Reversible gelation of rod-like viruses grafted with thermoresponsive polymers

Thermoreversible gels of hollow silica nanorod dispersions

Colloidal suspensions of anisotropic particles are ubiquitous in particle-based industries. Consequently, there is a need to quantify the effects of particle shape on equilibrium phases and kinetic state transitions, particularly at lower aspect ratios (L/D ≈ 1-10). We present a new, colloidal system comprised of hollow, octadecyl-coated silica rods with 40 nm diameter with controlled aspect ratio and thermoreversible short-range attractions. Rheology and dynamic light scattering measurements on suspensions of these hollow adhesive hard rods with nominal aspect ratio ≈3 suspended in tetradecane exhibit thermoreversible gelation without complicating effects of gravitational settling. Small angle neutron scattering measurements of the microstructure are analyzed to determine the effective strength of attraction in the form of Baxter sticky parameter. Quantitative agreement is found with simulation predictions of the thermoreversible gel transition as a function of volume fraction, further validating a universal state diagram and providing guidance for the effects of aspect ratio on gelation.

Evaporation-induced self-assembly of liquid crystal biopolymers

Evaporation-induced self-assembly (EISA) is a process that has gained significant attention in recent years due to its fundamental science and potential applications in materials science and nanotechnology. This technique involves controlled drying of a solution or dispersion of materials, forming structures with specific shapes and sizes. In particular, liquid crystal (LC) biopolymers have emerged as promising candidates for EISA due to their highly ordered structures and biocompatible properties after deposition. This review provides an overview of recent progress in the EISA of LC biopolymers, including DNA, nanocellulose, viruses, and other biopolymers. The underlying self-assembly mechanisms, the effects of different processing conditions, and the potential applications of the resulting structures are discussed.

Colloidal Crystals of Charged-Polymer-Brush-Decorated Hybrid Particles in Low-Polarity Solvents

Colloidal crystals are self-assembled systems that are suitable as models for studying crystallization; they are also attractive as nanostructures with a periodic arrangement of materials that have different refractive indices. Here, we present a method of constructing colloidal crystals in an organic solvent using charged-polymer-brush-decorated core-shell-type hybrid particles synthesized by surface-initiated living radical polymerization. The core-shell-type hybrid particles consisted of a silica particle core surrounded by a shell of polymer brushes obtained by the polymerization of methyl methacrylate and a small amount of a cationic monomer, [2-(methacryloyloxy)ethyl]trimethylammonium chloride. When the core-shell-type hybrid particles were dispersed in a low-polarity solvent with a dielectric constant of ∼11, colloidal crystals formed when the particle volume fraction exceeded a certain threshold, and remarkably, the interparticle distance in the colloidal crystal reached more than several micrometers under certain colloidal crystallization conditions. The colloidal crystallization behavior depended upon the surface charge density of the hybrid particles, ionic strength of the suspension, and dielectric constant of the solvent. The proposed method to construct colloidal crystals using electrostatic interactions between charged polymer brushes will promote the development of systems exhibiting particle self-assembly.

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.

Colloidal Rod-Like Particles with Temperature-Driven Tunable Interactions

Temperature-sensitive rod-like colloidal particles were synthesized by grafting a temperature-responsive polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMA), on the surface of high aspect ratio silica rods by surface-initiated atom transfer radical polymerization. The stability of the grafted polymer on the surface of the particles in aqueous solutions was found to deteriorate with time, leading to a gradual decrease of the polymer content of the hybrid colloids, which was attributed to the mechanically activated hydrolysis of the labile bonds at the polymer-silica interface. The polymer degrafting was significantly suppressed by first growing a hydrophobic poly(methyl methacrylate) block onto the particle surface to act as a barrier layer for the penetration of water molecules at the polymer-particle interface, followed by chain-extension with the hydrophilic PDMA chains. Dynamic light scattering, microscopy, and rheological measurements revealed that the PDMA block conferred a temperature-responsive behavior to the rod-like particles, which formed aggregates at temperatures above the lower critical solution temperature (LCST) of the polymer. However, in contrast to their spherical counterparts, the polymer-grafted rod-like particles did not exhibit complete thermo-reversibility upon lowering the solution temperature below the LCST of PDMA, which was reflected by different values of the diffusion coefficient for the heating and cooling cycles, indicating an irreversible rod particle aggregation upon increasing the temperature.

