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Colloidal dispersions are prized as model systems to understand basic properties of materials, and are central to a wide range of industries from cosmetics to foods to agrichemicals. Among the key developments in using colloids to address challenges in condensed matter is to resolve the particle coordinates in 3D, allowing a level of analysis usually only possible in computer simulation. However in amorphous materials, relating mechanical properties to microscopic structure remains problematic. This makes it rather hard to understand, for example, mechanical failure. Here we address this challenge by studying the contacts and the forces between particles, as well as their positions. To do so, we use a colloidal model system (an emulsion) in which the interparticle forces and local stress can be linked to the microscopic structure. We demonstrate the potential of our method to reveal insights into the failure mechanisms of soft amorphous solids by determining local stress in a colloidal gel. In particular, we identify `force chains' of load-bearing droplets, and local stress anisotropy, and investigate their connection with locally rigid packings of the droplets., Comment: 12 pages

Colloidal gels constitute an important class of materials found in many contexts and with a wide range of applications. Yet as matter far from equilibrium, gels exhibit a variety of time-dependent behaviours, which can be perplexing, such as an increase in strength prior to catastrophic failure. Remarkably, such complex phenomena are faithfully captured by an extremely simple model - "sticky spheres". Here we review progress in our understanding of colloidal gels made through the use of real space analysis and particle resolved studies. We consider the challenges of obtaining a suitable experimental system where the refractive index and density of the colloidal particles is matched to that of the solvent. We review work to obtain a particle-level mechanism for rigidity in gels and the evolution of our understanding of time-dependent behaviour, from early-time aggregation to ageing, before considering the response of colloidal gels to deformation and then move on to more complex systems of anisotropic particles and mixtures. Finally we note some more exotic materials with similar properties., Comment: 55 pages, submitted to J. Phys.: Condens. Matter

We propose a novel microfluidic “opposed-flow” geometry in which the continuous fluid phase is fed into a junction in a direction opposite to the dispersed phase. This pulls out the dispersed phase into a micron-sized jet, which decays into micron-sized droplets. As the driving pressure is tuned to a critical value, the jet radius vanishes as a power law down to sizes below 1 μm. By contrast, the conventional “coflowing” junction leads to a first order jetting transition, in which the jet disappears at a finite radius of several μm, to give way to a “dripping” state, resulting in much larger droplets. We demonstrate the effectiveness of our method by producing the first microfluidic silicone oil emulsions with a sub micron particle radius, and utilize these droplets to produce colloidal clusters.

While proteins have been treated as particles with a spherically symmetric interaction, of course in reality the situation is rather more complex. A simple step towards higher complexity is to treat the proteins as non--spherical particles and that is the approach we pursue here. We investigate the phase behavior of enhanced green fluorescent protein (eGFP) under the addition of a non--adsorbing polymer, polyethylene glycol (PEG). From small angle x-ray scattering we infer that the eGFP undergoes dimerization and we treat the dimers as spherocylinders with aspect ratio $L/D-1 = 1.05$. Despite the complex nature of the proteins, we find that the phase behaviour is similar to that of hard spherocylinders with ideal polymer depletant, exhibiting aggregation and, in a small region of the phase diagram, crystallization. By comparing our measurements of the onset of aggregation with predictions for hard colloids and ideal polymers [S.V. Savenko and M. Dijkstra, J. Chem. Phys 124, 234902 (2006) and F. lo Verso et al., Phys. Rev. E 73, 061407 (2006)] we find good agreement, which suggests that the eGFP proteins are consistent with hard spherocylinders and ideal polymer., 12 pages

We investigate the reversible gelation of poly(N-isopropylacrylamide) (pNIPAM) microgels in the presence of triblock-copolymer (PEO-PPO-PEO type) surfactant. We demonstrate that the association of these polymers with the microgel particles at elevated temperature is responsible for the gelation, due to the temperature responsive nature of the components. This is highlighted by an increase in the apparent hydrodynamic diameter of the particles in dynamic light scattering experiments, which only occurs above the volume phase transition temperature of pNIPAM. The gels that result shrink over a time period much larger than that of the collapse of pNIPAM microgels, and retain the shape of the container they form in. We investigate the mechanism that leads to this gelation and the structure of the gels that result. Confocal microscopy experiments show that both polymers are present in the gel network, indicating that an associative mechanism is responsible for the gelation. We vary the pNIPAM particle architecture to further investigate the gelation process, and find that the cross-link distribution plays a key role in the gelation mechanism, where for uniformly cross-linked particles the gelation is not observed. This shows that the fuzzy corona of the pNIPAM microgels is involved in the association of the polymers, allowing the triblock-copolymer to penetrate the outer corona of the microgels and bridge the particles. The phase transition observed is close to physiological conditions, so these gels have the potential for use in biomedical applications, including tissue engineering.

