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Liquid capillarity through porous media can be enhanced by a rational design of hierarchically porous constructs that suggest sufficiently large liquid pathways from an upper-level hierarchy as well as capillary pressure enabled by a lower hierarchy. Here, we demonstrate a material design strategy utilizing a new class of self-assembled soft materials, called bicontinuous interfacially jammed emulsion gels (bijels), to produce hierarchically porous copper, which enables the unique combination of unprecedented control over both macropores and mesopores in a regular, uniform, and continuous arrangement. The dynamic droplet topologies on the hierarchically copper pores prove the significant enhancement in liquid capillarity compared to homogeneous porous structures. The role of nanoscale morphology in liquid infiltration is further investigated through environmental scanning electron microscopy, in which wetting through the mesopores occurs at the beginning, followed by liquid transport through macropores. This understanding on capillary wicking will allow us to design better hierarchically porous media that can address performance breakthroughs in interfacial applications, ranging from battery electrodes, cell delivery in biomedical devices, to capillary-fed thermal management systems.

Porous electrodes are widely used in the industry because of their high surface area to volume ratio. However, the stochastic morphology of most commercially available porous electrodes results in poor electrical connections in the solid phase and inefficient mass transport through the pore phase. This can be especially detrimental for gas-evolving processes such as water electrolysis for hydrogen and oxygen generation. Bicontinuous interfacially jammed emulsion gels (bijels) offer templates from which to create porous electrodes with robust solid-state interconnectivity and a uniform pore structure that facilitate improved electron and mass transport. In this study, gas release rates and electrochemical experiments are utilized to study the effects of powder- and bijel-derived microstructures on hydrogen generation by water electrolysis. The bijel-derived electrodes are shown to expel product gas faster and require up to 25% less overpotential to drive water electrolysis over the range of current densities tested (-5 to -40 mA/cm

The discovery of bicontinuous interfacially jammed emulsion gels (bijels) in 2007 motivated the development of processing techniques to harness their unique morphological attributes in applications such as electrochemical energy storage and conversion, catalysis, and regenerative biomaterials. These techniques are primarily based on selective polymerization of one phase, and subsequent chemical processing of the resultant scaffold into porous, micro-architectured materials. A significant limitation of these protocols is the need to transport polymer precursors into one of the fluid phases after bijel formation, a time-consuming step that can also impose disruptive gravitational and interfacial stresses, sometimes causing a complete breakdown of the bijel backbone. Here, we introduce a class of intrinsically polymerizable bijels (IPBs) comprising partially miscible mixtures of solvent and poly(ethylene glycol) precursor, which can be directly transformed into bijel-templated materials (BTMs), completely bypassing the precursor transport step and relaxing the associated limitations of previous protocols. To achieve selective polymerization, we incorporated into the mixture a common fluorescent dye, sodium fluorescein, which had strong affinity for the monomer-poor phase. Spectrophotometry experiments demonstrated a local photon quenching effect due to the fluorescent dye, which in turn curtailed activation of the photoinitiator and thus prevented polymerization in the monomer-poor phase. We establish the generality of our approach by using different monomers and monomer blends, and demonstrate how this modularity enables tuning of the mechanical properties of BTMs, measured by flexural testing. Our protocol establishes a scalable and efficient platform for producing BTMs, paving the way for their protential applications in emerging technologies.

We present rheological evidence demonstrating the glass-like nature of bicontinuous interfacially jammed emulsion gels (bijels). Under small amplitude oscillatory shear, bijels exhibited rheological signatures akin to

We introduce laser cavitation rheology (LCR) as a minimally-invasive optical method to characterize mechanical properties within the interior of biological and synthetic aqueous soft materials at high strain-rates. We utilized time-resolved photography to measure cavitation bubble dynamics generated by the delivery of focused 500 ps duration laser radiation at λ = 532 nm within fibrin hydrogels at pulse energies of Ep = 12, 18 µJ and within polyethylene glycol (600) diacrylate (PEG (600) DA) hydrogels at Ep = 2, 5, 12 µJ. Elastic moduli and failure strains of fibrin and PEG (600) DA hydrogels were calculated from these measurements by determining parameter values which provide the best fit of the measured data to a theoretical model of cavitation bubble dynamics in a Neo-Hookean viscoelastic medium subject to material failure. We demonstrate the use of this method to retrieve the local, interior elastic modulus of these hydrogels and both the radial and circumferential failure strains.

Secondary zinc/zinc oxide electrodes have traditionally suffered from electrochemically driven material redistribution during recharge, leading to cell failure through short circuits or compromised...

