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Job Thijssen

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Contactless interfacial rheology: Probing shear at liquid–liquid interfaces without an interfacial geometry via fluorescence microscopy

Iain Muntz, James A. Richards, Sam Brown, Andrew B. Schofield, Marcel Rey and Job H. J. Thijssen

Journal of Rheology 67, 67 (2023)

Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method, we have measured steady shear material parameters such as interfacial elastic moduli for interfaces with solidlike behavior and interfacial viscosities for fluidlike interfaces. The accuracy of this method has been verified relative to a double-wall ring geometry. Moreover, using our contactless method, we are able to measure lower interfacial viscosities than those that have previously been reported using a double-wall ring geometry. A further advantage is the simultaneous combination of macroscopic rheological analysis with microscopic structural analysis. Our analysis directly visualizes how the interfacial response is strongly correlated to the particle surface coverage and their interfacial assembly. Furthermore, we capture the evolution and irreversible changes in the particle assembly that correspond with the rheological response to steady shear.

DOI: 10.1122/8.0000559

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Bicontinuous Soft Solids with a Gradient in Channel Size

David J. French, Andrew B. Schofield and Job H. J. Thijssen

Advanced Materials Interfaces , 2102307 (2022)

This paper presents examples of bicontinuous interfacially jammed emulsion gels (“bijels”) with a designed gradient in the channel size along the sample. These samples are created by quenching binary fluids which have a gradient in particle concentration along the sample, since the channel size is determined by the local particle concentration. A gradient in local particle concentration is achieved using a two-stage loading process, with different particle volume fractions in each stage. Confocal microscopy and image analysis are used to quantitatively measure the channel size of the bijels. Bijels with a gradient in channel size of up to 2.8% mm−1 have been created. Such tailored soft materials could act as templates for energy materials optimized for both high ionic transport rates (high power) and high interfacial area (high energy density), potentially making them useful in novel energy applications.

DOI: 10.1002/admi.202102307

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Participation, performance, and outcomes in an undergraduate physics degree: Perspectives on gender and socioeconomic factors

Rachel E. Harrington, Job H. J. Thijssen and Judy Hardy

Physical Review Physics Education research 18, 010114 (2022)

The academic performance of undergraduate physics students at a research-intensive UK university were studied to identify whether there were any significant differences between underrepresented and overrepresented groups. Exit qualification, degree classification, average yearly marks, and course marks were analyzed to determine statistically significant associations with gender or widening participation status. Significant association was found between gender and the integrated masters qualification, with a higher proportion of female students attaining a first class or second class (upper) classification. A performance gap was identified between widening participation and non-widening participation students in second year, but no significant association was found between widening participation status and exit qualification or degree classification. In examining the intersection of gender and widening participation status, no significant association was found between these groups and exit qualification. These results contrast with previous studies, which suggest that the effects of underrepresentation adversely impact academic performance. The reasons why this might be the case, with particular consideration of the context of the sample studied, are discussed.

DOI: 10.1103/PhysRevPhysEducRes.18.010114

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Inclusion in the time of COVID: 14 ways to seize the moment for change

Carla Cebula, Katie Nicoll Baines, Catherine Lido, Job H. J. Thijssen, Karen Halliday, Nicki Hedge, Helen Mulvana and Caroline Gauchotte Lindsay

Nature Index 9 Feb (2021)

Flux in the system is a chance to create new and better opportunities in academic STEMM careers for marginalized groups.

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Surface pressure of liquid interfaces laden with micron-sized particles

Rudi Mears, Iain Muntz and Job H. J. Thijssen

Soft Matter 16, 9347-9356 (2020)

We consider the surface pressure of a colloid–laden liquid interface. As micron-sized particles of suitable wettability can be irreversibly bound to the liquid interface on experimental timescales, we use the canonical ensemble to derive an expression for the surface pressure of a colloid–laden interface. We use this expression to show that adsorption of particles with only hard-core interactions has a negligible effect on surface pressures from typical Langmuir-trough measurements. Moreover, we show that Langmuir-trough measurements cannot be used to extract typical interparticle potentials. Finally, in the case of relatively weakly interacting sterically stabilized particles at a liquid interface, we argue that the dependence of measured surface pressure on surface fraction can be explained by particle coordination number at low to intermediate particle surface fractions. At high surface fractions, where the particles are jammed and cannot easily rearrange, we argue that contact-line sliding and/or deformations of the liquid interface at the length scale of the particles might play a pivotal role.

DOI: 10.1039/D0SM01229G

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Soft matter science and the COVID-19 pandemic

Wilson C. K. Poon, Aidan T. Brown, Susana O. L. Direito, Daniel J. M. Hodgson, Lucas Le Nagard, Alex Lips, Cait E. MacPhee, Davide Marenduzzo, John R. Royer, Andreia F. Silva, Job H. J. Thijssen and Simon Titmuss

Soft Matter 16, 8310-8324 (2020)

Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes controls their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. We survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.

