Rheology of gelling and yielding soft matter systems

Jamming on convex deformable surfaces

Jamming is a fundamental transition that governs the behavior of particulate media, including sand, foams and dense suspensions. Upon compression, such media change from freely flowing to a disordered, marginally stable solid that exhibits non-Hookean elasticity. While the jamming process is well established for fixed geometries, the nature and dynamics of jamming for a diverse class of soft materials and deformable substrates, including emulsions and biological matter, remains unknown. Here we propose a new scenario, metric jamming, where rigidification occurs on a surface that has been deformed from its ground state. Unlike classical jamming processes that exhibit discrete mechanical transitions, surprisingly we find that metric jammed states possess mechanical properties continuously tunable between those of classically jammed and conventional elastic media. The compact and curved geometry significantly alters the vibrational spectra of the structures relative to jamming in flat Euclidean space, and metric jammed systems also possess new types of vibrational mode that couple particle and shape degrees of freedom. Our work provides a theoretical framework that unifies our understanding of solidification processes that take place on deformable media and lays the groundwork to exploit jamming for the control and stabilization of shape in self-assembly processes.

The Effect of Temperature and Shear on the Gelation of Cellulose Nanocrystals in Deep Eutectic Solvents

With the flourishing development of 3D printing technology, the demand for printing materials has been increasing rapidly in recent years. In particular, physical gels formed by cellulose nanocrystals (CNCs) exhibit suitable shear-thinning behavior, high storage moduli, and high yield stresses for extrusion-based printing. While most studies use water as the dispersing medium to form CNC percolated gels, the dispersing behavior of CNCs in alternative solvents, such as deep eutectic solvents (DESs), has not been fully explored. Especially, DESs have low volatility and good ionic conductivity to form functional ionogels. Precise control of the rheological properties and selection of suitable dispersion processes continue to pose significant challenges. In light of this, we have devised a novel dispersion process employing thermal and shear treatments to facilitate the gelation of CNCs within DESs. A crude dispersion of CNCs in the DES underwent thermal treatment to partially remove the surface sulfate ester on CNCs. As a result, the repulsive force between CNCs decreases. A second shear then significantly increases the strength of CNC/DES gels potentially because of the increased rod-rod contacts. This approach enables the formation of high-strength gels at low concentrations of CNCs. Both thermal treatment and a second shear are crucial to forming strong percolated CNC gels. In short, we showed a simple strategy to facilitate the dispersion and gelation of CNCs for direct ink writing.

Rheology and dispensing of real and vegan mayo: the chickpea or egg problem

The rheology, stability, texture, and taste of mayonnaise, a dense oil-in-water (O/W) emulsion, are determined by interfacially active egg lipids and proteins. Often mayonnaise is presented as a challenging example of an egg-based food material that is hard to emulate using plant-based or vegan ingredients. In this contribution, we characterize the flow behavior of animal-based and plant-based mayo emulsions, seeking to decipher the signatures that make the real mayonnaise into such an appetizing complex fluid. We find that commercially available vegan mayos can emulate the apparent yield stress and shear thinning of yolk-based mayonnaise by the combined influence of plant-based proteins (like those extracted from chickpeas) and polysaccharide thickeners. However, we show that the dispensing and dipping behavior of egg-based and vegan mayos display striking differences in neck shape, sharpness, and length. The ratio of apparent extensional to shear yield stress value is found to be larger than the theoretically predicted square root of three for all mayo emulsions. The analysis of neck radius evolution of these extension thinning yield stress fluids reveals that even when the power law exponent governing the intermediate pinching dynamics is similar to the exponent obtained from the shear flow curve, the terminal pinching dynamics show strong local effects, possibly influenced by interstitial fluid properties, finite drop size and deformations, and capillarity.