High internal phase emulsions (HIPEs) comprise highly faceted droplets separated by thin films of fluid. Though surfactants are traditionally used in formulating HIPEs, growing interest in solid-stabilized HIPEs calls for a better understanding of how particles may affect the coalescence of droplets at high volume fractions of the dispersed phase. In this study, we address the effect of particle size on this issue. Using confocal microscopy, we examine the microstructures of four different solid-stabilized emulsion series and quantify droplet coalescence in each. We show that, systematically, HIPEs stabilized with smaller particles show a greater propensity for film rupture and the presence of partially coalesced droplets, whereas the use of larger particles results in a higher fraction of bridged particle monolayers between neighboring droplets. This result is in contrast with the behavior of dilute emulsions, where the use of smaller particles has been shown to impart greater stability against droplet coalescence. Utilizing a simple model of film rupture, we rationalize our experimental findings in the context of the capillary pressure profile within a solid-stabilized liquid film, and show that bridged monolayer formation is directly linked to improved film stability at high volume fractions of the dispersed phase. Therefore, particle size can impact the stability of solid-stabilized HIPEs by influencing their propensity for monolayer formation.

Hypothesis The presence of colloidal particles on fluid interfaces can have a significant impact on the rheology of solid-stabilized high internal phase emulsions (HIPEs). Experiments Using dynamic oscillatory rheometry and confocal microscopy, we investigate a broad array of solid-stabilized HIPEs formulated along four different compositional trajectories in their ternary state diagram, using particles of three different sizes and two different surface chemistries. Findings We unveil three important consequences of the use of particles, in lieu of surfactants, on the rheology of HIPEs. First, particle excluded volume interactions take a pronounced role in the transition to solid-like rheology due to crowding. An effective dispersed phase volume fraction, taking into account the particle three-phase contact angle, must be defined to account for the dependence of the mixture’s rheology on its composition. Second, weak, chemistry-dependent attractive colloidal interactions through the continuous phase result in a finite elastic modulus at low effective dispersed phase volume fractions. Third, we observe a secondary rise in the mixture’s elastic modulus at increasingly high dispersed-to-continuous-phase volumetric ratios. We postulate these interactions stem from attractive lateral capillary interactions between the particles, due to thinning of the continuous fluid film between faceted droplets.

We use rheo-microscopy to directly investigate the dynamics of solid-stabilized droplets subjected to shear flow of a surrounding bulk fluid. In our system, the stabilizing particles are weakly attractive through the continuous fluid phase and along the droplet interface. Under shear, droplets stabilized by these particles at near-complete surface coverage exhibit a number of previously unforeseen phenomena, including negative-then-positive deviations from the predictions of Taylor and the behavior of bare droplets, evolution toward spherocylindrical shapes, and an earlier onset of rupture than their bare counterparts, which we explain in light of the weak attractive interparticle interactions along the droplet interface. We also demonstrate the formation of long-lived anisotropic particle-coated droplets by flow cessation, and provide evidence that this is due to the formation of a jammed, disordered particle network along the interface at surface coverage lower than the starting conditions. Importantly, these newly observed phenomena are shown to be sensitive to the droplets' initial surface coverage. Our findings provide new technologically-relevant insights into the physics of particle-coated droplets under fluidic or other external stresses, and introduce avenues for future research to better understand the roles of interparticle interactions and surface coverage in mediating the interfacial rheology of particle-laden interfaces and solid-stabilized emulsions.

The microstructure of Pickering emulsion gels features a tenuous network of faceted droplets, bridged together by shared monolayers of particles. In this investigation, we use standard oscillatory rheometry in conjunction with confocal microscopy to gain a more comprehensive understanding of the role particle bridged interfaces have on the rheology of Pickering emulsion gels. The zero-shear elastic modulus of Pickering emulsion gels shows a non-monotonic dependence on particle loading, with three separate regimes of power-law and linear gel strengthening, and subsequent gel weakening. The transition from power-law to linear scaling is found to coincide with a peak in the volume fraction of particles that participate in bridging, which we indirectly calculate using measureable quantities, and the transition to gel weakening is shown to result from a loss in network connectivity at high particle loadings. These observations are explained via a simple representation of how Pickering emulsion gels arise from an initial population of partially-covered droplets. Based on these considerations, we propose a combined variable related to the initial droplet coverage, to be used in reporting and rationalizing the rheology of Pickering emulsion gels. We demonstrate the applicability of this variable with Pickering emulsions prepared at variable fluid ratios and with different-sized colloidal particles. The results of our investigation have important implications for many technological applications that utilize solid stabilized multi-phase emulsions and require a priori knowledge or engineering of their flow characteristics.

Mitigation of the foreign body response (FBR) and successful tissue integration are essential to ensuring the longevity of implanted devices and biomaterials. The use of porous materials and coatings has been shown to have an impact, as the textured surfaces can mediate macrophage interactions with the implant and influence the FBR, and the pores can provide space for vascularization and tissue integration. In this study, we use a new class of implantable porous biomaterials templated from bicontinuous interfacially jammed emulsion gels (bijels), which offer a fully percolating, non-constricting porous network with a uniform pore diameter on the order of tens of micrometers, and surfaces with consistent curvature. We demonstrate that these unique morphological features, inherent to bijel-templated materials (BTMs), can enhance tissue integration and vascularization, and reduce the FBR. Cylindrical polyethylene glycol diacrylate (PEGDA) BTMs, along with PEGDA particle-templated materials (PTMs), and non-templated materials (NTMs), were implanted into the subcutaneous space of athymic nude mice. After 28 days, implants were retrieved and analyzed via histological techniques. Within BTMs, blood vessels of increased size and depth, changes in collagen deposition, and increased presence of pro-healing macrophages were observed compared to that of PTM and NTM implants. Bijel templating offers a new route to biomaterials that can improve the function and longevity of implantable devices. Statement of Significance All implanted biomaterials are subject to the foreign body response (FBR) which can have a detrimental effect on their efficacy. Altering the surface chemistry can decrease the FBR by limiting the amount of proteins adsorbed to the implant. This effect can be enhanced by including pores in the biomaterial to allow new tissue growth as the implant becomes integrated in the body. Here, we introduce a new class of self-assembled biomaterials comprising a fully penetrating, non-constricting pore phase with hyperbolic (saddle) surfaces for enhanced tissue integration. These unique morphological characteristics result in dense blood vessel formation and favorable tissue response properties demonstrated in a four-week implantation study.