Non-aqueous microgel particles are commonly synthesised in water, dried, and then redispersed in non-aqueous solvents. An important factor to consider when synthesising such particles is the initiator, which can determine how well the particles disperse in solvents. Polystyrene microgel particles were made with three different initiators. When a neutral, oil soluble initiator (azobisisobutyronitrile) was used the particles dispersed in toluene as well as cyclohexane and decalin. In contrast, anionic, water-soluble initiators (potassium persulfate or azobis(4-cyanovaleric acid)) created particles that only redispersed in toluene and not the other two solvents. Of the three considered, toluene is the best solvent for polystyrene and also has the highest polarizability, making it most effective at redispersing particles with polar or ionisable functional groups. Zeta potential and conductivity measurements, however, did not detect a direct relationship between particle charging and redispersibility. Oil soluble initiators result in “inside out” polymerisation where the initiator groups are buried inside the growing particle, whereas water-soluble initiators result in “outside in” polymerisation, with the polar initiator groups residing on the particle surface. By tailoring the ratio between water and oil soluble initiators, it may be possible to synthesise microgel particles with uniform or designed charge profiles from the core to the surface. Electronic supplementary material The online version of this article (doi:10.1007/s00396-017-4023-y) contains supplementary material, which is available to authorized users.

Crosslinked polystyrene (PS) particles were dispersed in diisopropyl adipate, a non-volatile, good solvent for PS. Depletion attractions between particles were induced by adding linear PS, with a polymer/particle size ratio of 0.3. For hard particles, with a high crosslink density, the colloid-polymer mixtures displayed phase separation in agreement with predictions for hard sphere - polymer mixtures. For similar sized but weakly crosslinked, soft microgel particles however, a significantly higher concentration of linear PS needed to be added to observe phase separation. This is because the non-adsorbing polymer can penetrate the soft microgels which weakens the depletion interaction.In ternary mixtures of the hard and soft particles and linear PS, confocal microscopy reveals that mixed particle networks are formed. Upon adding soft particles to a hard particle gel network, there is only modest variation in the flow properties, with the moduli decreasing somewhat, yet viscosity increasing. It is argued that the effect of an increase of volume fraction is offset by a reduction in depletion interaction strength. This demonstrates that, in these ternary mixtures, microgels act like particles rather than polymer depletants; yet microgel addition allows high volume fraction polymer - colloid mixtures to be made whilst avoiding the viscosity increase that would normally result.

Understanding the phase behaviour of mixtures continues to pose challenges, even for systems that might be considered "simple". Here we consider a very simple mixture of two colloidal and one non-adsorbing polymer species which can be simplified even further to a size-asymmetrical binary mixture, in which the effective colloid-colloid interactions depend on the polymer concentration. We show that this basic system exhibits surprisingly rich phase behaviour. In particular, we enquire whether such a system features only a liquid-vapor phase separation (as in one-component colloid-polymer mixtures) or whether, additionally, liquid-liquid demixing of two colloidal phases can occur. Particle-resolved experiments show demixing-like behaviour, but when combined with bespoke Monte Carlo simulations, this proves illusory, and we reveal that only a single liquid-vapor transition occurs. Progressive migration of the small particles to the liquid phase as the polymer concentration increases gives rise to composition inversion - a maximum in the large particle concentration in the liquid phase. Near criticality the density fluctuations are found to be dominated by the larger colloids., Comment: submitted to J. Chem. Phys

Microgels are cross-linked polymer latex particles that can form stable colloidal dispersions. Their typical sizes range from 10 to 1000 nm and they can swell in response to their external environment (pH, temperature and solvency). This swelling behaviour is central to many potential applications for microgels. The existing literature is dominated by studies of the properties of aqueous microgel dispersions. In contrast, this review focusses on the development of microgel particles in non-aqueous systems, looking at the challenges of studying these particles as well as their swelling behaviour. The five main mechanisms of producing microgel particles will be discussed and examples of materials used for microgels that swell in non-aqueous solvents will be given. Finally some examples of applications for non-aqueous microgels are given.