DOI: 10.1039/D0SM01223H

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Electrostatic potential between charged particles at an oil-water interface

Alexander Morozov, Iain Muntz, Job H. J. Thijssen and Davide Marenduzzo

Physical Review E 102, 020801(R) (2020)

Electrostatic interactions between point charges embedded into interfaces separating dielectric media are omnipresent in soft matter systems and often control their stability. Such interactions are typically complicated and do not resemble their bulk counterparts. For instance, the electrostatic potential of a point charge at an air-water interface falls off as 1/r^3, where r is the distance from the charge, exhibiting a dipolar behaviour. This behaviour is often assumed to be generic, and is widely referred to when interpreting experimental results. Here we explicitly calculate the in-plane potential of a point charge at an interface between two electrolyte solutions with different, finite dielectric permittivities and Debye screening lengths, such as oil and water. We show that the asymptotic behaviour of this potential is neither a dipole, which characterises the potential at air-water interfaces, nor a screened monopole, which describes the bulk behaviour in a single electrolyte solution. By considering the same problem in arbitrary dimensions, we find that the physics behind this difference can be traced to the asymmetric propagation of the interaction in the two media. Our results should be relevant to understand the effective potential acting between interfacial proteins in biofilms, and the self-assembly of charged colloids at droplet surfaces in oil-water emulsions. © 2020 American Physical Society

DOI: 10.1103/PhysRevE.102.020801

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Interaction between nearly hard colloidal spheres at an oil-water interface

Iain Muntz, Franceska Waggett, Michael Hunter, Andrew B. Schofield, Paul Bartlett, Davide Marenduzzo and Job H. J. Thijssen

Physical Review Research 2, 023388 (2020)

We show that the interaction potential between sterically stabilized, nearly hard-sphere [poly(methyl methacrylate)–poly(lauryl methacrylate) (PMMA-PLMA)] colloids at a water-oil interface has a negligible unscreened-dipole contribution, suggesting that models previously developed for charged particles at liquid interfaces are not necessarily applicable to sterically stabilized particles. Interparticle potentials, U(r), are extracted from radial distribution functions [g(r), measured by fluorescence microscopy] via Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results are then validated by particle tracking in a blinking optical trap. Using a Bayesian model comparison, we find that our PMMA-PLMA data is better described by a screened monopole only rather than a functional form having a screened monopole plus an unscreened dipole term. We postulate that the long range repulsion we observe arises mainly through interactions between neutral holes on a charged interface, i.e., the charge of the liquid interface cannot, in general, be ignored. In agreement with this interpretation, we find that the interaction can be tuned by varying salt concentration in the aqueous phase. Inspired by recent theoretical work on point charges at dielectric interfaces, which we explain is relevant here, we show that a screened 1/r^2 term can also be used to fit our data. Finally, we present measurements for poly(methyl methacrylate)–poly(12-hydroxystearic acid) (PMMA-PHSA) particles at a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an additional contribution to the interaction potential. This is in line with our optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate that they are slightly charged.

DOI: 10.1103/PhysRevResearch.2.023388

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Cs3Bi2I9 as high-performance electrode material achieving high capacitance and stability in an economical supercapacitor

Keir Adams, John Mallows, Tianyue Li, Dimitrios Kampouris, Job H. J. Thijssen and Neil Robertson

JPhys Energy 1, 034001 (2019)

Supercapacitors are well-known as promising energy storage devices capable of bridging the gap between conventional electrolytic capacitors and batteries to deliver both high power and energy densities for applications in electric vehicles and a smart energy grid. However, many reported instances of high-capacitance pseudocapacitors employ strong Faradaic reactions that hinder fast charge-discharge cycles and long-term stability, limiting their commercial viability. In this study, we utilize an economical and solution-processable procedure to fabricate a Cs3Bi2I9-based symmetric supercapacitor employing both electric double layer capacitance and pseudocapacitance with an aqueous NaClO4 electrolyte to deliver an outstanding device areal capacitance of 2.4 F cm-2 and specific capacitance of 280 F g-1. The Cs3Bi2I9 device achieves an excellent 88% capacitance retention after 5000 charge-discharge cycles, proving its long-term cycle stability and promise as a practical supercapacitor. We characterize the time-dependent charge storage mechanisms through cyclic voltammetry and electrochemical impedance spectroscopy to find that electrostatic charge accumulation predominates at high potentials (0.3-0.6 V) whereas weak, Faradaic charge adsorption and pore penetration bolster charge storage at lower potentials (0.0-0.2 V).

DOI: 10.1088/2515-7655

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Thiourea Bismuth Iodide: Crystal structure, Characterization and High Performance as an Electrode Material for Supercapacitors

Tianyue Li, John Mallows, Keir Adams, Gary S. Nichol, Job H. J. Thijssen and Neil Robertson

Batteries & Supercaps 2, 568 (2019)

This paper reports on the synthesis, crystal structure and application of a novel hybrid bismuth-halide complex: (CN2SH5)3BiI6 (TBI) for supercapacitor applications, featuring merits including high areal capacitance, low cost, solution-processability and non-toxicity. Single crystal X-ray diffraction reveals that TBI crystallizes in the monoclinic system, with discrete [BiI6]3- octahedra as the inorganic motif. Utilizing TBI as the active supercapacitor electrode material with carbon cloth current collector and aqueous NaClO4 electrolyte, an electrode areal capacitance of 3.32 F/cm2 and a systemic specific capacitance of 1030 F/g was achieved when the device operates as an electric double-layer capacitor (EDLC). The supercapacitor device shows excellent capacitance retention even after 5,000 charge-discharge cycles. The powder XRD patterns, Raman spectroscopy and SEM images of TBI electrodes were compared before and after the cycling test to demonstrate the material stability and investigate the film morphology.

