Quantifying electron transport in aggregated colloidal suspensions in the strong flow regime.

Electron transport in complex fluids, biology, and soft matter is a valuable characteristic in processes ranging from redox reactions to electrochemical energy storage. These processes often employ conductor-insulator composites in which electron transport properties are fundamentally linked to the microstructure and dynamics of the conductive phase. While microstructure and dynamics are well recognized as key determinants of the electrical properties, a unified description of their effect has yet to be determined, especially under flowing conditions. In this work, the conductivity and shear viscosity are measured for conductive colloidal suspensions to build a unified description by exploiting both recent quantification of the effect of flow-induced dynamics on electron transport and well-established relationships between electrical properties, microstructure, and flow. These model suspensions consist of conductive carbon black (CB) particles dispersed in fluids of varying viscosities and dielectric constants. In a stable, well-characterized shear rate regime where all suspensions undergo self-similar agglomerate breakup, competing relationships between conductivity and shear rate were observed. To account for the role of variable agglomerate size, equivalent microstructural states were identified using a dimensionless fluid Mason number, [Formula: see text], which allowed for isolation of the role of dynamics on the flow-induced electron transport rate. At equivalent microstructural states, shear-enhanced particle-particle collisions are found to dominate the electron transport rate. This work rationalizes seemingly contradictory experimental observations in literature concerning the shear-dependent electrical properties of CB suspensions and can be extended to other flowing composite systems.

Quantifying the hydrodynamic contribution to electrical transport in non-Brownian suspensions.

Electrical transport in semiconducting and metallic particle suspensions is an enabling feature of emerging grid-scale battery technologies. Although the physics of the transport process plays a key role in these technologies, no universal framework has yet emerged. Here, we examine the important contribution of shear flow to the electrical transport of non-Brownian suspensions. We find that these suspensions exhibit a strong dependence of the transport rate on the particle volume fraction and applied shear rate, which enables the conductivity to be dynamically changed by over 107 decades based on the applied shear rate. We combine experiments and simulations to conclude that the transport process relies on a combination of charge and particle diffusion with a rate that can be predicted using a quantitative physical model that incorporates the self-diffusion of the particles.

In situ investigation of the rheological and dielectric properties of a cross-linking carbon nanotube-thermosetting epoxy.

Radio-frequency (RF) heating of thermosetting epoxies is an agile method to decouple the extrudability of epoxy resins from their buildability for additive manufacturing. Through this method, the resin is extruded in the liquid state at the early stages of curing. Then, an RF applicator induces a rapid and uniform increase in temperature of the resin, accelerating the solidification of the printed feature. Understanding the evolution of the resin's RF heating response as it cures is therefore critical in meeting the demands of additive manufacturing. In this work, we show that the high-frequency dielectric loss, determined using in situ rheo-dielectric measurements, of both neat and carbon nanotube (CNT) filled resins is correlated to the heating response at different temperatures throughout curing. Furthermore, we show that the presence of CNTs within the resin augments the heating response and that their dispersion quality is critical to achieving rapid heating rates during the cure.

Extracting microscopic insight from transient dielectric measurements during large amplitude oscillatory shear.

Probing the transient microstructure of soft matter far from equilibrium is an ongoing challenge to understanding material processing. In this work, we investigate inverse worm-like micelles undergoing large amplitude oscillatory shear using time-resolved dielectric spectroscopy. By controlling the Weissenburg number, we compare the non-linear microstructure response of branched and unbranched worm-like micelles and isolate distinct elastic effects that manifest near flow reversal. We validate our dielectric measurements with small angle neutron scattering and employ sequence of physical processes to disentangle the elastic and viscous contributions of the stress.

Branching and alignment in reverse worm-like micelles studied with simultaneous dielectric spectroscopy and RheoSANS.