Insight into the Mechanism of Porcine Myofibrillar Protein Gel Properties Modulated by κ-Carrageenan

The purpose of this study is to explain the mechanism of porcine myofibrillar protein gel properties modulated by κ-carrageenan. The textural properties results showed that the stress at fracture of the composite gel with 0.4% κ-carrageenan had the highest value (91.33 g), which suggested that the 0.4% κ-carrageenan addition was the limitation. The strain at fracture was significantly reduced with κ-carrageenan addition. The composite gel with 0.4% κ-carrageenan had the lowest proportion of T22 (7.85%) and the shortest T21 relaxation time (252.81 ms). The paraffin section showed that the phase separation behavior of the composite gel transformed from single-phase behavior to dispersed phase behavior to bi-continuous phase behavior, and the ratio of CG/MP phase area significantly increased from 0.06 to 1.73. The SEM showed that the three-dimensional network of myofibrillar protein transformed from a loose structure to a compact structure to an unaggregated structure with κ-carrageenan addition. The myofibrillar protein network of the treatment with 0.4% κ-carrageenan had the highest DF value (1.7858) and lowest lacunary value (0.452). The principal component analysis was performed on the data of microstructure and textural properties, and the results showed that the dispersed phase behavior and moisture stabilization promoted the aggregation of myofibrillar protein and the composite gel had better water holding capacity and textural properties, while bi-continuous phase behavior hindered the aggregation of myofibrillar protein and the composite gel had worse water holding capacity and textural properties.

Inhomogeneous steady shear dynamics of a three-body colloidal gel former

We investigate the stationary flow of a colloidal gel under an inhomogeneous external shear force using adaptive Brownian dynamics simulations. The interparticle forces are derived from the Stillinger-Weber potential, where the three-body term is tuned to enable network formation and gelation in equilibrium. When subjected to the shear force field, the system develops remarkable modulations in the one-body density profile. Depending on the shear magnitude, particles accumulate either in quiescent regions or in the vicinity of maximum net flow, and we deduce this strong non-equilibrium response to be characteristic of the gel state. Studying the components of the internal force parallel and perpendicular to the flow direction reveals that the emerging flow and structure of the stationary state are driven by significant viscous and structural superadiabatic forces. Thereby, the magnitude and nature of the observed non-equilibrium phenomena differ from the corresponding behavior of simple fluids. We demonstrate that a simple power functional theory reproduces accurately the viscous force profile, giving a rationale of the complex dynamical behavior of the system.

Beyond simple self-healing: How anisotropic nanogels adapt their shape to their environment

The response of soft colloids to crowding depends sensitively on the particles’ compressibility. Nanogel suspensions provide model systems that are often studied to better understand the properties of soft materials and complex fluids from the formation of colloidal crystals to the flow of viruses, blood, or platelet cells in the body. Large spherical nanogels, when embedded in a matrix of smaller nanogels, have the unique ability to spontaneously deswell to match their size to that of the nanogel composing the matrix. In contrast to hard colloids, this self-healing mechanism allows for crystal formation without giving rise to point defects or dislocations. Here, we show that anisotropic ellipsoidal nanogels adapt both their size and their shape depending on the nature of the particles composing the matrix in which they are embedded. Using small-angle neutron scattering with contrast variation, we show that ellipsoidal nanogels become spherical when embedded in a matrix of spherical nanogels. In contrast, the anisotropy of the ellipsoid is enhanced when they are embedded in a matrix of anisotropic nanogels. Our experimental data are supported by Monte Carlo simulations that reproduce the trend of decreasing aspect ratio of ellipsoidal nanogels with increasing crowding by a matrix of spherical nanogels.

Slip-stick transitions of soft permeable particles near a repulsive wall

The behavior of permeable, elastic particles sliding along a repulsive wall is examined computationally. It is found that particles will stick or slip depending on the interplay of elastohydrodynamic and repulsive forces, and the flow in the porous particle. Particles slip when either the elastohydrodynamic lift or repulsive forces are large and create a supporting lubricating film of fluid. However, for lower values of elastohydrodynamic lift or repulsive forces, the flow within the porous particle reduces the pressure in the thin film, resulting in the particles making contact and sticking to the surface. The criteria for the slip-stick transition is presented, which can be used to design systems to promote or suppress slip for such suspensions.