DOI: 10.1002/batt.201900005

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Particle-Stabilized Janus Emulsions that Exhibit pH-Tunable Stability

Tao Li, Andrew B Schofield, Ke Chen, >Job H. J. Thijssen, Jure Dobnikar and Paul Clegg

Chemical Communications 55, 5773 (2019)

By developing a deeper understanding of the formation mechanism and the origin of the stability, we report a simple and large-scale fabrication approach to create Janus emulsions that can be controlled in size, geometry and stability.

DOI: 10.1039/C8CC09842E

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Facile Synthesis and Characterization of Bi13S18I2 Films as a Stable Supercapacitor Electrode Material

Keir Adams, Alba Franco Gonzalez, John Mallows, Tianyue Li, Job H. J. Thijssen and Neil Robertson

Journal of Materials Chemistry A 7, 1638 (2018)

Electrical double layer capacitors (EDLCs) featuring low-cost and solution-processable electrode materials have attracted significant research interest for their green and economical applications in energy harvesting and storage devices. Here, we demonstrate a novel synthetic route for films of an underexplored 3-D hexagonal bismuth chalcohalide, Bi13S18I2, and investigate its potential as the active electrode material in EDLC-type supercapacitors. The synthetic procedure has been optimised and comprises the lowest annealing temperature (150°C) and the shortest processing time (1 h) currently reported. When integrated in a symmetrical EDLC with an aqueous NaClO4 electrolyte, the Bi13S18I2-based device achieves a remarkable areal capacitance of 210.68 mF cm-2 with 99.7% capacitance retention after 5000 cycles. Both the Bi13S18I2 powder and thin-film electrodes have been characterized through XRD, XPS, Raman spectroscopy, and SEM. The superior stability, low-cost, and facile synthesis of Bi13S18I2 proves its promising potential for supercapacitor applications.

DOI: 10.1039/C8TA11029H

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SI

Direct transformation of bijels into bicontinuous composite electrolytes using a pre-mix containing lithium salt

Dongyu Cai, Felix H. Richter, Job H. J. Thijssen, Peter G. Bruce and Paul Clegg

Materials Horizon 5, 499 (2018)

We report a general strategy for making bicontinuous conducting composite materials in a controllable fashion. Our approach begins with a bicontinuous interfacially jammed emulsion gel (bijel) fabricated from a pre-mix containing a salt, here bis(trifluoromethane)sulfonimide lithium salt (LiTFSI). The resulting structure has interpenetrating ionic conducting and non-conducting domains comprised of an ethylene carbonate (EC)-rich phase and a p-xylene (xylene)-rich phase of roughly equal volumes. This is the first time that bijel fabrication has been carried out with the underlying two-fluid phase diagram modified by a salt. Diffusing polystyrene (PS) into the xylene-rich phase enables the facile formation of a PS-filled bijel in place of a multi-step polymerization of added monomers. Drying the bijel results in the selective removal of xylene, reducing the total sample volume without compromising the morphology of the EC domain. Electrochemical impedance spectroscopy of the composite electrolytes confirms the existence of ion conducting pathways.

DOI: 10.1039/C7MH01038A

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Hybrid electrolytes with 3D bicontinuous ordered ceramic and polymer microchannels for all-solid-state batteries

Stefanie Zekoll, Cassian Marriner-Edwards, A. K. Ola Hekselman, Jitti Kasemchainan, Christian Kuss, David E. J. Armstrong, Dongyu Cai, Robert J. Wallace, Felix H. Richter, Job H. J. Thijssen and Peter G. Bruce

Energy & Environmental Science 11, P185 (2018)

Hybrid solid electrolytes, composed of 3D ordered bicontinuous conducting ceramic and insulating polymer microchannels are reported. The ceramic channels provide continuous, uninterrupted pathways, maintaining high ionic conductivity between the electrodes, while the polymer channels permit improvement of the mechanical properties from that of the ceramic alone, in particular mitigation of the ceramic brittleness. The conductivity of a ceramic electrolyte is usually limited by resistance at the grain boundaries, necessitating dense ceramics. The conductivity of the 3D ordered hybrid is reduced by only the volume fraction occupied by the ceramic, demonstrating that the ceramic channels can be sintered to high density similar to a dense ceramic disk. The hybrid electrolytes are demonstrated using the ceramic lithium ion conductor Li1.4Al0.4Ge1.6(PO4)3 (LAGP). Structured LAGP 3D scaffolds with empty channels were prepared by negative replication of a 3D printed polymer template. Filling the empty channels with non-conducting polypropylene (PP) or epoxy polymer (epoxy) creates the structured hybrid electrolytes with 3D bicontinuous ceramic and polymer microchannels. Printed templating permits precise control of the ceramic to polymer ratio and the microarchitecture; as demonstrated by the formation of cubic, gyroidal, diamond and spinodal (bijel) structures. The electrical and mechanical properties depend on the microarchitecture, the gyroid filled with epoxy giving the best combination of conductivity and mechanical properties. An ionic conductivity of 1.6 × 10−4 S cm−1 at room temperature was obtained, reduced from the conductivity of a sintered LAGP pellet only by the volume fraction occupied by the ceramic. The mechanical properties of the gyroid LAGP–epoxy electrolyte demonstrate up to 28% higher compressive failure strain and up to five times the flexural failure strain of a LAGP pellet before rupture. Notably, this demonstrates that ordered ceramic and polymer hybrid electrolytes can have superior mechanical properties without significantly compromising ionic conductivity, which addresses one of the key challenges for all-solid-state batteries.