Topology and branching play an important but poorly understood role in controlling the mechanical and flow properties of worm-like micelles (WLMs). To address the challenge of characterizing branching during flow of WLMs, dielectric spectroscopy, rheology, and small-angle neutron scattering (dielectric RheoSANS) experiments are performed simultaneously to measure the concurrent evolution of conductivity, permittivity, stress, and segmental anisotropy of reverse WLMs under steady-shear flow. Reverse WLMs are microemulsions comprised of the phospholipid surfactant lecithin dispersed in oil with water solubilized in the micelle core. Their electrical properties are independently sensitive to the WLM topology and dynamics. To isolate the effects of branching, dielectric RheoSANS is performed on WLMs in n-decane, which show fast breakage times and exhibit a continuous branching transition for water-to-surfactant ratios above the corresponding maximum in zero-shear viscosity. The unbranched WLMs in n-decane exhibit only subtle decreases in their electrical properties under flow that are driven by chain alignment and structural anisotropy in the plane perpendicular to the electric field and incident neutron beam. These results are in qualitative agreement with additional measurements on a purely linear WLM system in cyclohexane despite differences in breakage kinetics and a stronger tendency for the latter to shear band. In contrast, the branched micelles in n-decane (higher water content) undergo non-monotonic changes in permittivity and more pronounced decreases in conductivity under flow. The combined steady-shear electrical and microstructural measurements are capable, for the first time, of resolving branch breaking at low shear rates prior to alignment-driven anisotropy at higher shear rates.

Clustering and Percolation in Suspensions of Carbon Black.

High-structured carbon fillers are ubiquitous as the conductive additive comprising suspension-based electrochemical energy storage technologies. Carbon black networks provide the necessary electrical conductivity as well as mechanical percolation in the form of a yield stress. Despite their critical role in determining system performance, a full mechanistic understanding of the relationship between the electrical transport characteristics of the percolated, conductive networks of carbon black, and the rheological properties is lacking, which hinders the rational design and optimization of flowable electrodes and the processing of electrolytes for batteries. Here, we report on the microstructural origin of the rheological and electrical properties of two commonly used conductive additives in neat propylene carbonate. From quiescent mechanical and structural studies, we find that the gelation of these carbon black suspensions is best described by the dynamic arrest of a clustered fluid phase. In contrast, the temperature and frequency dependence of the ac conductivity near this transition shows that mesoscale charge transport is determined by hopping between localized states that does not require a stress-bearing network. This unique combination of microstructural characterization with rheological and electrical measurements enables testing prevailing theories of the connection between electrical and mechanical percolation as well as improving conductive additives to enhance electrochemical performance.

Designing Two-Dimensional Protein Arrays through Fusion of Multimers and Interface Mutations.

We have combined fusion of oligomers with cyclic symmetry and alanine substitutions to eliminate clashes and produce proteins that self-assemble into 2-D arrays upon addition of calcium ions. Using TEM, AFM, small-angle X-ray scattering, and fluorescence microscopy, we show that the designed lattices which are 5 nm high with p3 space group symmetry and 7.25 nm periodicity self-assemble into structures that can exceed 100 µm in characteristic length. The versatile strategy, experimental approach, and hexagonal arrays described herein should prove valuable for the engineering of functional nanostructured materials in 2-D.

Quantitative high-throughput measurement of bulk mechanical properties using commonly available equipment.

Machine learning approaches have introduced an urgent need for large datasets of materials properties. However, for mechanical properties, current high-throughput measurement methods typically require complex robotic instrumentation, with enormous capital costs that are inaccessible to most experimentalists. A quantitative high-throughput method using only common lab equipment and consumables with simple fabrication steps is long desired. Here, we present such a technique that can measure bulk mechanical properties in soft materials with a common laboratory centrifuge, multiwell plates, and microparticles. By applying a homogeneous force on the particles embedded inside samples in the multiwell plate using centrifugation, we show that our technique measures the fracture stress of gels with similar accuracy to a rheometer. However, our method has a throughput on the order of 103 samples per run and is generalizable to virtually all soft material systems. We hope that our method can expedite materials discovery and potentially inspire the future development of additional high-throughput characterization methods.

Microscopic Dynamics of Inverse Wormlike Micelles Probed Using X-ray Photon Correlation Spectroscopy.

Wormlike micelles (WLMs) are ubiquitous viscoelastic modifiers that share properties with polymer solutions. While their macroscopic rheology is well-understood, their microscopic dynamics remain difficult to measure because they span a large range of time and length scales. In this work, we demonstrate the use of X-ray photon correlation spectroscopy to interrogate the segmental dynamics of inverse WLM solutions swollen with a rubidium chloride solution. We observe a diffusive scaling of the dynamics and extract a temperature-dependent diffusion coefficient, which we associate with the thermal interactions of the slow segmental dynamics near entanglement points. We probe this relaxation process across the unbranched to branched topological transition and find no microstructural evidence of branch formation in the slow mode. Instead, we observe that the dynamics become more homogeneous and prominent as the temperature is reduced and water content increases.

Fast Dynamics of Inverse Wormlike Micelles Probed Using Mechanical and Dielectric Spectroscopy.