Understanding relations between rheology, tribology, and sensory perception of modified texture foods

The aim of this work was to examine relations between instrumental and sensory parameters in a texture modified food matrix, with and without saliva. Nine pureed carrot samples (eight thickened and a control) were developed with starch (0.4 and 0.8% wt/wt), xanthan (0.2 and 0.4% wt/wt) or starch-xanthan blends that met International Dysphagia Diet Standardisation Initiative (IDDSI) Level 4 guidelines using fork and spoon tests. Rheological and tribological tests were conducted on the food and simulated bolus prepared by adding fresh stimulated saliva to the food (1:5, saliva:food) to mimic oral processing. Perceived sensory properties were identified using a temporal dominance of sensations (TDS) test (n = 16) where panelists were given a list of nine attributes. The area under the curve was extracted from TDS curves for each attribute/sample and this was correlated with rheological (viscosity at 10 s^-1 , G', G″, and tan δ at 1 Hz) and tribological (friction coefficient in three regimes) data. The viscosity of the control sample decreased after adding hydrocolloids (except Starch_0.8%) and with saliva incorporation. G' and G″ either increased or were similar for xanthan and blends and decreased for starch-thickened samples. Hydrocolloid addition increased friction for all samples and was higher with saliva addition. Sensory results showed that samples with starch were perceived as thick and grainy while xanthan was perceived as smooth and slippery. A greater number of sensory attributes correlated with viscoelastic parameters compared to friction coefficients. Correlations were highest with the saliva added samples, further highlighting the importance of including saliva during instrumental testing.

Enhanced microscopic dynamics in mucus gels under a mechanical load in the linear viscoelastic regime

Mucus is a biological gel covering the surface of several tissues and ensuring key biological functions, including as a protective barrier against dehydration, pathogen penetration, or gastric acids. Mucus biological functioning requires a finely tuned balance between solid-like and fluid-like mechanical response, ensured by reversible bonds between mucins, the glycoproteins that form the gel. In living organisms, mucus is subject to various kinds of mechanical stresses, e.g., due to osmosis, bacterial penetration, coughing, and gastric peristalsis. However, our knowledge of the effects of stress on mucus is still rudimentary and mostly limited to macroscopic rheological measurements, with no insight into the relevant microscopic mechanisms. Here, we run mechanical tests simultaneously to measurements of the microscopic dynamics of pig gastric mucus. Strikingly, we find that a modest shear stress, within the macroscopic rheological linear regime, dramatically enhances mucus reorganization at the microscopic level, as signaled by a transient acceleration of the microscopic dynamics, by up to 2 orders of magnitude. We rationalize these findings by proposing a simple, yet general, model for the dynamics of physical gels under strain and validate its assumptions through numerical simulations of spring networks. These results shed light on the rearrangement dynamics of mucus at the microscopic scale, with potential implications in phenomena ranging from mucus clearance to bacterial and drug penetration.

Light-Controlled Aggregation and Gelation of Viologen-Based Coordination Polymers

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.

Viscoelastic and Deformation Characteristics of Structurally Different Commercial Topical Systems

Rheological characteristics and shear response have potential implication in defining the pharmaceutical equivalence, therapeutic equivalence, and perceptive equivalence of commercial topical products. Three creams (C1 and C3 as oil-in-water and C2 as water-in-oil emulsions), and two gels (G1 and G2 carbomer-based) were characterized using the dynamic range of controlled shear in steady-state flow and oscillatory modes. All products, other than C3, met the Critical Quality Attribute criteria for high zero-shear viscosity (η₀) of 2.6 × 10⁴ to 1.5 × 10⁵ Pa∙s and yield stress (τ₀) of 55 to 277 Pa. C3 exhibited a smaller linear viscoelastic region and lower η₀ (2547 Pa∙s) and τ₀ (2 Pa), consistent with lotion-like behavior. All dose forms showed viscoelastic solid behavior having a storage modulus (G′) higher than the loss modulus (G″) in the linear viscoelastic region. However, the transition of G′ > G″ to G″ > G′ during the continual strain increment was more rapid for the creams, elucidating a relatively brittle deformation, whereas these transitions in gels were more prolonged, consistent with a gradual disentanglement of the polymer network. In conclusion, these analyses not only ensure quality and stability, but also enable the microstructure to be characterized as being flexible (gels) or inelastic (creams).

