of structural order, and highlight the importance of particle shape in controlling colloidal assembly thus materials properties.
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http://dx.doi.org/10.1039/c7sm01761hDOI Listing
November 2017

Two-Dimensional Melting of Colloidal Hard Spheres.

Phys Rev Lett 2017 Apr 10;118(15):158001. Epub 2017 Apr 10.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.

We study the melting of quasi-two-dimensional colloidal hard spheres by considering a tilted monolayer of particles in sedimentation-diffusion equilibrium. In particular, we measure the equation of state from the density profiles and use time-dependent and height-resolved correlation functions to identify the liquid, hexatic, and crystal phases. We find that the liquid-hexatic transition is first order and that the hexatic-crystal transition is continuous. Furthermore, we directly measure the width of the liquid-hexatic coexistence gap from the fluctuations of the corresponding interface, and thereby experimentally establish the full phase behavior of hard disks.
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http://dx.doi.org/10.1103/PhysRevLett.118.158001DOI Listing
April 2017

Dynamic heterogeneities and non-Gaussian behavior in two-dimensional randomly confined colloidal fluids.

Phys Rev E 2017 Mar 6;95(3-1):032602. Epub 2017 Mar 6.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.

A binary mixture of superparamagnetic colloidal particles is confined between glass plates such that the large particles become fixed and provide a two-dimensional disordered matrix for the still mobile small particles, which form a fluid. By varying fluid and matrix area fractions and tuning the interactions between the superparamagnetic particles via an external magnetic field, different regions of the state diagram are explored. The mobile particles exhibit delocalized dynamics at small matrix area fractions and localized motion at high matrix area fractions, and the localization transition is rounded by the soft interactions [T. O. E. Skinner et al., Phys. Rev. Lett. 111, 128301 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.128301]. Expanding on previous work, we find the dynamics of the tracers to be strongly heterogeneous and show that molecular dynamics simulations of an ideal gas confined in a fixed matrix exhibit similar behavior. The simulations show how these soft interactions make the dynamics more heterogeneous compared to the disordered Lorentz gas and lead to strong non-Gaussian fluctuations.
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http://dx.doi.org/10.1103/PhysRevE.95.032602DOI Listing
March 2017

Self-diffusion in two-dimensional binary colloidal hard-sphere fluids.

Phys Rev E 2017 Jan 31;95(1-1):012614. Epub 2017 Jan 31.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.

We present a systematic experimental study of the dynamic behavior of monodisperse and bidisperse two-dimensional colloidal hard-sphere fluids. We consider the diffusive behavior of the two types of particles for systems with a variety of compositions and total area fractions. In particular, we measure the short- and long-time diffusion coefficients for both species independently. We find that the short-time self-diffusion coefficients show an approximately linear dependence on the area fraction and that the long-time self-diffusion coefficients are well described by an expression dependent upon only the area fraction and contact value of the radial distribution function. Finally, we consider the effect of composition change and find some variation in the long-time self-diffusion coefficients, which we ascribe to the complex packing effects exhibited by binary systems.
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http://dx.doi.org/10.1103/PhysRevE.95.012614DOI Listing
January 2017

The effect of colloidal aggregates on fat crystal networks.

Food Funct 2017 Jan;8(1):352-359

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK.

We investigate the role of a colloidal model system in the crystallization and network formation of lipids. This system consists of fractal fumed silica aggregates. We look at the influence of different solid fat concentrations and fumed silica concentrations on the resulting gel network. Oscillatory rheology shows that the addition of silica to fat-in-oil gels does not significantly affect the magnitude of the storage modulus within the linear viscoelastic region. Interestingly, the range of this region is increased. Differential scanning calorimetry shows that the presence of silica leads to slightly earlier crystallization, though no significant effect on the melting profile of the formed network is found. Based on these observations, we propose that composite gel network structures have been formed. These results show that we have created reduced solid fat alternatives with similar rheological behaviour and thermal properties as the full-fat systems through the addition of colloidal silica.
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http://dx.doi.org/10.1039/c6fo01622gDOI Listing
January 2017

Colloidal liquid crystals in square confinement: isotropic, nematic and smectic phases.

J Phys Condens Matter 2017 Feb 21;29(6):064003. Epub 2016 Dec 21.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK.

We report on the confinement of colloidal liquid crystals in three dimensional chambers with a square footprint. To this end we use colloidal silica rods and exploit their relatively large density difference with respect to the dispersing solvent to study isotropic, nematic and smectic phases confined into a single chamber. Combining laser scanning confocal microscopy and soft-lithography techniques enables us to characterize the configurations down to the single particle level. We will focus on the smectic phase and compare to recent theories and simulations.
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http://dx.doi.org/10.1088/1361-648X/29/6/064003DOI Listing
February 2017

Equilibrium Grain Boundary Segregation and Clustering of Impurities in Colloidal Polycrystalline Monolayers.