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We report measurements on the ageing dynamics of a colloid–polymer mixture with a large polymer–colloid size ratio of 0.62. Quenched into a two-phase region the system gels and forms a network with a characteristic radius Rc. We find three distinct regimes in the time evolution of Rc(t), reminiscent of the linear, late and gravity-dominated regimes of coarsening seen in classical spinodal decomposition kinetics of binary fluids. In the early stages of gelation, we observe a peak in the time-dependent structure factor S(q, t) which is stationary in q and grows in intensity characteristic of a linear-Cahn regime. The domain size then coarsens continuously with the age t of the sample. In the late stages the domain size follows the approximate algebraic law, Rc ∼ tθ. The growth exponent θ is a strong function of the quench depth: for small polymer concentrations θ is significantly larger than for large polymer concentrations. The gel networks formed are transient, and in the final stages of phase separation, collapse under gravity when the correlation length of the gel becomes ∼ 2π times the capillary length.

BACKGROUND: For an oil adjuvant to enhance uptake of a particulate active ingredient (AI), it is hypothesised that closer association between the two should result in higher uptake. Accordingly, factors important for the spray deposit size on grapevine leaves have been investigated for a series of model suspoemulsion formulations containing colloidal crystalline AI or fluorescent pigment particles and an emulsion of an oil adjuvant with different degrees of wetting and different spray volumes. RESULTS: Low spray volumes (

The link between the rheological properties and syneresis of weak colloidal particle gels formed either by non-adsorbing polymer or gelling clays is investigated with reference to crop protection formulations. Suspensions of particles (1.5 μm) in aqueous xanthan solutions showed an increase in the relaxation modulus for particle volume fractions greater than 0.05–0.07 due to the formation of a continuous, space filling network of particles. The rate of syneresis here appeared to be governed by the time taken for the polymer molecules to diffuse through the particle network. Volume fractions below this limit showed no change in the relaxation modulus and appeared to consist of individual particles or small aggregates dispersed in xanthan solution. Suspensions of particles in gelling clays (attapulgite and montmorillonite) showed much higher relaxation modulus values with very long relaxation times, characteristic of viscoelastic gels. Ultrasound velocity volume fraction—height sedimentation profiles showed the suspensions to collapse at the base by a consolidation process. Comparison of the sedimentation velocity of these three types of system with the zero shear viscosity obtained from stress relaxation measurements showed three different correlations corresponding to each system.

In this paper, the role of both non-adsorbing (depleting) and adsorbing (stabilizing) polymers is investigated in the process of sedimentation in aqueous suspension concentrates. A range of polymers with different molecular masses have been used. By measuring the rheology of these systems and also studying the sedimentation of model polystyrene latex and real agrochemical suspensions of fluquinconazole, we have observed that the sedimentation rate is related to the nature of the particle flocs and that the rate of sedimentation is significantly reduced once a continuous network of particles is formed. Rheologically this corresponds to the formation of a weak elastic gel. When the attraction between particles is sufficient, this network is formed via a diffusion-limited aggregation mechanism, has an open structure and sediments slowly. The network, however, is able to rearrange into a more dense reaction-limited structure which sediments more rapidly. This transition from a slow to a faster sedimentation rate takes up to four days and depends on the strength of the attraction between the particles, the volume fraction of particles and molecular mass of the non-adsorbing polymer. Soft adsorbed polymer stabilizing layers lead to weaker flocculation, require more non-adsorbing polymer for network formation and could result in more rapid sedimentation. A range of suspensions containing different molecular mass non-adsorbing polymers and with the same sedimentation rate have different rheologies, including the zero shear viscosity. Consequently, the flocculated network is not the only factor in sedimentation and the non-adsorbing polymer also contributes. Furthermore, there is no direct correlation between rheology and sedimentation. This difference could be due to the non-adsorbing polymer molecules having to diffuse through the network of particles before the particle clusters in the network can rearrange and sedimentation occur.