Investigation of excipients influence on rheological behaviour of hydrogels with dimethindene maleate and dexpanthenol: conditions of controlled shear rate

Every year, the number of cases of hypersensitivity to insect bites increases. Thus, particular attention needs to be paid to the treatment of insect allergy in children, as scratching the bites can complicate the healing process and lead to infection. Therefore, a topical issue for modern medicine and pharmacy is the development of a gel of combined anti-allergic, reparative and anti-inflammatory action for the treatment of local manifestations of allergic skin reactions. Rheological studies are one of the stages of pharmaceutical development of soft dosage forms. In this study, we perform rheological studies of hydrogels containing 0.1% dimethindene maleate and 3.0% dexpanthenol made on different carriers of dispersed structures: Carbopol™ Polymers carbomer Ultrez 10 NF, Hydroxypropyl methylcellulose (HPMC) of brand Metolose SR-90SH-100000SR, Ziboxan F200 xanthan gum. The rheological studies were undertaken using a rheometer in controlled shear rate mode CSR. Basic structural and mechanical indices of the dispersed systems were determined. These included: yield stress, structural viscosity, viscosity at infinite shear rate, the hysteresis loop area. Moreover, dynamic liquefaction coefficients have been calculated. Based on the rheological studies of hydrogels containing 0.1% dimethindene maleate and 3.0% dexpanthenol prepared with various carriers, the use of 1.8% and 2.0% HPMC hydrogels, 2.0% and 2.5% xanthan gum hydrogels, 0.5% and 0.75% carbomer gels is recommended for further biopharmaceutical studies. The application of one of these formulations as the final composition, provides adhesion and will not complicate the scaling-up of the process.

Slip of soft permeable particles near a wall

The slip and stick of soft permeable particles sliding near a smooth surface is determined by computing flow, pressure and shape of a particle pressed against a surface due to the osmotic pressure of the surrounding suspension and its translation at constant velocity parallel to the surface. We present a poro-elastohydrodynamic lubrication theory that accounts for the interplay of the viscous pressure force on the elastic deformation of the particle and the flow through the particle pores. At high particle velocities, the particles move along an elastohydrodynamic film of fluid causing the particles to slip on the surface. For finite particle permeability, there is a critical particle velocity determined by the permeability relative to the thickness of the film and a ratio of the viscous and elastic forces that cause a portion of the particle to contact the surface and stick. In this case the magnitude of pressure in the lubricated film is lower compared to their impermeable counterpart sliding against a smooth surface at the same speed. The particle pores offer an alternative route for the fluid in the film, reducing the lubrication pressure resulting in the particle contacting the surface. A universal function is deduced to predict this transition for a range of poro-elastohydrodynamic interactions. The drag force of the particle sliding along the surface up to the contact is also determined and found to follow a universal function. These results demonstrate the possibility of dynamic stick-slip transitions via control of particle properties instead of wall surface treatments.

Assessment of 3D printability of composite dairy matrix by correlating with its rheological properties

In this study, the potential of heat desiccated milk powder (HDMP) in a composite dairy matrix with semi skimmed milk powder (SSMP) was assessed for hot melt extrusion-based 3D printing. The rheological characteristics of formulations at three stages i.e., pre-printing, printing, and post-printing were investigated. The shear thinning with rapid shear recovery and thermoresponsive behavior of the formulations were analyzed to mimic the prevailing conditions of pre-processing, processing, and post-processing of formulations to understand the temperature induced variations in their rheological characteristics during each stage. The rheological properties were correlated with printability through assessment of the consistency of straight lines (1D), average area of lattice scaffolds (2D), and dimensional stability of the 3D printed constructs. Results demonstrated that an increase in the level of incorporation of SSMP and a decrease in the proportion of HDMP increased the shear thinning behavior, viscosity (ɳ), yield stress (τ₀), storage modulus (G') and a decline in the shear recoverability of the formulations. The thermoresponsive behavior of the formulations was established with gelation temperature ranging from 28.1 to 29.4˚C. The formulation SSMP (35): HDMP (25) resulted in sagging of the printed constructs, whereas the formulation SSMP (55): HDMP (5.0) exhibited the highest dimensional stability and shape retention post printing, owing to its maximum τ₀ (1211.8 Pa) and G' (7026.4 Pa). The results obtained could provide insight into improving the performance of an HME based 3D printing in the dairy and food industries.