Three-dimensional porous architectures of graphene are desirable for energy storage, catalysis, and sensing applications. Yet it has proven challenging to devise scalable methods capable of producing co-continuous architectures and well-defined, uniform pore and ligament sizes at length scales relevant to applications. This is further complicated by processing temperatures necessary for high quality graphene. Here, bicontinuous interfacially jammed emulsion gels (bijels) are formed and processed into sacrificial porous Ni scaffolds for chemical vapor deposition to produce freestanding three-dimensional turbostratic graphene (bi-3DG) monoliths with high specific surface area. Scanning electron microscopy (SEM) images show that the bi-3DG monoliths inherit the unique microstructural characteristics of their bijel parents. Processing of the Ni templates strongly influences the resultant bi-3DG structures, enabling the formation of stacked graphene flakes or fewer-layer continuous films. Despite the multilayer nature, Raman spectra exhibit no discernable defect peak and large relative intensity for the Raman 2D mode, which is a characteristic of turbostratic graphene. Moire patterns, observed in scanning tunneling microscopy images, further confirm the presence of turbostratic graphene. Nanoindentation of macroscopic pillars reveals a Young's modulus of 30 MPa, one of the highest recorded for sp2 carbon in a porous structure. Overall, this work highlights the utility of a scalable self-assembly method towards porous high quality graphene constructs with tunable, uniform, and co-continuous microstructure.

Colloidal gelation is a rheological transition from fluid-like to solid-like viscoelasticity in a particulate suspension and is often instigated by causing the net interparticle interaction to be attractive. In this article, three routes to colloidal gelation that have been discovered recently and involve interfacial phenomena at a fluid interface are reviewed. As in conventional systems, gelation is due to a percolating particle network that imparts elasticity to the mixture, but the network formation involves interfacial particle jamming or bridging, or capillary interactions along or across interfaces, in a mixture of immiscible fluids. Gelation imparts mechanical stability to these multiphase mixtures and paves the way for their use as templates for the synthesis of functional, microstructured materials and composites. The gel mechanical properties are mediated by the interfacial forces and the mixture's microstructure, and therefore show different dependencies on particle volume fraction across the three systems.

To compare the viscoelastic properties of calcium hydroxyapatite (CaHA) to carboxymethylcellulose (CMC) injectables used for injection laryngoplasty and determine if they are affected by heating and shearing.Experimental.University laboratory.Vocal fold injection laryngoplasty with CaHA is oftentimes challenging due to the amount of pressure necessary to push the injectate through a needle. Anecdotal techniques, such as heating the product, have been suggested to facilitate injection. The viscoelastic properties of CaHA and CMC were measured with a rheometer. The effects of heating and shearing on sample viscoelasticity were recorded.CaHA was 9.5 times more viscous than CMC (43,100 vs 4540 Pa·s). Heating temporarily decreased the viscosity of CaHA by 32%. However, it also caused the viscosity to subsequently increase after time. Shearing of CaHA reduced its viscosity by 26%. Heating and shearing together temporarily reduced the viscosity of CaHA by 52%.A combination of heating and shearing had a more profound effect than heating or shearing alone on the viscosity of CaHA, potentially making it easier to inject temporarily. Long-term and in vivo studies are required to further analyze the effect of heating and shearing on CaHA injectables.

SUMMARY Objectives: Whitening efficacy has been related to hydrogen peroxide (HP) diffusion into tooth structure. However, little information is available relating rheological properties to whitening efficacy. The purpose was to evaluate the whitening efficacy and HP penetration level of a 10% HP gel at three different viscosities and to compare them to a strip delivery system. Methods and Materials: Extracted molars (n=120) were randomly assigned into five groups (n=24/ group): NC_MED (negative control; median): medium viscosity gel without HP; LOW: 10% HP gel (low viscosity experimental gel, Ultradent Products Inc); MED: 10% HP gel (medium viscosity experimental gel, Ultradent); HIGH: 10% HP gel (high viscosity gel, Ultradent); and CWS: Crest 3D Whitestrips 1-Hour Express (Procter & Gamble). All teeth were subjected to five 60-minute whitening sessions. Instrumental color measurements were performed at baseline (T0), and 1-day after each application (T1-T5), and 1-month after whitening (T6). HP penetration was estimated with leucocrystal violet and horseradish peroxidase. A Kruskal-Wallis test and post hoc Bonferroni test were performed to assess the difference in tooth color change and HP penetration among the groups (α=0.05). Results: Hydrogen peroxide penetration levels and overall color changes at T6 were 0.24 μg/mL / 2.80; 0.48 μg/mL / 8.48; 0.44 μg/mL / 7.72; 0.35 μg/mL / 8.49; 0.36 μg/mL / 7.30 for groups NC, LOW, MED, HIGH, and CWS, respectively. There was a significant difference for HP penetration, while there was no significant difference among the four experimental groups for tooth color change. Conclusion: Rheological properties should be considered when developing new whitening formulations.