The rheology of wormlike micelle (WLM) solutions is tunable by engineering the micellar structure and topology. While much is known about how microscopic properties influence the rheological characteristics, questions remain regarding the quantification of fast relaxation processes, including Rouse and rotational modes. These fast processes are challenging to access using mechanical spectroscopy as bending modes dominate high-frequency mechanical measurements. In this work, we demonstrate the use of dielectric spectroscopy (DES) to directly interrogate these fast relaxation modes in solutions containing reverse WLMs. These consist of lecithin solutions in n-decane swollen with water. We develop an equivalent circuit model that separates the fast spectral features from the low-frequency processes and show that this relaxation feature is consistent with a combination of high-frequency Rouse and rotational modes. Further, we show that the low-frequency response is not determined by polymer dynamics alone. These findings demonstrate the potential of DES measurements to describe WLM behavior and pave the way toward in situ measurements under steady and transient shear flow.

Control over electroless plating of silver on silica nanoparticles with sodium citrate.

We describe the use of citrate to control the electroless plating of silver metal on silica nanoparticles. We find that the incorporation of relatively small amounts of citrate during the reduction of the Tollens' reagent in the presence of sensitized silica nanoparticles induces a continuous transition from conformal to raspberry particle coatings. This transition is dependent on both the citrate concentration and the silver precursor concentration. We characterize this transition using electron microscopy and spectroscopy and use these results to confirm citrate's ability to cap and restrict silver growth. We compliment these structural measurements with in-situ quartz crystal microbalance experiments to quantify citrate's role as a complexing agent to slow silver reduction kinetics. These results confirm citrate's dual role in controlling the morphology of silver deposits produced in this work.

A simple sheath-flow microfluidic device for micro/nanomanufacturing: fabrication of hydrodynamically shaped polymer fibers.

A simple sheath flow microfluidic device is used to fabricate polymer micro/nanofibers that have precisely controlled shapes and sizes. Poly(methylmethacrylate) (PMMA) was used as the model polymer for these experiments. The sheath-flow device uses straight diagonal and chevron-shaped grooves integrated in the top and bottom walls of the flow channel to move sheath fluid completely around the polymer stream. Portions of the sheath stream are deflected in such a way as to define the cross-sectional shape of the polymer core. The flow-rate ratio between the sheath and core solution determines the fiber diameter. Round PMMA fibers with a diameter as small as 300 nm and flattened fibers with a submicron thickness are demonstrated.

A strain-controlled RheoSANS instrument for the measurement of the microstructural, electrical, and mechanical properties of soft materials.

In situ measurements are an increasingly important tool to inform the complex relationship between nanoscale properties and macroscopic material measurements. Knowledge of these phenomena can be used to develop new materials to meet the performance demands of next generation technologies. Conductive complex fluids have emerged as an area of research where the electrical and mechanical properties are key design parameters. To study the relationship between microstructure, conductivity, and rheology, we have developed a small angle neutron scattering (SANS) compatible Couette rheological geometry capable of making impedance spectroscopy measurements under continuous shear. We have also mounted this geometry on a commercial strain controlled rheometer with a modified forced convection oven. In this manuscript, we introduce the simultaneous measurement of impedance spectroscopy, rheological properties and SANS data. We describe the validation of this dielectric RheoSANS instrument and demonstrate its operation using two systems-an ion gel comprising Pluronic® surfactant and ionic liquid, ethyl-ammonium nitrate, and poly(3-hexylthiophene) organogel prepared in a mixture of hexadecane and dichlorobenzene. In both systems, we use this new measurement capability to study the microstructural state of these materials under two different protocols. By monitoring their dielectric rheology at the same time as the SANS measurement, we demonstrate the capacity to directly probe structure-property relationships inherent to the macroscopic material response.

Dielectric RheoSANS - Simultaneous Interrogation of Impedance, Rheology and Small Angle Neutron Scattering of Complex Fluids.

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and microstructural properties of complex fluids is presented. The instrument consists of a Couette geometry contained within a modified forced convection oven mounted on a commercial rheometer. This instrument is available for use on the small angle neutron scattering (SANS) beamlines at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The Couette geometry is machined to be transparent to neutrons and provides for measurement of the electrical properties and microstructural properties of a sample confined between titanium cylinders while the sample undergoes arbitrary deformation. Synchronization of these measurements is enabled through the use of a customizable program that monitors and controls the execution of predetermined experimental protocols. Described here is a protocol to perform a flow sweep experiment where the shear rate is logarithmically stepped from a maximum value to a minimum value holding at each step for a specified period of time while frequency dependent dielectric measurements are made. Representative results are shown from a sample consisting of a gel composed of carbon black aggregates dispersed in propylene carbonate. As the gel undergoes steady shear, the carbon black network is mechanically deformed, which causes an initial decrease in conductivity associated with the breaking of bonds comprising the carbon black network. However, at higher shear rates, the conductivity recovers associated with the onset of shear thickening. Overall, these results demonstrate the utility of the simultaneous measurement of the rheo-electro-microstructural properties of these suspensions using the dielectric RheoSANS geometry.