A computational study of the behavior of colloidal gel networks at low volume fraction

Colloidal gel networks appear in different scientific and industrial applications because of their unique properties. Molecular dynamics simulations could reveal the relation between molecular level and macroscopic properties of these systems. Nevertheless, the predictions of numerical simulations might depend on the specific form and parameters of interaction potentials. In this paper, a new effective interaction potential is used for characterizing the mechanical behavior of low volume fraction colloidal gels under large shear deformation. The findings are compared with those obtained from other available forms of interaction potentials in order to determine gel characteristics that are interaction potential independent. Furthermore, the macroscopic stress-strain behavior is discussed in terms of the behavior of different terms of the proposed interaction potential. The correlation between the stretch of interparticle bonds and their alignment in the direction of the maximum principal stress is also computed in order to provide microscopic explanations for the initial strain softening behavior. It is concluded that, in addition to topology, local mechanical interactions between colloidal particles are important in defining the mechanical response of soft gels.

UV-Resistant Self-Healing Emulsion Glass as a New Liquid-like Solid Material for 3D Printing

Directly writing 3D structures into supporting mediums is a relatively new developing technology in additive manufacturing. In this work, durable and recyclable liquid-like solid (LLS) materials are developed as supporting mediums that are stable for both UV and thermal solidification. Our LLS material is comprised of densely packed oil droplets in a continuous aqueous medium, known as emulsion glass. Its elastic nature emerges from the jammed structure of oil droplets, which offers this LLS material rapidly self-healing ability. Moreover, the yield stress of the glass is relatively low and can be tuned by the viscosity and weight percentage of oil. The capability of the emulsion glass as supporting mediums is successfully demonstrated by directly writing and then curing designed structures. The emulsion glass has been repeatedly used at least 6 times upon exposure to UV irradiation and heat, implying it can expand the applications of supporting medium to the writing process involving UV- and thermal-curable inks simultaneously.

Colloidal particle gel models using many-body potential interactions

Many-body effective interactions are commonly used in a molecular dynamics simulation study of gel networks formed by colloidal particles. Here we report an interaction potential that can be used to investigate the mechanical response of colloidal gel networks under shear deformation. We then investigate the dependence of the numerical simulation results on the form of mathematical expression used to define the interparticle interactions. This work reveals insight into particle gel models by discussing the physical origins of their mechanical response.

A review of nanocrystalline cellulose suspensions: Rheology, liquid crystal ordering and colloidal phase behaviour

Nanocrystalline cellulose (NCC) is a colloidal rigid rod, referred to by various terms in the literature including cellulose whisker (CW) and cellulose nanocrystal (CNC). These charged colloidal rods exhibit complex colloidal phase and rheological behaviours in aqueous suspensions, that are dependent on volume fraction and interparticle forces. A major shortcoming in the literature of NCC is that the dimensions and morphology of NCC particles vary significantly with the type of raw material and manufacturing conditions, which causes inconsistencies in suspension rheology and colloidal behaviours reported between different works. In this review, we consider the theory and experimentally-determined rheological and colloidal phase behaviours of charged rod suspensions in general, with a focus in particular on NCC. Dilute and semi-dilute NCC suspensions are isotropic liquids, in which NCC particles follow diffusional dynamics. The rheology of these isotropic NCC suspensions can be described by theoretical models that account for the effects of rod dimensions and surface charge, including those based on Doi and Edwards' theory. With increasing NCC concentration, the isotropic phase can undergo a transition to a liquid crystalline state (isotropic-nematic transition) or a transition to a dynamically arrested solid (liquid-solid transition). The liquid crystal ordering and gelation/glass transition are of particular interest because they respectively form an ordered structure and allow a solid-like mechanical response at relatively low solids fraction. For conditions at which the isotropic-nematic and liquid-solid transitions coincide, the formation of an anisotropic structure within a soft solid suspension is possible. Investigation of these two competing transitions led to the discovery of liquid crystal re-entrancy and existence of an anisotropic soft solid (liquid crystal hydroglass, LCH). LCH has a biphasic structure with an attractive glass matrix and a co-existing liquid crystal phase, providing similar viscoelastic properties to hydrogels but permitting reversible orientation of the colloidal rods in the liquid crystalline phase by shear forces; i.e. their structural ordering is programmable. The liquid crystal transition and gelation/glass transitions are quantitatively dependent on rod dimensions i.e. respectively proportional to L²D and L/D. Phase transitions in NCC suspensions including liquid crystal re-entrancy and formation of LCH can be fully described as a function of rod dimension, volume fraction and interparticle forces. This behaviour is independent of NCC source, allowing development of a generalised phased diagram in which separately-reported phase transitions converge to consistent phase boundaries. This validates a key hypothesis for the study of NCC suspensions, that variation in NCC concentration and interparticle forces can explain the complex phase behaviours observed within suspensions formulated using NCC obtained from different sources.