DOI: 10.1039/C7EE02723K

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Interfacial rheology of model particles at liquid interfaces and its relation to (bicontinuous) Pickering emulsions

J. H. J. Thijssen and J. Vermant

Journal of Physics: Condensed Matter 40, E023002 (2018)

Interface-dominated materials are commonly encountered in both science and technology, and typical examples include foams and emulsions. Conventionally stabilised by surfactants, emulsions can also be stabilised by micron-sized particles. These so-called Pickering–Ramsden (PR) emulsions have received substantial interest, as they are model arrested systems, rather ubiquitous in industry and promising templates for advanced materials. The mechanical properties of the particle-laden liquid–liquid interface, probed via interfacial rheology, have been shown to play an important role in the formation and stability of PR emulsions. However, the morphological processes which control the formation of emulsions and foams in mixing devices, such as deformation, break-up, and coalescence, are complex and diverse, making it difficult to identify the precise role of the interfacial rheological properties. Interestingly, the role of interfacial rheology in the stability of bicontinuous PR emulsions (bijels) has been virtually unexplored, even though the phase separation process which leads to the formation of these systems is relatively simple and the interfacial deformation processes can be better conceptualised. Hence, the aims of this topical review are twofold. First, we review the existing literature on the interfacial rheology of particle-laden liquid interfaces in rheometrical flows, focussing mainly on model latex suspensions consisting of polystyrene particles carrying sulfate groups, which have been most extensively studied to date. The goal of this part of the review is to identify the generic features of the rheology of such systems. Secondly, we will discuss the relevance of these results to the formation and stability of PR emulsions and bijels.

DOI: 10.1088/1361-648X/aa9c74

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Interfacial Rheology of Sterically Stabilized Colloids at Liquid Interfaces and Its Effect on the Stability of Pickering Emulsions

R. Van Hooghten, V. E. Blair, A. Vananroye, A. B. Schofield, J. Vermant and J. H. J. Thijssen

Langmuir 33, P4107 (2017)

Particle-laden interfaces can be used to stabilize a variety of high-interface systems, from foams over emulsions to polymer blends. The relation between the particle interactions, the structure and rheology of the interface, and the stability of the system remains unclear. In the present work, we experimentally investigate how micron-sized, near-hard-sphere-like particles affect the mechanical properties of liquid interfaces. In particular, by comparing dried and undried samples, we investigate the effect of aggregation state on the properties of the particle-laden liquid interface and its relation to the stability of the corresponding Pickering emulsions. Partially aggregated suspensions give rise to a soft-solid-like response under shear, whereas for stable PMMA particulate layers a liquid-like behavior is observed. For interfacial creep-recovery measurements, we present an empirical method to correct for the combined effect of the subphase drag and the compliance of the double-wall ring geometry, which makes a significant contribution to the apparent elasticity of weak interfaces. We further demonstrate that both undried and dried PMMA particles can stabilize emulsions for months, dispelling the notion that particle aggregation, in bulk or at the interface, is required to create stable Pickering emulsions. Our results indicate that shear rheology is a sensitive probe of colloidal interactions but is not necessarily a predictor of the stability of interfaces, e.g., in quiescent Pickering emulsions, as in the latter the response to dilatational deformations can be of prime importance.

DOI: 10.1021/acs.langmuir.6b04365

Compressing a spinodal surface at fixed area: the bijel in a centrifuge

K. A. Rumble, J. H. J. Thijssen, A. B. Schofield and P. S. Clegg

Soft Matter 12, P4375 (2016)

Bicontinuous interfacially jammed emulsion gels (bijels) are solid-stabilised emulsions with two inter-penetrating continuous phases. Employing the method of centrifugal compression we find that macroscopically the bijel yields at relatively low angular acceleration. Both continuous phases escape from the top of the structure making any compression immediately irreversible. Microscopically, the bijel becomes anisotropic with the domains aligned perpendicular to the compression direction which inhibits further liquid expulsion; this contrasts strongly to the sedimentation behaviour of colloidal gels. The original structure can, however, be preserved close to the top of the sample and thus the change to an anisotropic structure suggests internal yielding. Any air bubbles trapped in the bijel are found to aid compression by forming channels aligned parallel to the compression direction which provide a route for liquid to escape.

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DOI: 10.1039/C6SM00168H

Quantitative Morphological Characterization of Bicontinuous Pickering Emulsions via Interfacial Curvatures

M. Reeves, K. Stratford, J. H. J. Thijssen

Soft Matter 12, P4082 (2016)

Bicontinuous Pickering emulsions (bijels) are a physically interesting class of soft materials with many potential applications including catalysis, microfluidics and tissue engineering. They are created by arresting the spinodal decomposition of a partially-miscible liquid with a (jammed) layer of interfacial colloids. Porosity L (average interfacial separation) of the bijel is controlled by varying the radius (r) and volume fraction (f) of the colloids (L ~ r/f). However, to optimize the bijel structure with respect to other parameters, e.g. quench rate, characterizing by L alone is insufficient. Hence, we have used confocal microscopy and X-ray CT to characterize a range of bijels in terms of local and area-averaged interfacial curvatures; we further demonstrate that bijels are bicontinuous using an image-analysis technique known as `region growing'. In addition, the curvatures of bijels have been monitored as a function of time, which has revealed an intriguing evolution up to 60 minutes after bijel formation, contrary to previous understanding.