Mixed Ionic/Electronic Conducting Surface Layers Adsorbed on Colloidal Silica for Flow Battery Applications.

Slurry based electrodes have shown promise as an energy dense and scalable storage technology for electrochemical flow batteries. Key to their efficient operation is the use of a conductive additive which allows for volumetric charging and discharging of the electrochemically active species contained within the electrodes. Carbon black is commonly used for this purpose due to the relatively low concentrations needed to maintain electrical percolation. While carbon black supplies the desirable electrical properties for the application, it contributes detrimentally to the rheology characteristics of these concentrated suspensions. In this work, we develop a synthesis protocol to produce inorganic oxide particles with electrostatically adsorbed poly(3,4-ethylenedioxithiophene):polystyrenesulfonate ( PEDOT: PSS). Using a combination of small angle neutron scattering (SANS), electron microscopy, and thin-film conductivity, we show that the synthesis scheme provides a flexible platform to form conductive PEDOT: PSS-SiO2 nanoparticle dispersions. Based on these measurements, we demonstrate that these particles are stable when dispersed in propylene carbonate. Using a combination of rheology and dielectric spectroscopy, we show that these stable dispersions facilitate electrical percolation at concentrations below their mechanical percolation threshold, and this percolation is maintained under flow. These results demonstrate the potential for strategies which seek to decouple mechanical and electrical percolation to allow for the development of higher performance conductive additives for slurry based flow batteries.

Correlating structure and photocurrent for composite semiconducting nanoparticles with contrast variation small-angle neutron scattering and photoconductive atomic force microscopy.

Aqueous dispersions of semiconducting nanoparticles have shown promise as a robust and scalable platform for the production of efficient polymer/fullerene active layers in organic photovoltaic applications. Semiconducting nanoparticles are a composite of both an n-type and p-type semiconductor contained within a single nanoparticle. In order to realize efficient organic solar cells from these materials, there is a need to understand how the size and internal distribution of materials within each nanoparticle contributes to photocurrent generation in a nanoparticle-derived device. Therefore, characterizing the internal distribution of conjugated polymer and fullerene within the dispersion is the first step to improving performance. To date, study of polymer/fullerene structure within these nanoparticles has been limited to microscopy techniques of deposited nanoparticles. In this work, we use contrast variation with small-angle neutron scattering to determine the internal distribution of poly(3-hexylthiophene) and [6,6]phenyl-C61-butyric acid methyl ester inside the composite nanoparticles as a function of formulation while in dispersion. On the basis of these measurements, we connect the formulation of these nanoparticles with their internal structure. Using electrostatic deposited monolayers of these nanoparticles, we characterize intrinsic charge generation using photoconductive atomic force microscopy and correlate this with structures determined from small-angle neutron scattering measurements. These techniques combined show that the best performing composite nanoparticles are those that have a uniform distribution of conjugated polymer and fullerene throughout the nanoparticle volume such that electrons and holes are easily transported out of the particle.

Aqueous dispersions of colloidal poly(3-hexylthiophene) gel particles with high internal porosity.

This work outlines the development of nano-porous, sub-micron poly(3-hexylthiophene) (P3HT) gel particles as solution-processable inks for applications in polymer solar cells. These dispersions are produced by emulsifying bulk P3HT organogels into water containing surfactant. The optical characteristics and stability of the resulting gel particles are assessed and their structure characterized. The P3HT within the gel particles is shown to retain its crystallinity with no evidence of doping. The gel particles are shown to be stable against aggregation due to the presence of surfactant at the oil/water interface. The fracture of the gel network during emulsification produces a bimodal distribution of particles that increase in size with increasing P3HT concentration in the 'parent' organogel. Small Angle Neutron Scattering measurements show that the particles maintain the structure of the bulk gels with high specific surface area. Spray-coating the gel particle dispersions produces uniform thin-films, which have been used to fabricate polymer/fullerene solar cells with a fully spray-coated active layer.