Application of Bacteriophages in Nanotechnology

Bacteriophages (phages for short) are viruses, which have bacteria as hosts. The single phage body virion, is a colloidal particle, often possessing a dipole moment. As such, phages were used as perfectly monodisperse systems to study various physicochemical phenomena (e.g., transport or sedimentation in complex fluids), or in the material science (e.g., as scaffolds). Nevertheless, phages also execute the life cycle to multiply and produce progeny virions. Upon completion of the life cycle of phages, the host cells are usually destroyed. Natural abilities to bind to and kill bacteria were a starting point for utilizing phages in phage therapies (i.e., medical treatments that use phages to fight bacterial infections) and for bacteria detection. Numerous applications of phages became possible thanks to phage display-a method connecting the phenotype and genotype, which allows for selecting specific peptides or proteins with affinity to a given target. Here, we review the application of bacteriophages in nanoscience, emphasizing bio-related applications, material science, soft matter research, and physical chemistry.

Irreversible Shear-Activated Gelation of a Liquid Crystalline Polyelectrolyte

We report irreversible, shear-activated gelation in liquid crystalline solutions of a rigid polyelectrolyte that forms rodlike assemblies (rods) in salt-free solution. At rest, the liquid crystalline solutions are kinetically stable against gelation and exhibit low viscosities. Under steady shear at, or above, a critical shear rate, a physically cross-linked, nematic gel network forms due to linear growth and branching of the rods. Above a critical shear rate, the time scale of gelation can be tuned from hours to nearly instantaneously by varying the shear rate and solution concentration. The shear-activated gels are distinct in their structure and rheological properties from thermoreversible gels. At a fixed concentration, the induction time prior to gelation decreases exponentially with the shear rate. This result indicates that shear-activated thermalization of the electrostatically stabilized rods overcomes the energy barrier for rod-rod contact, enabling rod fusion and subsequent irreversible network formation.

Shear-Induced Isotropic-Nematic Transition in Poly(ether ether ketone) Melts

In a previous work on a poly(ether ether ketone) (PEEK) melt, above its nominal melting temperature (T_m ≅ 335 °C), a severe Cox-Merz rule failure was observed. The abrupt decrease in the apparent shear viscosity was ascribed to the formation of flow-induced crystallization precursors. Here shear rheology and reflection polariscope experiments are utilized to unravel the structural changes occurring under shear on a similar PEEK melt above T_m. Three regimes of the flow curve were identified from low (0.01 s^-1) to high shear rates (1000 s^-1): (I) an isotropic structure with weak birefringence due to polymer chain orientation and mild shear thinning for γ̇ < 1 s^-1, (II) an isotropic-nematic transition accompanied by strong birefringence, two steady-state viscosities, and large nematic polydomain director fluctuations, and (III) shear-thinning behavior with an η ∼ γ̇^-0.5 dependence for γ̇ > 20 s^-1, typically found in nematic fluids. The findings reported in this experimental work suggest that the nematic phase may represent the early stage of the formation of shear-induced crystallization precursors.

Polymer-Peptide Conjugates Convert Amyloid into Protein Nanobundles through Fragmentation and Lateral Association

The assembly of proteins into amyloid fibrils has become linked not only with the progression of myriad human diseases, but also important biological functions. Understanding and controlling the formation, structure, and stability of amyloid fibrils is therefore a major scientific goal. Here we utilize electron microscopy-based approaches combined with quantitative statistical analysis to show how recently developed kind of amyloid modulators-multivalent polymer-peptide conjugates (mPPCs)-can be applied to control the structure and stability of amyloid fibrils. In doing so, we demonstrate that mPPCs are able to convert 40-residue amyloid beta fibrils into ordered nanostructures through a combination of fragmentation and bundling. Fragmentation is shown to be consistent with a model where the rate constant of fibril breakage is independent of the fibril length, suggesting a local and specific interaction between fibrils and mPPCs. Subsequent bundling, which was previously not observed, leads to the formation of sheet-like nanostructures which are surprisingly much more uniform than the starting fibrils. These nanostructures have dimensions independent of the molecular weight of the mPPC and retain the molecular-level ordering of the starting amyloid fibrils. Collectively, we reveal quantitative and nanoscopic understanding of how mPPCs can be applied to control amyloid structure and stability, and demonstrate approaches to elucidate nanoscale amyloid phase behavior in the presence of functional macromolecules and other modulators.