Numerous processing techniques aim to impart interconnected, porous structures within regenerative medicine materials to support cell delivery and direct tissue growth. Many of these techniques lack predictable control of scaffold architecture, and rapid prototyping methods are often limited by time-consuming, layer-by-layer fabrication of micro-features. Bicontinuous interfacially jammed emulsion gels (bijels) offer a robust, self-assembly-based platform for synthesizing a new class of morphologically unique cell delivery biomaterials. Bijels form via kinetic arrest of temperature-driven spinodal decomposition in partially miscible binary liquid systems. These non-equilibrium soft materials are comprised of co-continuous, fully percolating, non-constricting liquid domains separated by a nanoparticle monolayer. Through the selective introduction of biocompatible precursors, hydrogel scaffolds displaying the morphological characteristics of the parent bijel can be formed. We report using bijel templating to generate structurally unique, fibrin-loaded polyethylene glycol hydrogel composites. Demonstration of composite bijel-templated hydrogels (CBiTHs) as a new cell delivery system was carried out in vitro using fluorescence-based tracking of cells delivered to previously acellular fibrin gels. Imaging analysis confirmed repeatable delivery of normal human dermal fibroblasts to acellular fibrin gels.

Collagen is the most abundant protein in extracellular matrices and is commonly used as a tissue engineering scaffold. However, collagen and other biopolymers from native sources can exhibit limitations when tuning mechanical and biological properties. Cysteines do not naturally occur within the triple-helical region of any native collagen. We utilized a novel modular synthesis strategy to fabricate variants of recombinant human collagen that contained 2, 4, or 8 non-native cysteines at precisely defined locations within each biopolymer. This bottom-up approach introduced capabilities using sulfhydryl chemistry to form hydrogels and immobilize bioactive factors. Collagen variants retained their triple-helical structure and supported cellular adhesion. Hydrogels were characterized using rheology, and the storage moduli were comparable to fibrillar collagen gels at similar concentrations. Furthermore, the introduced cysteines functioned as anchoring sites, with TGF-β1-conjugated collagens promoting myofibroblast differentiation. This approach demonstrates the feasibility to produce custom-designed collagens with chemical functionality not available from native sources.

We utilize a custom-built shear cell mounted on a confocal microscope to directly visualize and quantify the microdynamic mechanisms that mediate the rheology of a nearly jammed colloidal suspension under constant-stress deformation, with and without attractive interparticle interactions. The application of external stresses systematically increases particle mobility, as well as the ease by which the colloids can escape from topological cages formed by their nearest neighbors. We quantify the characteristic size and timescale of microstructural rearrangements within the suspension and show that these relaxation events become less spatiotemporally heterogeneous as the applied stress is increased. When interparticle attraction is introduced, the colloids tend to move more congruently under low stresses and the characteristic size of dynamically cooperative clusters increases. However, particle displacements become decorrelated under large external loads, with an abrupt transition occurring at the yield stress. In contrast, the repulsive system shows a more gradual transition from creep deformation to flow, with a nonmonotonic dependence of the particle displacement correlation function on the applied stress. Our results contribute to a better understanding of the jamming phase diagram in disordered colloidal materials, and the connection between the microstructure and nonlinear rheology of colloidal gels and glasses.

The geometric arrangement of pore features is a critical aspect of three-dimensional (3D) materials design for a diverse set of applications. Transport properties such as diffusivity and conductivity are intimately linked to this pore arrangement, thus, significant effort has been invested into designing processes and systems that offer predictable final morphologies. Minimal surface structures comprising bicontinuous, symmetric phases are predicted to provide optimal transport properties. The bicontinuous interfacially jammed emulsion gel (bijel) is a class of soft matter that forms by kinetically arresting spinodal decomposition phase separation. Driven by the reduction of interfacial energy between phases, bijel morphology develops in a dynamically self-similar fashion with a near-minimal surface (spinodal) interface. Here, bijels were used as template structures to generate carbon and polymer scaffolds, and morphological characterization of distinctive features was carried out on 3D reconstructions of micro-computed tomography (µCT) data. Specific emphasis is placed on the characterization of size distribution, interfacial curvature, continuity, tortuosity, and self-similarity exhibited by pore networks within these structures. Microstructural attributes are compared to three additional porous media to demonstrate bijel-derived materials as near-minimal surface structures with high transport potential.