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Supplementary information

DOI: 10.1039/C5SM03102H

Particle-size effects in the formation of bicontinuous Pickering emulsions

M. Reeves, A. T. Brown, A. B. Schofield, M. E. Cates and J. H. J. Thijssen

Physical Review E 92, E032308 (2015)

We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway, i.e., nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.

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DOI: 10.1103/PhysRevE.92.032308

Featured in PRE's September 2015 Kaleidoscope.

Bicontinuous Emulsions Stabilized by Colloidal Particles

J. W. Tavacoli, J. H. J. Thijssen and P. S. Clegg

Chapter 6 in: Particle-Stabilized Emulsions and Colloids - Formation and Applications, RSC (2015)

The liquid-liquid phase separation of binary fluids, induced by a temperature quench, can be arrested by colloidal particles trapped at the interface. The arrested structure, a novel soft solid known as a bicontinuous interfacially jammed emulsion gel (bijel), was first predicted by computer simulations and subsequently realized in the lab. Initially the mechanical properties are controlled by the interfacial tension between the two fluid domains (e.g. a deeper quench yields a stronger bijel) and the volume fraction of particles. Reversing the temperature quench results in the two fluids becoming miscible again. Whether or not this leaves a colloidal gel in place which is stable without a liquid-liquid interface (a 'monogel') depends on the age of the gel and the choice of liquids. In this chapter we describe the current state of bijel research using experimental, theoretical and computational approaches. We discuss possible areas of application and, finally, we contrast the physical route with which the bijel is prepared with a related material that is prepared via direct particle-particle interactions.

DOI: 10.1039/9781782620143-00129

Making non-aqueous high internal phase Pickering emulsions: influence of added polymer and selective drying

D. Cai, J. H. J. Thijssen and P. S. Clegg

ACS Applied Materials and Interfaces 23, P9214 (2014)

We report the first example of a non-aqueous (oil-in-oil) Pickering high internal phase emulsion (HIPE) stabilized by chemically modified fumed silica. In this case, a 75 vol % ethylene carbonate (EC)-rich internal phase is emulsified in 25 vol % p-xylene (xylene)-rich continuous phase using interfacial nanoparticles. It is revealed that no phase inversion takes place during the HIPE formation process when using the appropriate wettability of solid particles. Incorporating polystyrene (PS) into xylene enables one-step formation of PS-filled HIPEs in place of a multi-step polymerization of the continuous phase. We observe that the size of droplets changes with the addition of PS, and we associate this with the change in the viscosity of the continuous xylene-rich phase. Drying the pure HIPE results in the selective removal of xylene and coalescence of EC-rich droplets. With the PS in the xylene-rich continuous phase, we show that EC-rich droplets can be retained even though the xylene is evaporated off, and a new semi-solid composite containing both liquid phase and solid phase is formed via this non-aqueous Pickering-HIPE template.

DOI: 10.1021/am501328r

Squeezing particle-stabilized emulsions into biliquid foams – equation of state

L. Maurice, R. A. Maguire, A. B. Schofield, M. E. Cates, P. S. Clegg and J. H. J. Thijssen

Soft Matter 9, P7757 (2013)

Using a centrifuge, we measure the (pressure vs. density) equation of state of Pickering emulsions stabilized by hard-sphere colloids, in order to elucidate the particle contribution to their mechanical properties. Moreover, we have developed a transparent Pickering emulsion, allowing us to determine local volume fraction as a function of distance within the sediment using confocal microscopy, thus extracting an entire equation of state from one centrifugation cycle. We can explain and predict trends in our data using a quantitative model incorporating interdroplet films with a thickness on the scale of the (micron-sized) particles and repulsive interactions across these films. We suggest that the effective repulsion between droplets is due to the deformation of the liquid–liquid interface between particles on one droplet due to compression against a neighbouring droplet.

Making a Robust Interfacial Scaffold: Bijel Rheology and its Link to Processability

M. N. Lee, J. H. J. Thijssen, J. A. Witt, P. S. Clegg and A. Mohraz

Advanced Functional Materials 23, P417 (2013)

Confocal microscopy and rheology studies of two bijel systems are presented to elucidate relationships between the physicochemical properties of bijels and their ability to be utilized as soft matter templates for materials synthesis. For the first time, the origins of viscoelasticity in these systems are investigated using conventional rheometry and a direct correspondence between the elastic storage modulus, particle loading, and the departure from criticality is observed. Further, the rheological transitions that accompany fluid re-mixing in bijels are characterized, providing key insights into the synergistic role of interfacial tension and interparticle interactions in mediating their mechanical robustness. Bijels that are predominantly stabilized by interfacial tension are also highly sensitive to gradients in chemical composition and more easily prone to mechanical failure during processing. Despite this increased sensitivity, a modified strategy for processing these more delicate systems is developed and its efficacy is demonstrated by synthesizing a bicontinuous macroporous hydrogel scaffold.