Understanding the multiscale self-assembly of metal-organic polyhedra towards functionally graded porous gels

Spatial heterogeneity and gradients within porous materials are key for controlling their mechanical properties and mass/energy transport, both in biological and synthetic materials. However, it is still challenging to induce such complexity in well-defined microporous materials such as crystalline metal-organic frameworks (MOFs). Here we show a method to generate a continuous gradient of porosity over multiple length scales by taking advantage of the amorphous nature of supramolecular polymers based on metal-organic polyhedra (MOPs). First, we use time-resolved dynamic light scattering (TRDLS) to elucidate the mechanism of hierarchical self-assembly of MOPs into colloidal gels and to understand the relationship between the MOP concentrations and the architecture of the resulting colloidal networks. These features directly impact the viscoelastic response of the gels and their mechanical strength. We then show that gradients of stiffness and porosity can be created within the gel by applying centrifugal force at the point of colloidal aggregation. These results with the creation of asymmetric and graded pore configuration in soft materials could lead to the emergence of advanced properties that are coupled to asymmetric molecule/ion transport as seen in biological systems.

A Way to Predict Gold Nanoparticles/Polymer Hybrid Microgel Agglomeration Based on Rheological Studies

In this work, a detailed rheological study of hybrid poly(acrylamide-co-acrylic acid) P(AAm-co-AAc) aqueous microgel dispersions is performed. Our intention is to understand how the presence of gold nanoparticles, AuNP, embedded within the microgel matrix, affects the viscoelastic properties, the colloidal gel structure formation, and the structure recovery after cessation of the deformation of the aqueous microgel dispersions. Frequency sweep experiments confirmed that hybrid microgel dispersions present a gel-like behavior and that the presence of AuNP content within microgel matrix contributes to the elasticity of the microgel dispersions. Strain sweep test confirmed that hybrid microgels aqueous dispersion also form colloidal gel structures that break upon deformation but that can be recovered when the deformation decreases. The fractal analysis performed to hybrid microgels, by applying Shih et al. and Wu and Morbidelli's scaling theories, evidenced that AuNP significantly affects the colloidal gel structure configuration ending up with the formation of agglomerates or microgel clusters with closer structures in comparison to the reference P(AAm-co-AAc) aqueous microgel dispersions.

Tribology and its growing use toward the study of food oral processing and sensory perception

Here we provide a comprehensive review of the knowledge base of soft tribology, the study of friction, lubrication, and wear on deformable surfaces, with consideration for its application toward oral tribology and food lubrication. Studies on "soft-tribology" have emerged to provide knowledge and tools to predict oral behavior and assess the performance of foods and beverages. We have shown that there is a comprehensive set of fundamental literature, mainly based on soft contacts in the Mini-traction machine with rolling ball on disk configuration, which provides a baseline for interpreting tribological data from complex food systems. Tribology-sensory relationships do currently exist. However, they are restricted to the specific formulations and tribological configuration utilized, and cannot usually be applied more broadly. With a careful and rigorous formulation/experimental design, we envisage tribological tools to provide insights into the sensory perception of foods in combination with other in vitro technique such as rheology, particle sizing or characterization of surface interactions. This can only occur with the use of well characterized tribopairs and equipment; a careful characterization of simpler model foods before considering complex food products; the incorporation of saliva in tribological studies; the removal of confounding factors from the sensory study and a global approach that considers all regimes of lubrication.