Short and Soft: Multidomain Organization, Tunable Dynamics, and Jamming in Suspensions of Grafted Colloidal Cylinders with a Small Aspect Ratio

The yet virtually unexplored class of soft colloidal rods with a small aspect ratio is investigated and shown to exhibit a very rich phase and dynamic behavior, spanning from liquid to nearly melt state. Instead of the nematic order, these short and soft nanocylinders alter their organization with increasing concentration from isotropic liquid with random orientation to small domains with preferred local orientation and eventually a multidomain arrangement with a local orientational order. The latter gives rise to a kinetically suppressed state akin to structural glass with detectable terminal relaxation, which, on further increasing concentration, reveals features of hexagonally packed order as in ordered block copolymers. The respective dynamic response comprises four regimes, all above the overlapping concentration of 0.02 g/mL:(I) from 0.03 to 0.1 g/mol, the system undergoes a liquid-to-solidlike transition with a structural relaxation time that grows by 4 orders of magnitude. (II) From 0.1 to 0.2 g/mL, a dramatic slowing-down is observed and is accompanied by an evolution from isotropic to a multidomain structure. (III) Between 0.2 and 0.6 g/mol, the suspensions exhibit signatures of shell interpenetration and jamming, with the colloidal plateau modulus depending linearly on concentration. (IV) At 0.74 g/mL, in the densely jammed state, the viscoelastic signature of hexagonally packed cylinders from microphase-separated block copolymers is detected. These properties set short and soft nanocylinders apart from long colloidal rods (with a large aspect ratio) and provide insights for fundamentally understanding the physics in this intermediate soft colloidal regime and for tailoring the flow properties of nonspherical soft colloids.

Effects of particle stiffness on the extensional rheology of model rod-like nanoparticle suspensions

The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semiflexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is compared in detail with theoretical predictions for worm-like chains. The extensional properties are compared to Batchelor's theory, generalized for the shear thinning nature of the suspensions. Theoretical predictions agree well with the measured complex moduli at low concentrations as well as the nonlinear shear and elongational viscosities at high flow rates. The results in this work provide guidelines for enhancing the elongational viscosity based on purely frictional effects in the absence of strong normal forces which are characteristic for high molecular weight polymers.

Filamentous phages as building blocks for reconfigurable and hierarchical self-assembly

Filamentous bacteriophages such as fd-like viruses are monodisperse rod-like colloids that have well defined properties of diameter, length, rigidity, charge and chirality. Engineering these viruses leads to a library of colloidal rods, which can be used as building blocks for reconfigurable and hierarchical self-assembly. Their condensation in an aqueous solution with additive polymers, which act as depletants to induce attraction between the rods, leads to a myriad of fluid-like micronic structures ranging from isotropic/nematic droplets, colloid membranes, achiral membrane seeds, twisted ribbons, π-wall, pores, colloidal skyrmions, Möbius anchors, scallop membranes to membrane rafts. These structures, and the way that they shape-shift, not only shed light on the role of entropy, chiral frustration and topology in soft matter, but also mimic many structures encountered in different fields of science. On the one hand, filamentous phages being an experimental realization of colloidal hard rods, their condensation mediated by depletion interactions constitutes a blueprint for the self-assembly of rod-like particles and provides a fundamental foundation for bio- or material-oriented applications. On the other hand, the chiral properties of the viruses restrict the generalities of some results but vastly broaden the self-assembly possibilities.