We introduce a custom-built stress-controlled shear cell coupled to a confocal microscope for direct visualization of constant-stress shear deformation in soft materials. The torque generator is a cylindrical Taylor–Couette system with a Newtonian fluid between a rotating inner bob and a free-to-move outer cup. A spindle/cone assembly is coaxially coupled to the cup and transfers the torque exerted by the fluid to the sample of interest in a cone-and-plate geometry. We demonstrate the performance of our device in both steady-state and transient experiments with different viscoelastic materials. Our apparatus can conduct unidirectional constant-stress experiments as accurately as most commercial rheometers, with the capability to directly visualize the flow field using tracer particles. Further, our step-stress experiments on viscoelastic materials are devoid of creep ringing, which is an advantageous aspect of our torque generation mechanism. We believe that the device presented here could serve as a powerful and cost-effective tool to investigate the microstructural determinants of nonlinear rheology in complex fluids.

© 2016 The Royal Society of Chemistry. Emerging demands for national security, transportation, distributed power, and portable systems call for energy storage and conversion technologies that can simultaneously deliver large power and energy densities. To this end, here we report three-dimensional Ni/Ni(OH)2composite electrodes derived from a new class of multi-phase soft materials with uniform, co-continuous, and tunable internal microdomains. These remarkable morphological attributes combined with our facile chemical processing techniques allow the electrode's salient morphological parameters to be independently tuned for rapid ion transport and a large volumetric energy storage capacity. Through microstructural design and optimization, our composite electrodes can simultaneously deliver energy densities equal to that of batteries and power densities equivalent to or greater than that of the best supercapacitors, bridging the gap between these modern technologies. Our synthesis procedure is robust and can be extended to a myriad of other chemistries for next generation energy storage materials.

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

Viscoelastic fluids can dramatically change the stress field around rigid spherical particles in shear flows, and these changes can cause microstructure and particle chain formation as a result of hydrodynamic interaction between particles. This paper introduces a new microscopic based approach for quantifying the kinetics of particle chaining. By monitoring the number of particles in the form of singlets, doublets, triplets, and multiplets, an absolute measure of string formation is presented. This approach enables finding the shear induced microstructural length scales in the medium. The concept of a String Factor (SF) is defined as a direct and reliable measure of chaining strength. The SF, which can be plotted over time, considers and accordingly weights the long range orientational correlations within each cluster. Real-time monitoring of particles is carried out by confocal microscopy imaging in a cone and plate setup that ensures a constant shear rate all over the field. Monodisperse polystyrene spheres with 5 μm diameter in a viscoelastic solution of polyethylene oxide in DI water (0.8% PEO, 1.7% PEO) are used as test suspensions to demonstrate quantifiable differences in chain dynamics over a small range of Weissenberg number.

A macroscopically homogeneous emulsion with bicontinuous microstructure is created by simple shaking of a mixture containing polymers, nanoparticles and two immiscible fluids.

We experimentally characterize the microstructure and rheology of a carefully designed mixture of immiscible fluids and near-neutral-wetting colloidal particles. Particle bridging across two fluid interfaces provides a route to highly stable gel-like emulsions at volume fractions of the dispersed phase well below the random close-packing limit for spheres. We investigate the microstructural origins of this behavior by confocal microscopy and reveal a percolating network of colloidal particles that serves as a cohesive scaffold, bridging together droplets of the dispersed phase. Remarkably, the mixture's salient rheological characteristics are governed predominantly by the solids loading and can be tailored irrespective of the droplet volume fraction. The identification of this rheological hallmark could provide a means toward the improved design of modern products that utilize solid-stabilized interfaces.

We investigate the link between the microstructure, dynamics, and rheological properties in dense (phi = 0.3) mixtures of charge-stabilized colloidal silica and oppositely charged poly(ethylene imine) polymer in a mixed DMSO/H(2)O solvent. Over a finite range of polymer concentrations, the addition of polymer results in the formation of sample-spanning, self-supporting gel networks. As the polymer concentration is increased, a reentrant rheological transition is observed where the gel's elastic modulus and yield stress initially increase and subsequently drop. The dynamic and microstructural changes associated with this transition are resolved using quantitative confocal microscopy. Within the initial regime, a biphasic system consisting of a mixture of arrested and diffusive particles is observed. We segregate the particles with high accuracy into mobile and arrested populations based on their dynamics. The addition of polymer in this regime systematically decreases the proportion of free particles, until all the particles are arrested. Concurrent with this transition, the elastic modulus and yield stress go through their corresponding maxima. However, over the range of polymer concentrations studied, the reentrant transition to weak gels is not captured by the particle dynamics but is instead accompanied by subtle changes in the microstructure of the arrested phase. We discuss two possible scenarios for this behavior in view of the strength of interparticle bonds.

