Programming Semiconductor Nanowire Composition with Sub-100 nm Resolution via the Geode Process.

We demonstrate the vapor-liquid-solid growth of single-crystalline i-Si, i-Si/n-Si, and SixGe1-x/SiyGe1-y nanowires via the Geode process. By enabling nanowire growth on the large internal surface area of a microcapsule powder, the Geode process improves the scalability of semiconductor nanowire manufacturing while maintaining nanoscale programmability. Here, we show that heat and mass transport limitations introduced by the microcapsule wall are negligible, enabling the same degree of compositional control for nanowires grown inside microcapsules and on conventional flat substrates. Efficient heat and mass transport also minimize the structural variations of nanowires grown in microcapsules with different diameters and wall thicknesses. Nanowires containing at least 16 segments and segment lengths below 75 nm are demonstrated.

Liquid Repellence of Phobic Fiber Networks.

The wetting behavior of fiber networks, which are central to many research and industrial applications, can be difficult to predict accurately owing to their complex, heterogeneous structure. The cylindrical pore model, widely used to interpret and predict the forced wetting of hydrophobic porous materials, often does not yield correct results when working with fibrous networks like paper substrates and non-woven fabrics. This is because these materials exhibit variation in pore size, fiber length, and fiber diameter, as well as a reentrant pore geometry. Quantitative prediction of the critical wetting resistance of hydrophobized papers to arbitrary entrant liquids requires a more sophisticated analytical approach that considers this unique fibrous structure and the effect of stochastic variations within the pore matrix. In this work, we directly measure the critical breakthrough pressure for different porous substrates across various wetting entrant liquids. To isolate the effects of the structure and stochastics on critical wetting behavior of fibrous networks, we analyze additional materials strategically chosen for their subsets of structural features. Ultimately, we formulate a method that demonstrates physical reasonableness, numerical accuracy, and the ability to elucidate the effects of pore size, pore size distribution, fiber diameter, fiber diameter distribution, surface wettability, and liquid surface tension on critical breakthrough pressure of liquids through hydrophobic fibrous networks.

Thickened Liquids Using Pureed Foods for Children with Dysphagia: IDDSI and Rheology Measurements.

Children with dysphagia, or swallowing disorder, are at an increased risk for developing respiratory compromise, failure to thrive, and aversion. Thickened liquids can be recommended for children with dysphagia, if shown to be effective on instrumental examination and if strategies/interventions with thin liquids are not successful. Thickened liquids have many benefits, including creating a more cohesive bolus, slowing oropharyngeal transit time, and reducing aspiration. However, preparing thickened liquids with commercially available thickeners can result in poor compliance due to concerns regarding taste, texture, accessibility, cost, thickness variability, and potential negative impact of these substances on a child's immature digestive tract. The purpose of this study was to determine if liquids could be successfully thickened with widely available, commercial pureed foods, and to assess how these mixtures compare to starch and gum based thickening agents. The International Dysphagia Diet Standardisation Initiative (IDDSI) flow test was performed for each sample of puree thickened liquids, gum based thickened water, and cornstarch based thickened water. In addition, rheology testing was performed on each category of the samples to measure viscosity at various shear rates and temperatures, and to assess the presence of yield stress. Results revealed that liquids thickened with smooth textured purees were comparable to commercial starch and gum based thickeners, and may be offered as a viable alternative.

Synergistic Reinforcement of Composite Hydrogels with Nanofiber Mixtures of Cellulose Nanocrystals and Chitin Nanofibers.

Simultaneous incorporation of cellulose nanocrystals (CNCs) and chitin nanofibers (ChNFs) into a polyvinyl alcohol (PVA) matrix opens possibilities for customization of more environmentally friendly composite materials. When used in tricomponent composite hydrogels, the opposite surface charges on CNCs and ChNFs lead to the construction of beneficial nanofiber structures. In this work, composite hydrogels containing CNCs, ChNFs, or their mixtures are produced using cyclic freeze-thaw (FT) treatments. When considering different compositions and FT cycling, tricomponent composite hydrogels containing a specific ratio of CNCs/ChNFs are shown to have promising mechanical performance in comparison to other samples. These results together with results from water absorption, rheological, and light scattering studies suggest that the CNC/ChNF structures produced property improvement by concurrently accessing the stronger interfacial interactions between CNCs and PVA and the longer lengths of the ChNFs for load transfer. Overall, these results provide insight into using electrostatically driven nanofiber structures in nanocomposites.

Rheology finds distinct glass and jamming transitions in emulsions.