Principles of pharmacology in the eye

The eye is a highly specialized organ that is subject to a huge range of pathology. Both local and systemic disease may affect different anatomical regions of the eye. The least invasive routes for ocular drug administration are topical (e.g. eye drops) and systemic (e.g. tablets) formulations. Barriers that subserve as protection against pathogen entry also restrict drug permeation. Topically administered drugs often display limited bioavailability due to many physical and biochemical barriers including the pre-corneal tear film, the structure and biophysiological properties of the cornea, the limited volume that can be accommodated by the cul-de-sac, the lacrimal drainage system and reflex tearing. The tissue layers of the cornea and conjunctiva are further key factors that act to restrict drug delivery. Using carriers that enhance viscosity or bind to the ocular surface increases bioavailability. Matching the pH and polarity of drug molecules to the tissue layers allows greater penetration. Drug delivery to the posterior segment is a greater challenge and, currently, the standard route is via intravitreal injection, notwithstanding the risks of endophthalmitis and retinal detachment with frequent injections. Intraocular implants that allow sustained drug release are at different stages of development. Novel exciting therapeutic approaches include methods for promoting transscleral delivery, sustained release devices, nanotechnology and gene therapy.

Tunable uptake/release mechanism of protein microgel particles in biomimicking environment

Microgels are intra-molecular crosslinked macromolecules that can be used as vehicles to deliver and release drugs at the point-of-need in the patient’s body. Here, gelatin microgels were formed from microfluidics droplets, stabilised by aldehydes and frozen into a spheroidal shape. Microgel morphology and response to external stimuli were characterised. It was found that the behaviour of the spheroidal microgels was sensitive to both pH and ionic strength and that the distribution of charges into the microgels affected the behaviour of swelling and uptake. The uptake of molecules such as Rhodamine B and Methylene Blue were investigated as a model for drug uptake/release mechanisms. Under physiological conditions, the uptake of Rhodamine was rapid and a uniform distribution of the fluorescent molecules was recorded inside the microgels. However, the mechanism of release became slower at lower pH, which mimics the stomach environment. Under physiological conditions, Methylene Blue release occurred faster than for Rhodamine. Anionic and neutral molecules were also tested. In conclusion, the dependence of uptake and release of model drugs on basic/acid conditions shows that microgels could be used for targeted drug delivery. Different shaped microgels, such as spheres, spheroids, and rods, could be useful in tissue engineering or during vascularisation.

Adjustment of the Chiral Nematic Phase Properties of Cellulose Nanocrystals by Polymer Grafting

The self-organization properties of sulfated cellulose nanocrystals, TEMPO-oxidized cellulose nanocrystals and polymer-decorated cellulose nanocrystals suspensions in water were investigated and compared. Polarized light optical microscopy observations showed that these three systems phase separated to form a lower anisotropic chiral-nematic phase and an upper isotropic phase following a nucleation and growth mechanism, proving that surface-grafted polymer chains did not inhibit the self-organization properties of CNCs. The phase diagrams and pitch of the suspensions were shown to strongly depend on the surface chemistry of the nanoparticles and the nature of the interacting forces. Especially, the entropic repulsion contribution of the polymer chains to the overall interactions forces resulted in a decrease of the critical volume fractions due to an increase of the effective diameter of the rods. Additionally, above a cellulose volume fraction of 3.5% v/v, the pitch was significantly smaller for polymer-decorated CNC suspensions than for sulfated as-prepared CNC ones, revealing stronger chiral interactions with the surface-grafted chains. In all cases, the addition of small quantities of monovalent salt induced an increase of the critical concentrations, but values for polymer-decorated CNCs were always the smallest ones due to entropic repulsion forces. Overall, results show that polymer grafting provides more tunability to the chiral-nematic phase properties of CNCs, including an enhanced expression of the chirality.

Theoretical Predictions of Temperature-Induced Gelation in Aqueous Dispersions Containing PEO-Grafted Particles

In this work, we utilize classical polymer density functional theory (DFT) to study gelation in systems containing colloidal particles onto which polymers are grafted. The solution conditions are such that the corresponding bulk system displays a lower critical solution temperature (LCST). We specifically compare our predictions with experimental results by Shay et al. (J. Rheol. 2001, 45, 913-927), who investigated temperature response in aqueous dispersions containing polystyrene particles (PS), with grafted 45-mer poly(ethylene oxide) (PEO) chains. Our DFT treatment is based on a model for aqueous PEO solutions that was originally developed by Karlström for bulk solutions. In this model, monomers are assumed to be in either of two classes of states, labeled A and B, where B is more solvophobic than A. On the other hand, the degeneracy of B exceeds that of A, causing the population of solvophobic monomers to increase with temperature. In agreement with experimental findings by Shay et al., we locate gelation at temperatures considerably below TΘ, and far below the LCST for such chain lengths. This gelation occurs also without any dispersion interactions between the PS particles. Interestingly, the polymer-induced interaction free energy displays a nonmonotonic dependence on the grafting density. At high grafting densities, bridging attractions between grafted layers take place (considerably below TΘ). At low grafting densities, on the other hand, the polymers are able to bridge across to the other particle surface. Shay et al. conducted their experiments at very low ionic strength, using deionized water as a solvent. We demonstrate that even minute amounts of adsorbed charge on the surface of the particles, can lead to dramatic changes of the gelation temperature, especially at high grafting densities. Another interesting prediction is the existence of elongated (chainlike) equilibrium structures, at low particle concentrations. We emphasize that our model does not rely upon any temperature-dependent interactions.