Several routes have recently been introduced for microscale patterning of materials in three dimensions, including multilayer photolithography, nanotransfer printing, LiGA, a german acronym for Lithographie–Galvanoformung–Abformung (lithography-electroplating-molding), microstereolithography, and multiphoton polymerization. Each of these routes typically yields a solid structure, yet novel porous architectures such as those assembled from colloidal building blocks, are required for applications ranging from microfluidic filters and mixing elements to catalyst supports. Direct-write assembly of colloidal inks offers a pathway for creating the desired porous structures. However, their minimum dimensions must exceed 100mm to maintain continuous ink flow during deposition. To overcome this limitation, we harness the power of multiphoton direct laser writing to locally define the interactions between photoswitchable colloidal microspheres suspended in an organic solvent. Through this novel approach, we create porous-walled 3D structures includingmicroscale rectangular cavities that exhibit size-selective permeability. Direct laser writing of photoresponsive colloids consists of three basic steps (see Fig. 1a–c). First, we produce a dense colloidal suspension via sedimentation of a dilute solution of photoresponsive microspheres (Fig. 1a). Next, we locally induce colloidal gelation by photoswitching these microspheres from a repulsive to an attractive state (Fig. 1b). We achieve this transformation by rastering a high-intensity near-IR pulsed laser that alters the polymer brush chemistry, and hence colloidal stability, via a two-photon absorption process. Finally, we remove the unexposed microspheres through a simple rinsing step, leaving behind the desired 3D structure (Fig. 1c). The photoresponsive microspheres are formed by grafting a copolymer brush 55 nm thick (dry thickness) onto silica colloids with 927 nm diameter (Fig. 1d). The brush layer is grown using surface-initiated atom-transfer radical polymerization (ATRP), and is composed of methyl methacrylate (MMA) containing 20% spirobenzopyran (SP) pendant groups poly(SP-co-MMA), as shown in Figure 1e. In the SP-form, the microspheres are sterically stabilized when suspended in a nonpolar solvent such as toluene. However, upon irradiation with UV or high-intensity near-IR pulsed radiation (utilizing two-photon absorption), the SP side groups photoisomerize into the polar, zwitterionicmerocyanine (MC) form (see Fig. 1f). In toluene, the exposed colloids, now coated with the MC-rich form of the copolymer, undergo rapid flocculation when they come into contact via Brownian motion. It has been postulated that MC–MMA, MC–SP, and H-stacked MC–MC aggregates are all present, and contribute to the strong intermolecular and interparticle bonding in this system. While the reverse reaction back to the sterically stabilized SP-form can be induced upon exposure to visible wavelengths (lmax1⁄4 585 nm), the colloidal microspheres remain flocculated, and significant mechanical agitation is required to break up the particle network. Figure 2 depicts two simple structures fabricated using direct laser writing that illustrate the power of this approach. The ‘‘MRL’’ structure is composed of high-aspect-ratio walls (Fig. 2a–c), and the ‘‘mushrooms’’ are examples of complex 3D self-supporting features (Fig. 2d–i). Spatially defined C O M M U N IC A IO N

The behavior of colloidal gels under pressure-driven flow in square microchannels is quantified by microscopic particle image velocimetry (muPIV) and compared to predictions of available rheological models. The gels consist of hydrophobically modified silica microspheres (phi = 0.15-0.33) suspended in a refractive index-matched fluid along with fluorescent tracers to aid visualization. Measured velocity flow profiles show a transition from plug flow to more fluid-like behavior with increasing volumetric flow rate (Q) at all phi. This transition is not captured by theoretical predictions of the flow profile based on the Herschel-Bulkley model. Rather, a model that accounts for gel breakup into a suspension of clusters at elevated shear rates by assuming a finite viscosity at infinite shear is needed to accurately predict the flow behavior of colloidal gels at large Q.

The internal dynamics of fractal cluster gels of colloidal boehmite rods with aspect ratios r = 3.9 , 8.6, and 30.1, and colloidal polystyrene spheres ( r = 1 ) are reported. Increasing r decreases the minimum colloid volume fraction for gelation. The behavior of the dynamic structure factor of rod gels is consistent with the internal dynamics of a constrained Brownian fractal object. Colloidal boehmite gels display an abrupt transition from floppy to brittle dynamics at ϕ ∼ 10 −4 . Moreover, the fractal cluster size of rod gels is not the determinant of the relaxation time of density fluctuations as it is in spherical particle gels. Instead, the relative behavior of the magnitude and time scale of the constrained fluctuations suggests that the fractal rod network is viscously coupled only on local, rather than cluster, scales. We hypothesize that noncentral forces between the anisometric particles are responsible for this anomalous behavior.

The transient structural evolution of polystyrene colloidal gels with fractal structure is quantified during start-up of steady shear flow by time-resolved small-angle light scattering and rheometry. Three distinct regimes are identified in the velocity-gradient plane: structural orientation, network breakup, and cluster densification. Structural anisotropy in the first regime is a universal function of applied strain. Flow cessation in this regime shows a lack of structural relaxation for Pe⪡1, where Pe is the Peclet number. In the second regime, the anisotropy attains a maximum value before monotonically decreasing. The volume fraction dependence of the critical strain for maximum anisotropy follows the scaling: 1+0.6γc,r∼ϕ(1−x)(3−D). Here x and D are the backbone and cluster fractal dimensions, respectively. This scaling agrees with the simple model of a gel network that ruptures after the cluster backbone is extended affinely to its full length. Rheological measurements demonstrate that the maximum an...