Attractive colloidal particles can form a disordered elastic solid or gel when quenched into a two-phase region, if the volume fraction is sufficiently large. When the interactions are comparable to thermal energies the stress-bearing network within the gel restructures over time as individual particle bonds break and reform. Typically, under gravity such weak gels show a prolonged period of either no or very slow settling, followed by a sudden and rapid collapse - a phenomenon known as delayed collapse. The link between local bond breaking events and the macroscopic process of delayed collapse is not well understood. Here we summarize the main features of delayed collapse and discuss the microscopic processes which cause it. We present a plausible model which connects the kinetics of bond breaking to gel collapse and test the model by exploring the effect of an applied external force on the stability of a gel., Comment: Accepted version: 10 pages, 7 figures

Metastable gels formed by weakly attractive colloidal particles display a distinctive two-stage time-dependent settling behavior under their own weight. Initially a space-spanning network is formed that for a characteristic time, which we define as the lag time $\taud$, resists compaction. This solid-like behavior persists only for a limited time. Gels whose age $\tw$ is greater than $\taud$ yield and suddenly collapse. We use a combination of confocal microscopy, rheology and time-lapse video imaging to investigate both the process of sudden collapse and its microscopic origin in an refractive-index matched emulsion-polymer system. We show that the height $h$ of the gel in the early stages of collapse is well described by the surprisingly simple expression, $h(\ts) = \h0 - A \ts^{3/2}$, with $\h0$ the initial height and $\ts = \tw-\taud$ the time counted from the instant where the gel first yields. We propose that this unexpected result arises because the colloidal network progressively builds up internal stress as a consequence of localized rearrangement events which leads ultimately to collapse as thermal equilibrium is re-established., Comment: 14 pages, 11 figures, final version

In this paper, the rheological properties, microscopic appearance and macroscopic sedimentation behaviour of 147- and 482-nm polystyrene latices in HEC solutions, bearing different adsorbed poly(ethyleneoxide)–poly(propyleneoxide)–poly(ethyleneoxide) (PEO–PPO–PEO) copolymers are presented and compared with previous results with a 67-nm latex (Langmuir 13 (1997) 2922). The ratio of the steric layer thickness to the particle radius varies from 1:5 to 1:40 for the three latex sizes covering a range of particle softness. The 147-nm latex showed gas, liquid and solid phases, including three phase coexistence with increasing concentrations of HEC. The solid phase was a viscoelastic gel that sedimented slowly and showed initially slow, then faster sedimentation rates for HEC concentrations close to the fluid–gel phase boundary. These properties depended on the adsorbed PEO–PPO–PEO copolymers; the fluid–gel phase boundary moved to lower HEC concentrations with increasing PEO chain length. Oscillatory shear measurements were sensitive to the floc network structure and showed the transition from the fluid to gel phases. The values for the elastic modulus in the gel region were independent of the PEO chain length in the stabiliser implying the presence of similar floc structures with each of the different PEO–PPO–PEO copolymers. Stress relaxation measurements gave long relaxation times (>10 3 s) and showed that the suspensions were viscoelastic fluids with high zero shear viscosities. Higher values were obtained with longer PEO chain lengths. Extrapolated yield stress values showed stronger flocculation with increasing PEO chain length in the adsorbed stabilising layer. There was good correlation between the extrapolated yield stress and the sedimentation velocity, indicating that the collapse of the floc network is governed by the strength of the interparticle bonds (for the same floc structure) and also with the long-time behaviour from the stress relaxation measurements. With the 482-nm latex there was less effect of the adsorbed copolymer both for the rheology and sedimentation behaviour, with the particles behaving more like hard spheres. The transition between hard and soft sphere behaviour, interpreted here as the ratio of the steric layer thickness to the particle radius at which the adsorbed stabilising layer starts to have an effect on the rheology and phase behaviour is estimated to be approximately 1:20.

Colloid–polymer mixtures may undergo either fluid–fluid phase separation or gelation. This depends on the depth of the quench (polymer concentration) and the polymer–colloid size ratio. We present a real-space study of dynamics in phase separating colloid–polymer mixtures with medium- to long-range attractions (polymer–colloid size ratio qR = 0.45–0.89), with the aim of understanding the mechanism of gelation as the range of attraction is changed. In contrast to previous studies of short-range attractive systems, where gelation occurs shortly after crossing the equilibrium phase boundary, we find a substantial region of fluid–fluid phase separation. Upon quenching deeper, the system undergoes a continuous crossover to gel formation. We identify two regimes, ‘classical’ phase separation, where single particle relaxation is much faster than the dynamics of phase separation, and ‘viscoelastic’ phase separation, where demixing is slowed down appreciably due to slow dynamics in the colloid-rich phase. Particles at the surface of the strands of the network exhibit significantly greater mobility than those buried inside the gel strand which presents a method for coarsening.