Colloidal particles at the interface between an isotropic liquid and a chiral liquid crystal

A. C. Pawsey, J. S. Lintuvuori, T. A. Wood, J. H. J. Thijssen, D. Marenduzzo and P. S. Clegg

Soft Matter 8, P8422 (2012)

We create an interface between a cholesteric liquid crystal (CLC) and an isotropic liquid (silicone oil) at which homeotropic anchoring leads to a well aligned cholesteric layer and the formation of the fingerprint texture. Fluorescent colloidal particles with planar surface anchoring are dispersed in the CLC and subsequently imaged using confocal microscopy. A majority of these particles decorate the interface between the CLC and the silicone oil. We present a detailed study of the position of the particles along the direction perpendicular to the interface: the final distribution of particles perpendicular to the interface has a clear dependence on the ratio between the particle size and the pitch of the CLC. This suggests, supported by simulations, that there is a particle size and pitch length dependent drive to expel particles, due to the elastic energy cost of remaining in the CLC. We use polarizing optical microscopy to observe changes to the fingerprint texture as the particles perturb the interface. This is combined with a qualitative study of the in-plane ordering of the particles. Chains of particles form perpendicular to the helical axis (parallel to the cholesteric layers), whereas disordered aggregates are seen where the direction of the helical axis is not uniform.

Hindered coarsening of a phase separating microemulsion due to dispersed colloidal particles

D. D. van't Zand, A. B. Schofield, J. H. J. Thijssen and P. S. Clegg

Langmuir 27, P13436 (2011)

The addition of sterically stabilized colloidal particles to a phase-separating microemulsion leads to dramatic changes in its demixing behavior, especially during the later stages. Our microemulsion is composed of reverse micelles of sodium dodecyl sulfate, pentanol, and water in a dodecane continuous phase which separates into micelle-rich and micelle-poor phases above a lower critical solution temperature. The poly(methyl methacrylate) particles preferentially partition into the less structured, micelle-poor phase. Nucleation of the minority phase or spinodal decomposition close to criticality continue to occur in the presence of particles, albeit with pronounced pretransitional clustering of particles when the micelle-poor phase is in the minority. The coalescence of micelle-poor droplets and the coarsening of micelle-rich domains are both strongly modified due to the presence of colloidal particles. We use our observations of the early stages of phase separation to understand these late stage changes.

Particle-stabilized oscillating diver: a self-assembled responsive capsule

J. W. Tavacoli, J. H. J. Thijssen and P. S. Clegg

Soft Matter 7, P7969 (2011)

We report the experimental discovery of a self-assembled capsule, with density set by interfacial glass beads and an internal bubble, that automatically performs regular oscillations up and down a vial in response to a temperature gradient. Similar composites featuring interfacial particles and multiple internal compartments could be the solution to a variety of application challenges.

How do fluorescent surfactants affect particle-stabilized emulsions?

J. H. J. Thijssen, A. B. Schofield and P. S. Clegg

Soft Matter 7, P7965 (2011)

We present the first confocal-microscopy study of synergistic effects in emulsions stabilized by both colloidal particles and a common fluorescent dye that acts as a surfactant. In situ microscopic imaging reveals surfactant adsorption onto the liquid-liquid interface and onto the colloidal particles, which changes the interfacial tension and the particle contact angle. This leads to emulsions that are more stable, more polydisperse and can incorporate more of the dispersed phase.

Novel, Robust, and Versatile Bijels of Nitromethane, Ethanediol, and Colloidal Silica: Capsules, Sub-Ten-Micrometer Domains, and Mechanical Properties

J. W. Tavacoli, J. H. J. Thijssen, A. B. Schofield and P. S. Clegg

Advanced Functional Materials 21, P2020 (2011)

Bicontinuous, interfacially jammed emulsion gels (bijels) are a class of soft solid materials in which interpenetrating domains of two immiscible fluids are stabilized by an interfacial colloidal monolayer. Such structures form through the arrest of the spinodal decomposition of an initially single-phase liquid mixture containing a colloidal suspension. With the use of hexalmethyldisilazane, the wetting character of silica colloids, ranging in size and dye content, can be modified for fabricating a novel bijel system comprising the binary liquid ethanediol-nitromethane. Unlike the preceding water-lutidine based system, this bijel is stable at room temperature and its fabrication and resultant manipulation are comparatively straightforward. The new system has facilitated three advancements: firstly, we use sub 100 nm silica particles to stabilize the first bijel made from low molecular weight liquids that has domains smaller than ten micrometers. Secondly, our new and robust bijel permits qualitative rheological work which reveals the bijel to be significantly elastic and self healing whilst its domains are able to break, reform and locally rearrange. Thirdly, we encapsulate the ethanediol-nitromethane bijel in Pickering drops to form novel particle-stabilized bicontinuous multiple emulsions that we christen bijel capsules. These emulsions are stimuli responsive - they liberate their contained materials in response to changes in temperature and solvency, and hence they show potential for controlled release applications.