The development of model materials and image processing methods to directly visualize and quantify colloidal rod assembly by means of confocal laser scanning microscopy (CLSM) is reported. Monodisperse fluorescent colloidal rods are prepared by the uniaxial extensional deformation of sterically stabilized microspheres at elevated temperatures. The particles are stably dispersed in refractive index matching mixed organic solvents for CLSM. An image processing algorithm is developed to detect rod backbones and extract particle centroids and orientation angles from the CLSM image volumes. By means of these methods we quantify the distribution of rod orientation angles in self-assembled structures of rods formed by sedimentation. We find the observations to be consistent with aspect-ratio-dependent jamming and orientational order/disorder transition in the rod sediments.

Silver monoliths with interconnected hierarchical pore networks and three-dimensional (3D) bicontinuous morphology are synthesized from a colloidal bicontinuous interfacially jammed emulsion gel (bijel) via reduction of silver ions within a nanoporous cross-linked polymer template. The pore sizes may be tuned independently and range from tens of nanometers to over a hundred micrometers. The method is straightforward as well as flexible and can pave the way to a host of hierarchical materials for current technologies.

We investigate the nonlinear rheology of dilute, depletion-induced colloidal gels and report that these systems yield via a two-step process. We propose the two yield points to be associated with interparticle bond rotation and bond breakage, respectively. These distinct yielding mechanisms lead to remarkable creep profiles at intermediate values of the applied stress, highlighted by an anisotropic shear-induced strengthening and flow arrest at very large accumulated strains (γ∼ 80). The possible microstructural origins of this behavior are discussed.

The transient microstructural response of dilute, depletion-induced colloidal gels to shear deformation is probed using a custom-built cone-and-plate shear cell mounted on a fast scanning confocal microscope. We directly unveil an important mode of nonlinear gel yielding, which marks the transition from an immobilized gel network to disconnected flowing clusters at large values of the accumulated strain ($\ensuremath{\gamma}\ensuremath{\sim}$ 0.7). The transition is mediated by a cascade of local rupture events that initiate at the sample's outer edges and propagate inward, resulting in transient dynamic and microstructural heterogeneities in the gel network. Based on the findings, we propose solvent-induced drag as the primary cause of nonlinear rupture, and discuss the coupling between the microstructural evolution and transient rheology in this system.

We investigate the structure and dynamics of biphasic colloidal mixtures composed of coexisting attractive and repulsive microspheres by confocal microscopy. Attractive gels formed in the presence of repulsive microspheres are more spatially homogeneous and, on average, are both more locally tenuous and have fewer large voids than their unary counterparts. The repulsive microspheres within these mixtures display heterogeneous dynamics, with some species exhibiting freely diffusive Brownian motion while others are trapped within the gel network during aggregation. Colloidal gels, which consist of attractive colloidal particles that stick together to form a system-spanning interconnected network, are widely used in ceramics processing 1, inks 2, catalyst supports 3, and membranes 4. To improve component performance, new strategies for designing concentrated gels with controlled structural and rheological properties are desired. The fundamental ties between local gel structure and bulk properties have received considerable attention, both theoretically and experimentally 5–7. The overall connectivity of the gel network provides finite bending and stretching elasticity 8, contributing to the system’s zero-shear elastic modulus. To date, most experimental studies have focused on colloidal gels composed of a single species with uniform interparticle interactions, in which the primary control parameters are the colloid volume fraction and the strength of the attractive interaction energy U 7,9,10. Surprisingly, mixtures in which interparticle interactions are independently tuned have not been systematically studied, even though such systems may open new routes for tailoring gel properties even when both and U remain fixed. In this Rapid Communication, we investigate the gel structure and suspension dynamics in biphasic colloidal mixtures using confocal laser scanning microscopy CLSM .W e selectively tune the interparticle interactions of attractive and repulsive microspheres, and create binary mixtures of these two species. In these mixtures, the attractive particles stick to one another yielding a gel network, while the repulsive particles are stable, sticking neither to the attractive particles nor to one another. This system differs from most colloidal dispersions in which attractive interactions are uniformly controlled via the addition of depletants 7,9,10, salt 11 ,o r acid or base to solution 2. By imaging and quantifying the two particle populations independently, we elucidate the role of the excluded volume of repulsive particles in mediating the local and the long-range structure of the gel phase as well as the effect of the arrested gel network on the dynamics of the repulsive particles. We use three separate particle batches of silica microspheres, all with similar diameters. Their average diameters, assessed from scanning electron microscopy photomicrographs, are 1 = 0.68 0.04 m, 2 = 0.72 0.02 m, and 3 = 0.62 0.03 m. The first batch diameter 1 is labeled with rhodamine dye, and rendered “attractive” by partially coating their surface with hydrophobic n-octyldimethylchlorosilane following a standard silanecoupling procedure 12. These particles rapidly flocculate when suspended in an index-matched polar mixture of H2O-dimethylsulfoxide. The second batch diameter 2 is labeled with fluorescein dye; these uncoated particles are “repulsive” due to their negatively charged surface 12. The third batch diameter 3 is synthesized without dye; however, their surfaces are rendered hydrophobic and thus they