Gel properties of T-shaped tetrathiafulvalene–pyridazine conjugates and F4TCNQ-induced morphological transformation

T-shaped organic π-conjugates and their F₄TCNQ charge-transfer (CT) complexes are developed as LMOGs. The morphological changes of the organogels occur obviously due to the CT interaction.

Gelation and mechanical response of patchy rods

We perform Brownian dynamics simulations to study the gelation of suspensions of attractive, rod-like particles. We show that in detail the rod-rod surface interactions can dramatically affect the dynamics of gelation and the structure and mechanics of the networks that form. If the attraction between the rods is perfectly smooth along their length, they will collapse into compact bundles. If the attraction is sufficiently corrugated or patchy, over time, a rigid space-spanning network will form. We study the structure and mechanical properties of the networks that form as a function of the fraction of the surface, f, that is allowed to bind. Surprisingly, the structural and mechanical properties are non-monotonic in f. At low f, there are not a sufficient number of cross-linking sites to form networks. At high f, rods bundle and form disconnected clusters. At intermediate f, robust networks form. The elastic modulus and yield stress are both non-monotonic in the surface coverage. The stiffest and strongest networks show an essentially homogeneous deformation under strain with rods re-orienting along the extensional axis. Weaker, more clumpy networks at high f re-orient relatively little with strong non-affine deformation. These results suggest design strategies for tailoring surface interactions between rods to yield rigid networks with optimal mechanical properties.

Anisotropic responsive microgels with tuneable shape and interactions

Highly monodisperse polystyrene/poly(N-isopropylmethacrylamide) (PS-PNIPMAM) core-shell composite microgels were synthesized and further nanoengineered in either ellipsoidal, faceted or bowl-shaped particles. Beside their anisotropy in shape, the microgel design enables an exquisite control of the particle conformation, size and interactions from swollen and hydrophilic to collapsed and hydrophobic using temperature as an external control variable. The post-processing procedures and the characterization of the different particles are first presented. Their potential as model systems for the investigation of the effects of anisotropic shape and interactions on the phase behavior is further demonstrated. Finally, the self-assembly of bowl-shaped composite microgel particles is discussed, where the temperature and an external AC electric field are employed to control the interactions from repulsive to attractive and from soft repulsive to dipolar, respectively.

Interfacial stability and shape change of anisotropic endoskeleton droplets

The delivery of suspended active ingredients to a surface is a central function of numerous commercial cosmetic, drug, and agricultural formulations. Many products use liquid droplets as a delivery vehicle but, because interfacial tension keeps droplets spherical, these materials cannot exploit the benefits of anisotropic shape and shape change offered by solid colloids. In this work, individual droplet manipulation is used to produce viscoelastic droplets that can stably retain non-spherical shapes by balancing the Laplace pressure of the liquid-liquid interface with the elasticity of an internal crystalline network. A stability criterion is developed for idealized spherocylindrical droplets and shown to agree with experimental data for varying droplet size and rheology. Shape change can be induced in the anisotropic droplets by upsetting the balance of droplet interfacial tension and internal rheology. Using dilution to increase the interfacial tension shows that external stimuli can trigger collapse and shape change in these droplets. The droplets wrap around substrates during collapse, improving contact and adhesion. The model is used to develop design criteria for production of droplets with tunable response.

Synthesis of end-functionalized boronic acid containing copolymers and their bioconjugates with rod-like viruses for multiple responsive hydrogels

End-functionalized boronic acid containing copolymers are grafted to a rod-like M13 virus. The resultant virus polymer can reversibly form hydrogels, which can be regulated by temperature, pH and glucose.