Dynamics of hard sphere suspensions using dynamic light scattering and X-ray photon correlation spectroscopy: Dynamics and scaling of the intermediate scattering function

V. A. Martinez, J. H. J. Thijssen, F. Zontone, W. van Megen and G. Bryant

Journal of Chemical Physics 134, E054505 (2011)

Intermediate scattering functions are measured for colloidal hard sphere systems using both dynamic light scattering and x-ray photon correlation spectroscopy. We compare the techniques, and discuss the advantages and disadvantages of each. Both techniques agree in the overlapping range of scattering vectors. We investigate the scaling behavior found by Segré and Pusey [Phys. Rev. Lett. 77, 771 (1996)] but challenged by Lurio et al. [Phys. Rev. Lett. 84, 785 (2000)]. We observe a scaling behavior over several decades in time but not in the long-time regime. Moreover, we do not observe long-time diffusive regimes at scattering vectors away from the peak of the structure factor and so question the existence of long-time diffusion coefficients at these scattering vectors.

Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

Emulsification in binary liquids containing colloidal particles: a structure-factor analysis

J. H. J.Thijssen and P. S. Clegg

Journal of Physics-Condensed Matter 22, E455102 (2010)

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.

Demixing, remixing and cellular networks in binary liquids containing colloidal particles

J. H. J. Thijssen and P. S. Clegg

Soft Matter 6, P1182 (2010)

We present a confocal-microscopy study of demixing and remixing in binary liquids containing colloidal particles. First, particle-stabilized emulsions have been fabricated by nucleation and growth of droplets upon cooling from the single-fluid phase. We show that their stability mainly derives from interfacial particles; the surplus of colloids in the continuous phase possibly provides additional stability. Upon heating these emulsions, we have observed the formation of polyhedral cellular networks of colloids, just before the system remixes. Given a suitable liquid-liquid composition, the initial emulsions cross the binary-liquid symmetry line due to creaming. Therefore, upon heating, the droplets do not shrink and they remain closely packed. The subsequent network formation relies on a delicate balance between the Laplace pressure and the pressure due to creaming/remixing. As high concentrations of colloids in the cell walls inhibit film thinning and rupture, the networks can be stabilized for more than 30 min. This opens up an avenue for their application in the fabrication of advanced materials.

Fabrication of large binary colloidal crystals with a NaCl structure

E. C. M. Vermolen, A. Kuijk, L. C. Filion, M. Hermes, J. H. J. Thijssen, M. Dijkstra and A. van Blaaderen

PNAS 106, P16063 (2009)

Binary colloidal crystals offer great potential for tuning material properties for applications in, for example, photonics, semiconductors and spintronics, because they allow the positioning of particles with quite different characteristics on one lattice. For micrometer-sized colloids, it is believed that gravity and slow crystallization rates hinder the formation of high-quality binary crystals. Here, we present methods for growing binary colloidal crystals with a NaCl structure from relatively heavy, hard-sphere-like, micrometer-sized silica particles by exploring the following external fields: electric, gravitational, and dielectrophoretic fields and a structured surface (colloidal epitaxy). Our simulations show that the free-energy difference between the NaCl and NiAs structures, which differ in their stacking of the hexagonal planes of the larger spheres, is very small (~0.002 kBT). However, we demonstrate that the fcc stacking of the large spheres, which is crucial for obtaining the pure NaCl structure, can be favored by using a combination of the above-mentioned external fields. In this way, we have successfully fabricated large, 3D, oriented single crystals having a NaCl structure without stacking disorder.

Comparing photonic band structure calculation methods for diamond and pyrochlore crystals

E. C. M. Vermolen*, J. H. J. Thijssen*, A. Mohroz, M. Megens and A. van Blaaderen

Optics Express 17, P6952 (2009)

The photonic band diagrams of close-packed colloidal diamond and pyrochlore structures, have been studied using Korringa-Kohn-Rostoker (KKR) and plane-wave calculations. In addition, the occurrence of a band gap has been investigated for the binary Laves structures and their constituent large- and small-sphere substructures. It was recently shown that these Laves structures give the possibility to fabricate the diamond and pyrochlore structures by self-organization. The comparison of the two calculation methods opens the possibility to study the validity and the convergence of the results, which have been an issue for diamond-related structures in the past. The KKR calculations systematically give a lower value for the gap width than the plane-wave calculations. This difference can partly be ascribed to a convergence issue in the plane-wave code when a contact point of two spheres coincides with the grid.

*Authors JHJT and ECMV contributed equally to this work.

This paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website (systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law):

Characterization of photonic colloidal crystals in real and reciprocal space

J. H. J. Thijssen

PhD thesis, Utrecht University, Soft Condensed Matter Group (2007)