Langmuir 2016 12 23;32(48):12716-12724. Epub 2016 Nov 23.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford , South Parks Road, Oxford OX1 3QZ, United Kingdom.

We investigate the segregation of impurities to grain boundaries in colloidal polycrystalline monolayers using video microscopy. A model colloidal alloy is prepared by embedding large spherical impurities in a polycrystalline monolayer of small host colloidal hard spheres, which stops grain growth at a finite grain size. The size ratio between the impurities and the host particles determines whether they behave as interstitial or substitutional impurities in the bulk crystal, akin to those in real alloys. We find that the partitioning of impurities between the grains and the grain boundaries is in very good agreement with the Langmuir-McLean adsorption model for equilibrium grain boundary segregation. This enables the direct measurement of the free energy of adsorption for the two types of impurities. Near saturation, we characterize the spatial distribution of the adsorbed impurities and find that it strongly depends on their interstitial or substitutional nature. This is because the relative importance of clustering and mixing due to nonadditivity is determined by geometrical constraints imposed by the crystalline host lattice.
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http://dx.doi.org/10.1021/acs.langmuir.6b02683DOI Listing
December 2016

Geometry-induced capillary emptying.

Proc Natl Acad Sci U S A 2016 Nov 24;113(45):12633-12636. Epub 2016 Oct 24.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom.

When a capillary is half-filled with liquid and turned to the horizontal, the liquid may flow out of the capillary or remain in it. For lack of a better criterion, the standard assumption is that the liquid will remain in a capillary of narrow cross-section, and will flow out otherwise. Here, we present a precise mathematical criterion that determines which of the two outcomes occurs for capillaries of arbitrary cross-sectional shape, and show that the standard assumption fails for certain simple geometries, leading to very rich and counterintuitive behavior. This opens the possibility of creating very sensitive microfluidic devices that respond readily to small physical changes, for instance, by triggering the sudden displacement of fluid along a capillary without the need of any external pumping.
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http://dx.doi.org/10.1073/pnas.1606217113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5111693PMC
November 2016

Core-Shell Particles for Simultaneous 3D Imaging and Optical Tweezing in Dense Colloidal Materials.

Adv Mater 2016 Sep 5;28(36):8001-8006. Epub 2016 Jul 5.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom.

A new colloidal system that consists of core-shell "probe" particles embedded in an optically transparent "host" particle suspension is developed. This system enables simultaneous fast confocal imaging and optical tweezing in dense 3D colloidal materials.
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http://dx.doi.org/10.1002/adma.201602137DOI Listing
September 2016

Finite particle size drives defect-mediated domain structures in strongly confined colloidal liquid crystals.

Nat Commun 2016 06 29;7:12112. Epub 2016 Jun 29.

Department of Systems Biophysics, FOM Institute AMOLF, Science Park 104, 1098XG Amsterdam, The Netherlands.

When liquid crystals are confined to finite volumes, the competition between the surface anchoring imposed by the boundaries and the intrinsic orientational symmetry-breaking of these materials gives rise to a host of intriguing phenomena involving topological defect structures. For synthetic molecular mesogens, like the ones used in liquid-crystal displays, these defect structures are independent of the size of the molecules and well described by continuum theories. In contrast, colloidal systems such as carbon nanotubes and biopolymers have micron-sized lengths, so continuum descriptions are expected to break down under strong confinement conditions. Here, we show, by a combination of computer simulations and experiments with virus particles in tailor-made disk- and annulus-shaped microchambers, that strong confinement of colloidal liquid crystals leads to novel defect-stabilized symmetrical domain structures. These finite-size effects point to a potential for designing optically active microstructures, exploiting the as yet unexplored regime of highly confined liquid crystals.
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http://dx.doi.org/10.1038/ncomms12112DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4931596PMC
June 2016

On the Gaussian approximation in colloidal hard sphere fluids.

Soft Matter 2016 05 11;12(18):4129-34. Epub 2016 Apr 11.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK.

We study the behaviour of the self-intermediate scattering function and self-van Hove correlation function for quasi-two-dimensional colloidal hard sphere fluids at a range of area fractions. We compute these functions first directly from the particle coordinates and secondly from the mean squared displacement via the Gaussian approximation. This allows us to test the validity of this approximation over a range of length and time scales, where we find that the Gaussian approximation holds if the hydrodynamic limits are appropriately probed. Surprisingly, only small deviations from Gaussian behaviour are seen at intermediate times, even for dense fluids. We next consider these deviations from Gaussian behaviour firstly via the non-Gaussian parameter and secondly by considering the relaxation times of the intermediate scattering function. From these measurements we develop a scaling relation in order to directly determine the combinations of wavevectors and times at which the non-Gaussian behavior is seen. This allows for the clear identification of the hydrodynamic regimes and thus provides new insight into the crossover between long- and short-time self-diffusion.
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http://dx.doi.org/10.1039/c5sm03049hDOI Listing
May 2016
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