We study the rheology of monodisperse and bidisperse emulsions with various droplet sizes (1-2 µm diameter). Above a critical volume fraction φc, these systems exhibit solid-like behavior and a yield stress can be detected. Previous experiments suggest that for small thermal particles, rheology will see a glass transition at φc = φg ≈ 0.58; for large athermal systems, rheology will see a jamming transition at φc = φJ ≈ 0.64. However, simulations point out that at the crossover of thermal and athermal regimes, the glass and jamming transitions may both be observed in the same sample. Here we conduct an experiment by shearing four oil-in-water emulsions with a rheometer. We observe both a glass and a jamming transition for our smaller diameter droplets, and only a jamming transition for our larger diameter droplets. The bidisperse sample behaves similarly to the small droplet sample, with two transitions observed. Our rheology data are well-fit by both the Herschel-Bulkley model and the three component model. Based on the fitting parameters, our raw rheological data would not collapse onto a master curve. Our results show that liquid-solid transitions in dispersions are not universal, but depend on particle size.

Solution-Based 3D Printing of Polymers of Intrinsic Microporosity.

Current additive manufacturing methods have significant limitations in the classes of compatible polymers. Many polymers of significant technological interest cannot currently be 3D printed. Here, a generalizable method for 3D printing of viscous tenary polymer solutions (polymer/solvent/nonsolvent) is applied to both "intrinsically porous" (a polymer of intrinsic microporosity, PIM-1) and "intrinsically nonporous" (cellulose acetate) polymers. Successful ternary ink formulations require balancing of solution thermodynamics (phase separation), mass transfer (solvent evaporation), and rheology. As a demonstration, a microporous polymer (PIM-1) incompatible with current additive manufacturing technologies is 3D printed into a high-efficiency mass transfer contactor exhibiting hierarchical porosity ranging from sub-nanometer to millimeter pores. Short contactors (1.27 cm) can fully purify (<1 ppm) toluene vapor (1000 ppm) in N2 gas for 1.7 h, which is six times longer than PIM-1 in traditional structures, and more than 4000 times the residence time of gas in the contactor. This solution-based additive manufacturing approach greatly extends the range of 3D-printable materials.

Underwater Oil Droplet Splitting on a Patterned Template.

Underwater oil droplets stretched and pinned by dual-dot oleophilic patterns on a superoleophobic substrate have been split into two nearly equal-volume daughter droplets using an underwater superoleophobic blade at substantially lower cutting speeds than reported in previous studies. A "liquid exchange model" based on Laplace pressure-driven liquid transport has been proposed to explain the mechanism of the underwater droplet split process. The dependence of droplet geometrical shape (curvature) and liquid properties (surface tension, viscosity) on the critical cutting speed that allows equal-volume split was investigated. Results demonstrate that critical cutting speed increases with increased curvature and surface tension of the split droplet, and decreases with increased droplet viscosity, which agrees with the proposed model. The ability to reproducibly split a single bulk oil droplet into daughter droplets with nearly equal volume facilitates the development of new functions for underwater microreactors.

Postprandial lymphatic pump function after a high-fat meal: a characterization of contractility, flow, and viscosity.

Dietary lipids are transported from the intestine through contractile lymphatics. Chronic lipid loads can adversely affect lymphatic function. However, the acute lymphatic pump response in the mesentery to a postprandial lipid meal has gone unexplored. In this study, we used the rat mesenteric collecting vessel as an in vivo model to quantify the effect of lipoproteins on vessel function. Lipid load was continuously monitored by using the intensity of a fluorescent fatty-acid analog, which we infused along with a fat emulsion through a duodenal cannula. The vessel contractility was simultaneously quantified. We demonstrated for the first time that collecting lymphatic vessels respond to an acute lipid load by reducing pump function. High lipid levels decreased contraction frequency and amplitude. We also showed a strong tonic response through a reduction in the end-diastolic and systolic diameters. We further characterized the changes in flow rate and viscosity and showed that both increase postprandially. In addition, shear-mediated Ca(2+) signaling in lymphatic endothelial cells differed when cultured with lipoproteins. Together these results show that the in vivo response could be both shear and lipid mediated and provide the first evidence that high postprandial lipid has an immediate negative effect on lymphatic function even in the acute setting.

Patterning of superhydrophobic paper to control the mobility of micro-liter drops for two-dimensional lab-on-paper applications.