The net charge of a colloidal particle was controlled using light and a new photocleavable self-assembled monolayer (SAM). The SAM contained a terminal ammonium group and a centrally located carboxylic acid group that was masked with an ortho-nitrobenzyl functionality. Once exposed to UV light, the 2-nitrobenzyl group was cleaved, therefore transforming the colloidal particle from a net positive (silica-SAM-NH3+) to a net negative (silica-SAM-COO-) charge. By varying the UV exposure time, their zeta potential could be tailored between +26 and -60 mV at neutral pH. To demonstrate a photoinduced gel-to-fluid phase transition, a binary colloidal suspension composed of silica-SAM-NH3+ and negatively charged, rhodamine-labeled silica particles was mixed to form a gel. Exposure to UV light rendered all of the particles negative and therefore converted the system into a colloidal fluid that settles to form a dense sediment.

The fractal structure of clusters formed by diffusion-limited aggregation of rodlike particles is characterized over three decades of the scattering vector q, and displays an unexpected dependence on the aspect ratio of the constituent monomers. Monte Carlo simulations of aggregating Brownian rods corroborate the experimental finding that the measured fractal dimension is an increasing function of the monomer aspect ratio. Moreover, increasing the rod aspect ratio eliminates the structural distinction between diffusion- and reaction-limited cluster aggregation that is observed for spheres.

Bijels are non-equilibrium solid-stabilized emulsions with bicontinuous arrangement of the constituent fluid phases. These multiphase materials spontaneously form through arrested spinodal decomposition in mixtures of partially miscible liquids and neutrally wetting colloids. Here, we present a new solid-stabilized emulsion with an overall bicontinuous morphology similar to a bijel, but with one continuous phase containing a network of colloid-bridged droplets. This dual morphology is the result of combined spinodal decomposition and nucleation and growth in a binary liquid mixture containing colloidal particles with off-neutral wetting properties and partial affinity for one liquid phase. The rheology of these systems, which we call bridged bijels, is nearly identical to their simple bijel counterparts, with a unique exponential dependence of the zero-shear elastic modulus on the colloid volume fraction. However, partitioning of the colloids between the spinodal surface and the fluid domains delays the onset of structural arrest, providing access to domain sizes much larger than available in simple bijels without loss of mechanical stability. This ability greatly expands the potential technological applications of these unique materials. In addition, our findings reveal new strategies for tuning the rheology of bijels and outline new directions for future fundamental research on this unique class of soft materials.

The shear-induced microstructure in dilute colloid–polymer mixtures, where the presence of polymer induces a tuneable attractive interaction between the colloids, is investigated using quantitative confocal microscopy, over a wide parameter space including the shear rate, the polymer concentration, and two different polymer molecular weights corresponding to polymer/colloid size ratios of approximately 0.02 and 0.05. Overall, the imposition of low shear rates radically transforms the relatively uniform quiescent structure into one marked by long-range heterogeneities and pronounced segregation of dense clusters and voids. Increasing the rate of deformation effects a consistent decrease in the average cluster size and a gradual transition towards a more homogeneous structure through the redistribution of voids. Interestingly, at high shear rates, the suspension microstructure for the large molecular weight polymer is nearly insensitive to the polymer concentration, and primarily determined by the shear rate alone. The prominent microstructural features of these shear-induced transformations are quantified in detail and discussed in light of the competition between interparticle attraction and microscopic shear forces.

Polystyrene colloids exhibit unique bimodal dynamics at the air–water interface in the presence of PEG polymers. We attribute this to agglomeration of the polymer into polymer-rich and polymer-sparse domains at the interface. Colloids adsorbed to polymer-depleted regions retain mobility and self-assemble into two-dimensional crystals with square symmetry, which is ascribed to an undulated contact line on the particles and the droplet's negative Gaussian curvature. To our knowledge, this is the first example of thermodynamically favoured two-dimensional square crystals formed by means of interfacial colloid assembly.

The time-resolved microstructural response of dilute, depletion-induced colloidal gels prepared in a density and refractive index matched solvent, to nonlinear shear deformation was investigated in 3D by fast scanning confocal microscopy in a custom-built cone-and-plate shear cell. Two sets of experiments were performed by manipulating the connectivity of the gel network with the stationary plate, thereby changing the flow boundary conditions. The gel structure evolves from its quiescent state via local rearrangement, rupture, and densification, first to a highly anisotropic network oriented near the extensional component of the shear flow field, and eventually to a mixture comprised of dense clusters and large voids. The transitions between these stages are highly sensitive to the boundary condition at the stationary plate. Our findings indirectly support the notion of soft pivot points along the backbone of dilute colloidal gels with centrosymmetric interactions, and will have important implications for the nonlinear rheology of colloidal gels and other structured fluids.