Effect of chain flexibility on cell adhesion: Semi-flexible model-based analysis of cell adhesion to hydrogels

Hydrogels have been developed and applied to various biomedical applications due to their biocompatibility. However, understanding of modulation between cells to hydrogel interface is still unclear, and parameters to explain the interaction are not sophisticated enough. In this report, we studied the effect of polymer chain flexibility on cell adhesion to various hydrogel constructs of collagen and fibrin gels. Specifically, novel method of semi-flexible model-based analysis confirmed that chain flexibility mediated microstructure of the hydrogels is a critical factor for cell adhesion on their surfaces. The proposed analysis showed possibility of more accurate prediction of biocompatibility of hydrogels, and it should be considered as one of the important criteria for polymer design and selections for enhancing both biocompatibility and biofunctionality.

Liquid crystal hydroglass formed via phase separation of nanocellulose colloidal rods

A new anisotropic soft material - a liquid crystal 'hydroglass' (LCH) - is created from aqueous suspensions of nanocrystalline cellulose (NCC) colloidal rods. Under specific conditions, the NCC suspension separates into a colloid-rich attractive glass matrix phase and a coexisting liquid crystal phase. LCH provides similar viscoelastic properties to polymer and colloidal gels, but permits reversibly-orientating the colloidal rods through shear forces.

Supramolecular birefringent metallogels formed by trinuclear copper(ii) complexes with myo-inositol and bipyridyl ligands

Birefringent metallohydrogels have been prepared by a fibrous supramolecular assembly resulting from the trinuclear copper(ii) complexes of myo-inositol (ins) and 2,2′-bipyridine (bipy), [Cu₃(ins)(bipy)₃]X₃, with formate, acetate and propionate as anions, X.

Viscosity of liquid and semisolid materials: Establishing correlations between instrumental analyses and sensory characteristics

Considering the importance of texture for food products, we aimed to evaluate viscosity of different liquid and semisolid materials through instrumental analyses (rheometer and texture analyzer), as well sensory descriptive analysis, and establishing correlations between all these analyses. Eight materials were used: water, strawberry yogurt, condensed milk, honey, UHT cream, creamy dairy dessert, petit suisse strawberry flavor (a traditional French cheese from Normandy region, and sold as an infant product), and dulce de leche (obtained through cooking of a can of condensed milk, during 15 min under pressure, resulting in a brownish color product and more consistent than condensed milk). All materials were submitted to rheological analysis, analysis on texture analyzer, and descriptive sensory analysis. All techniques of measurement discriminated the texture of samples. The visual viscosity, defined as a sensory attribute evaluated by visual observation, was negatively correlated to apparent viscosity measured through rheological analysis with shear rate at 10 s^-1 . Oral viscosity and body (both defined as sensory attributes evaluated by oral perception) were positively correlated with areas from graphs obtained in the texture analyzer, and with apparent viscosity measured through rheological analysis at shear rate of 10 s^-1 , although positive strong correlation was also found between body and apparent viscosity at higher shear rates (50 and 100 s^-1 ). The strong correlations enable application of these instrumental tests as indicators of the sensory texture of liquid and semisolid materials. PRACTICAL APPLICATIONS: Texture has a minor importance to liquid and semisolid materials in comparison to meat and crunchy products. However, the relevance of texture to these kinds of products has growing up recently. Therefore, measuring and understanding viscosity of liquid and semisolid materials, using different ways of evaluation, brings relevant information to the area. Moreover, establishing correlations between instrumental and sensory analyses may indicate which instrumental analysis and which analysis condition would be more adequate to correlate with sensory perception of texture, allowing a convergence for future studies and for discussion of results.

In situ follow-up of hybrid alginate-silicate microbeads formation by linear rheology

Hybrid alginate-silicate microbeads of about 10-20 μm were synthesized by combining alginate crosslinking, silica condensation in a one pot approach using a food grade emulsion as template. A fine tuning of the formulation composition (alginate, silica and calcium sources) is necessary in order to obtain core-shell microbeads instead of unshaped and irregular fragments or even perforated spherical beads. Importantly, in situ linear rheology provides insights into the reaction mechanism as a result of the rheological fingerprint profile obtained during beads formation.