In this thesis, we present experimental work on the characterization of photonic colloidal crystals in real and reciprocal space. Photonic crystals are structures in which the refractive index varies periodically in space on the length scale of the wavelength of light. Self-assembly of colloidal particles is a promising route towards three-dimensional (3-D) photonic crystals. However, fabrication of photonic band-gap materials remains challenging, so calculations that predict their optical properties are indispensable. Our photonic band-structure calculations on binary Laves phases have led to a proposed route towards photonic colloidal crystals with a band gap in the visible region. Furthermore, contrary to results in literature, we found that there is no photonic band gap for inverse BCT crystals. Finally, optical spectra of colloidal crystals were analyzed using band-structure calculations. Self-assembled photonic crystals are fabricated in multiple steps. Each of these steps can significantly affect the 3-D structure of the resulting crystal. X-rays are an excellent probe of the internal structure of photonic crystals, even if the refractive-index contrast is large. In Chapter 3, we demonstrate that an angular resolution of 0.002 mrad is achievable at a third-generation synchrotron using compound refractive optics. As a result, the position and the width of Bragg reflections in 2D diffraction patterns can be resolved, even for lattice spacings larger than a micrometer (corresponding to approximately 0.1 mrad). X-ray diffraction patterns and electron-microscopy images are used in Chapter 4 to determine the orientation of hexagonal layers in convective-assembly colloidal crystals. Quantitative analysis revealed that, in our samples, the layers were not exactly hexagonal and the stacking sequence was that of face-centered cubic (FCC) crystals, though stacking faults may have been present. In Chapter 5, binary colloidal crystals of organic spheres (polystyrene, PMMA) and/or inorganic spheres (silica) are introduced as promising templates for strongly photonic crystals. To prevent melting of the template, we used atomic layer deposition (ALD) to infiltrate polystyrene and PMMA templates with alumina, after which chemical vapor deposition (CVD) was used to further enhance the refractive-index contrast. Binary colloidal crystals of silica spheres can be infiltrated by CVD directly, but they often have a layer of colloidal fluid on top. Preliminary etching experiments demonstrated that it may be possible to etch silica templates with plasmas or with adhesive tape. As described in Chapter 6, sedimentation of colloidal silica spheres in an external, high-frequency electric field lead to mm-scale BCT crystals with up to 25 layers. In addition, electric fields were used as an external control to switch between BCT and close-packed (CP) crystal structures within seconds. We also developed two procedures to invert BCT crystals without loss of structure - colloidal particles were immobilized by diffusion-polymerization or photo-induced polymerization of the surrounding solvent. Some BCT crystals were even infiltrated with silicon using CVD. We demonstrate in Chapter 7 that X-ray diffraction can be used to determine the 3-D structure of such photonic colloidal crystals at the various stages of their fabrication. Excellent agreement was found with confocal and electron-microscopy images.

Self-assembly route for photonic crystals with a bandgap in the visible region

A. P. Hynninen, J. H. J. Thijssen, E. C. M. Vermolen, M. Dijkstra and A. van Blaaderen

Nature Materials 6, P202 (2007)

Three-dimensional photonic crystals, or periodic materials, that do not allow the propagation of photons in all directions with a wavelength in the visible region have not been experimentally fabricated, despite there being several potential structures and the interesting applications and physics that this would lead to(1). We show using computer simulations that two structures that would enable a bandgap in the visible region, diamond and pyrochlore, can be self-assembled in one crystal structure from a binary colloidal dispersion. In our approach, these two structures are obtained as the large (Mg) and small (Cu) sphere components of the colloidal analogue of the MgCu2 Laves phase(2), whose growth can be selected and directed using appropriate wall patterning. The method requires that the particles consist of different materials, so that one of them can be removed selectively after drying (for example, by burning or dissolution). Photonic calculations show that gaps appear at relatively low frequencies indicating that they are robust and open for modest contrast, enabling fabrication from-more materials.

Characterization of photonic colloidal single crystals by microradian X-ray diffraction

J. H. J. Thijssen, A. V. Petukhov, D. C. 't Hart, A. Imhof, C. H. M. van der Werf, R. E. I. Schropp and A. van Blaaderen

Advanced Materials 18, P1662 (2006)

Close-packed and non-close-packed colloidal photonic crystals of silica spheres have been imaged in real space to reveal their 3D structure. Although the lattice spacings are of the order of a micrometer, these crystals can also be characterized in reciprocal space using small-angle X-ray scattering (see figure). After infiltration with silicon (see inset), the internal 3D structure of these photonic crystals can only be probed using X-ray scattering.

Microradian X-ray diffraction in colloidal photonic crystals

A. V. Petukhov, J. H. J. Thijssen, D. C. 't Hart, A. Imhof, A. van Blaaderen, I. P. Dolbnya, A. Snigirev, A. Moussaid, I. Snigireva

Journal of Applied Crystallography 39, P137 (2006)

Ultra-high-resolution small-angle X-ray scattering in various colloidal photonic crystals is reported. It is demonstrated that an angular resolution of about two microradians is readily achievable at a third-generation synchrotron source using compound refractive optics. The scheme allows fast acquisition of two-dimensional X-ray diffraction data and can be realised at sample-detector separations of only a few metres. As a result, diffraction measurements in colloidal crystals with interplanar spacings larger than a micrometre, as well as determination of the range of various order parameters from the width of the Bragg peaks, are made possible.

Large-area electric-field-induced colloidal single crystals for photonic applications

A. Yethiraj, J. H. J. Thijssen, A. Wouterse and A. van Blaaderen

Advanced Materials 16, P596 (2004)

A technique to grow millimeter-scale (3 mm by 0.5 mm) electric-field-induced colloidal single crystals and a polymerization process that immobilizes them, allowing drying and reversal of the refractive-index contrast, is presented. The Figure shows a 2D diffraction pattern under white-light illumination from a crystal; the hexagonal symmetry pattern is characteristic of a close-packed lattice.