Depletion interactions give rise to phase separation in colloid-polymer mixtures at sufficiently high concentrations of both species. Similarly a fluid-solid phase separation is expected for binary hard sphere (BHS) mixtures, however in practice it appears such mixtures are very prone to form metastable (jammed) states. Here pNIPAM microgel particles are used to induce depletion attractions between polystyrene latex spheres. Such microgels can be modelled as hard spheres with a polymer brush-like outer layer. Mixtures with a microgel/polystyrene size ratio of 0.11 showed a fluid-solid phase separation, in good agreement with predictions for BHS. Gel states were obtained on further increase of concentration, around the position of the predicted metastable BHS fluid-fluid binodal, echoing previous claims of the significance of a hidden fluid-fluid phase boundary as the precursor to gelation. It thus appears that the slight deformability of the microgel particles reduces the tendency for mixtures to remain in jammed states relative to binary hard sphere systems. Nevertheless the mixtures did tend to form gels more easily than mixtures of the same polystyrene spheres with linear polymer coils at a similar polymer/colloid size ratio.

By delayed sedimentation is meant the rapid collapse of a colloidal gel following a period of either little or no sedimentation. The delay or collapse time can range from seconds to many months, depending upon the strength of the gel. Delayed collapse is generally presumed to be a consequence of steady coarsening which somehow brings the gel to a critical state. Earlier work on very weak depletion gels demonstrated an exponential dependence of the collapse time on depletant concentration, implying that the activation energy for coarsening is equal to the mean binding energy of the particles, θ. This correlation is not however followed by the somewhat stronger secondary-minimum gels studied originally by Partridge, who first reported the phenomenon of delayed collapse. It is demonstrated here that the scaling on the mean binding energy becomes progressively more and more sub-Arrhenius as the latter is increased above ca. 10 kBT, also that the width of the potential well has an important effect. Between them, these two effects account for the faster than expected collapse shown by the secondary-minimum gels and by more strongly-flocculated depletion gels. It is suggested that apparent departures from Arrhenius scaling are a consequence of heterogeneity which causes the mean activation energy and the mean binding energy to differ as the energies become larger.

Addition of CNR (R = Me or But) to the MM complex [{M(CO)2(η-C5H5)}2](M = Mo or W) results in addition of one isonitrile across the MM bond and the formation of [M2(µ-η2-CNR)(CO)4(η-C5H5)2]. The But molybdenum derivative crystallises as black platey needles [orthorhombic, a= 31.75(3), b= 8.171(11), c= 15.722(17)A; R converged to 0.0357 for 1 775 independent reflections for which I/σ(I) > 3.0]. The molecule consists of two Mo(CO)2(η-C5H5) groups linked by a Mo–Mo single bond and bridged by the CNBut group in a µ-η2 four-electron donor fashion. The bonding is viewed as donation of the carbon long pair to one molybdenum atom and donation of CN π-electron density to the second. Prolonged reaction times of [Mo2(µ-η2-CNR)(CO)4(η-C5H5)2] with excess CNR results in cleavage of the CN–Mo π-donor interaction and formation of the symmetrically disubstituted complex [{Mo(CNR)(CO)2(η-C5H5)}2]. The But derivative crystallises as dark red prisms [monoclinic, a= 10.354(7), b= 11.672(5), c= 10.793(5)A, β= 103.68(4)°; R converged to 0.0519 for 1 102 independent reflections for which I/σ(I) > 3.0]. The molecule consists of two Mo(CNBut)(CO)2(η-C5H5) groups linked through a Mo–Mo single bond with a crystallographically imposed centre of symmetry midway along the Mo–Mo bond. Decarbonylation by thermolysis of this compound results in the formation of [Mo2(CNBut)(µ-η2-CNBut)(CO)3(η-C5H5)2], containing one terminal and one bridging isonitrile. Reductive cleavage of [{Mo(CNR)(CO)2(η-C5H5)}2] with sodium amalgam gives the anions [Mo(CNR)(CO)2(η-C5H5)]– which react with SnPh3Cl to give the tin complexes [Mo(SnPh3)(CNR)(CO)2(η-C5H5)], as mixtures of cis and trans isomers.