Rheology and DWS microrheology of concentrated suspensions of the semiflexible filamentous fd virus

Microrheology measurements were performed on suspensions of bacteriophage fd with diffusive wave spectroscopy in the concentrated regime, at different values of ionic strength. Viscosity vs. shear rate was also measured, and the effect of bacteriophage concentration and salt addition on shear thinning was determined, as well as on the peaks in the viscosity vs. shear curves corresponding to a transition from tumbling to wagging flow. The influence of concentration and salt addition on the mean square displacement of microspheres embedded in the suspensions was determined, as well as on their viscoelastic moduli up to high angular frequencies. Our results were compared with another microrheology technique previously reported where the power spectral density of thermal fluctuations of embedded micron-sized particles was evaluated. Although both results in general agree, the diffusive wave spectroscopy results are much less noisy and can reach larger frequencies. A comparison was made between measured and calculated shear modulus. Calculations were made employing the theory for highly entangled isotropic solutions of semiflexible polymers using a tube model, where various ways of calculating the needed parameters were used. Although some features are captured by the model, it is far from the experimental results mainly at high frequencies.

Aqueous suspensions of natural swelling clay minerals. 2. Rheological characterization

We report in this article a comprehensive investigation of the viscoelastic behavior of different natural colloidal clay minerals in aqueous solution. Rheological experiments were carried out under both dynamic and steady-state conditions, allowing us to derive the elasticity and yield stress. Both parameters can be renormalized for all sizes, ionic strength, and type of clay using in a first approach only the volume of the particles. However, applying such a treatment to various clays of similar shapes and sizes yields differences that can be linked to the repulsion strength and charge location in the swelling clays. The stronger the repulsive interactions, the better the orientation of clay particles in flows. In addition, a master linear relationship between the elasticity and yield stress whose value corresponds to a critical deformation of 0.1 was evidenced. Such a relationship may be general for any colloidal suspension of anisometric particles as revealed by the analysis of various experimental data obtained on either disk-shaped or lath- and rod-shaped particles. The particle size dependence of the sol-gel transition was also investigated in detail. To understand why suspensions of larger particles gel at a higher volume fraction, we propose a very simplified view based on the statistical hydrodynamic trapping of a particle by an another one in its neighborhood upon translation and during a short period of time. We show that the key parameter describing this hydrodynamic trapping varies as the cube of the average diameter and captures most features of the sol-gel transition. Finally, we pointed out that in the high shear limit the suspension viscosity is still closely related to electrostatic interactions and follows the same trends as the viscoelastic properties.

Stress enhancement in the delayed yielding of colloidal gels

Networks of aggregated colloidal particles are solidlike and can sustain an applied shear stress while exhibiting little or no creep; however, ultimately they will catastrophically fail. We show that the time delay for this yielding decreases in two distinct exponential regimes with applied stress. This behavior is universal and found for a variety of colloidal gel systems. We present a bond-rupture model that quantitatively describes this behavior and highlights the role of mesoscopic structures. Our result gives new insight into the nature of yielding in these soft solid materials.

Charge reversal of the rodlike colloidal fd virus through surface chemical modification

There is increasing interest in the use of viruses as model systems for fundamental research and as templates for nanomaterials. In this work, the rodlike fd virus was subjected to chemical modifications targeting different solvent-exposed functional groups in order to tune its surface properties, especially reversing the surface charge from negative to positive. The carboxyl groups of fd were coupled with different kinds of organic amines by carbodiimide chemistry, resulting in modified viruses that are positively charged over a wide range of pH. Care was taken to minimize intervirus cross linking, which often occurs because of such modifications. The surface amino groups were also grafted with poly(ethylene glycol) (PEG) end-functionalized with an active succinimidyl ester in order to introduce a steric stabilization effect. By combining charge reversal with PEG grafting, a reversible attraction between positively and negatively charged PEG-grafted fd viruses could be realized, which was tuned by the ionic strength of the solution. In addition, a charge-reversed fd virus forms only a pure nematic phase in contrast to the cholesteric phase of the wild type. These modified viruses might be used as model systems in soft condensed matter physics, for example, in the study of polyelectrolyte complexes or lyotropic liquid-crystalline phase behavior.