"Liquid, gel and soft glass" phase transitions and rheology of nanocrystalline cellulose suspensions as a function of concentration and salinity

The colloidal size and rod morphology of nanocrystalline cellulose (NCC) lead to suspensions with useful phase and gelation behaviours as well as complex rheologies. However, these have not been comprehensively evaluated previously. Here we report the detailed phase behaviour of sulphonated NCC aqueous suspensions as a function of concentration and salinity. Four phases - liquid, viscoelastic, repulsive glass and attractive glass/gel - are identified in terms of their distinct rheological behaviours. The liquid-solid transitions (LSTs) are determined rheologically, and these are supported by a simplified model based on the DLVO theory that indicates the importance of charge in determining the phase behaviour. Rheology is also used to investigate the solid-solid transition from a repulsive glass to an attractive gel with increasing salt at high NCC concentrations. A time-dependent aging phenomenon is observed in suspensions with a composition just below the LSTs, and the implications of this on the dynamics occurring during gelation processes are discussed. This work can be directly applied to the development of structure-function relationships and the expanding utilisation of NCC suspensions, whilst also providing a basis for the study of charged colloidal rods more generally and evaluation of theoretical models.

Ionic liquid and nanoparticle hybrid systems: Emerging applications

Having novel electronic and optical properties that emanate from their nano-scale dimensions, nanoparticles are central to numerous applications. Ionic liquids can confer to nanoparticle chemical protection and physicochemical property enhancement through intermolecular interactions and can consequently improve the stability and reusability of nanoparticle for various operations. With an aim to combine the novel properties of nanoparticles and ionic liquids, different structures have been generated, based on a balance of several intermolecular interactions. Such ionic liquid and nanoparticle hybrids are showing great potential in diverse applications. In this review, we first introduce various types of ionic liquid and nanoparticle hybrids, including nanoparticle colloidal dispersions in ionic liquids, ionic liquid-grafted nanoparticles, and nanoparticle-stabilized ionic liquid-based emulsions. Such hybrid materials exhibit interesting synergisms. We then highlight representative applications of ionic liquid and nanoparticle hybrids in the catalysis, electrochemistry and separations fields. Such hybrids can attain better stability and higher efficiency under a broad range of conditions. Novel and enhanced performance can be achieved in these applications by combining desired properties of ionic liquids and of nanoparticles within an appropriate hybrid nanostructure.

Particle-wall tribology of slippery hydrogel particle suspensions

Slip is an important phenomenon that occurs during the flow of yield stress fluids like soft materials and pastes. Densely packed suspensions of hydrogel microparticles are used to show that slip is governed by the tribological interactions occurring between the samples and shearing surfaces. Both attractive/repulsive interactions between the dispersed particles and surface, as well as the viscoelasticity of the suspension, are found to play key roles in slip occurring within rheometric flows. We specifically discover that for two completely different sets of microgels, the sliding stress at which slip occurs scales with both the modulus of the particles and the bulk suspension modulus. This suggests that hysteresis losses within the viscoelastic particles contribute to friction forces and thus slip at the particle-surface tribo-contact. It is also found that slip during large amplitude oscillatory shear and steady shear flows share the same generic features.

Highly Elastic and Self-Healing Composite Colloidal Gels

Composite colloidal gels are formed by the pH-induced electrostatic assembly of silica and gelatin nanoparticles. These injectable and moldable colloidal gels are able to withstand substantial compressive and tensile loads, and exhibit a remarkable self-healing efficiency. This study provides new, critical insight into the structural and mechanical properties of composite colloidal gels and opens up new avenues for practical application of colloidal gels.

Bimodal colloid gels of highly size-asymmetric particles

We report a type of colloidal gel, induced by a minute incremental addition of mutually attractive small particles (size ∼12 nm) to a suspension of highly charged large particles (size ∼500 nm). The gel's morphological behavior does not follow the typical power-law scaling for fractal clusters. Its unique scaling behavior has two distinct power-law indices, based on particle volume fraction. We show the unique scaling behavior arises when nonfractal networks of large particles are bridged by small-particle clusters, which occurs between a lower and upper critical boundary of small particle volume fraction.