Superhydrophobic paper substrates were patterned with high surface energy black ink using commercially available desktop printing technology. The shape and size of the ink islands were designed to control the adhesion forces on water drops in two directions, parallel ('drag-adhesion') and perpendicular ('extensional-adhesion') to the substrate. Experimental data on the adhesion forces shows good agreement with classical models for 'drag' (Furmidge equation) and 'extensional' adhesion (modified Dupré equation). The tunability of the two adhesion forces was used to implement four basic unit operations for the manipulation of liquid drops on the paper substrates: storage, transfer, mixing and sampling. By combining these basic functionalities it is possible to design simple two-dimensional lab-on-paper (LOP) devices. In our 2D LOP prototype, liquid droplets adhere to the porous substrate, rather than absorbing into the paper; as a result, liquid droplets remain accessible for further quantitative testing and analysis, after performing simple qualitative on-chip testing. In addition, the use of commercially available desktop printers and word processing software to generate ink patterns enable end users to design LOP devices for specific applications.

Three-dimensional in situ temperature measurement in microsystems using Brownian motion of nanoparticles.

We report an in situ method for three-dimensionally resolved temperature measurement in microsystems. The temperature of the surrounding fluid is correlated from Brownian diffusion of suspended nanoparticles. We use video-microscopy in combination with image analysis software to selectively track nanoparticles in the focal plane. This method is superior with regards to reproducibility and reduced systematic errors since measuring Brownian diffusivity does not rely on fluorescence intensity or lifetime of fluorophores. The efficacy of the method is demonstrated by measuring spatial temperature profiles in various microfluidic devices that generate temperature gradients and by comparing these results with numerical simulations. We show that the method is accurate and can be used to extract spatial temperature variations in three dimensions. Compared to conventional methods that require expensive multiphoton optical sectioning setups, this technique is simple and inexpensive. In addition, we demonstrate the capability of this method as an in situ tool for simultaneously observing live cells under the microscope and monitoring the local temperature of the cell medium without biochemical interference, which is crucial for quantitative studies of cells in microfluidic devices.

Physical aging and phase behavior of multiresponsive microgel colloidal dispersions.

Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following approximately 1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pK(a), the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pK(a), repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.

Humidity-tolerant rate-dependent capillary viscous adhesion of bee-collected pollen fluids.

We report a two-phase adhesive fluid recovered from pollen, which displays remarkable rate tunability and humidity stabilization at microscopic and macroscopic scales. These natural materials provide a previously-unknown model for bioinspired humidity-stable and dynamically-tunable adhesive materials. In particular, two immiscible liquid phases are identified in bioadhesive fluid extracted from dandelion pollen taken from honey bees: a sugary adhesive aqueous phase similar to bee nectar and an oily phase consistent with plant pollenkitt. Here we show that the aqueous phase exhibits a rate-dependent capillary adhesion attributed to hydrodynamic forces above a critical separation rate. However, the performance of this adhesive phase alone is very sensitive to humidity due to water loss or uptake. Interestingly, the oily phase contributes scarcely to the wet adhesion. Rather, it spreads over the aqueous phase and functions as a barrier to water vapor that tempers the effects of humidity changes and stabilizes the capillary adhesion.

Sustainable and long-time 'rejuvenation' of biomimetic water-repellent silica coating on polyester fabrics induced by rough mechanical abrasion.

The economical use of water-repellent coatings on polymeric materials in commercial and industrial applications is limited by their mechanical wear robustness and long-term durability. In this study, we demonstrate that polyethylene terephthalate (PET) fabric modified with inorganic, methyltrimethoxysilane (MTMS)-based coatings shows excellent resistance against various types of wear damage, thereby mimicking superhydrophobic biological materials. These features were facilitated by the rational design of coating processing that also enabled tunable hierarchical surface structure. A series of custom and standard testing protocols revealed that coating-to-substrate adhesion was remarkably high, as was the resistance to various mechanical abradents. The most intriguing characteristic observed during aging and abrasion cycles was the enhancement in non-wettability or 'rejuvenation' reflected by water droplet roll-off behavior, a characteristic of self-cleaning materials. Water-repellent properties of coated polyester were also enhanced by prolonged thermal annealing and were maintained after custom laundry. The developed technology offers opportunities to design low cost, durable and functional textiles for both indoor and outdoor applications.

Underwater Curvature-Driven Transport between Oil Droplets on Patterned Substrates.

Roughness contrast patterns were generated on copper surfaces by a simple one-step site-selective oxidation process using a felt-tipped ink pen masking method. The patterned surface exhibited strong underwater oil wettability contrast which allows oil droplet confinement. Oil droplets placed on two patterned smooth dots (reservoirs) connected by a patterned smooth channel will spontaneously exchange liquid as a result of Laplace pressure differences until their shapes have reached equilibrium. In our experiments, residual solubility of the oil in water was overcome by using saturated oil-in-water solutions as the aqueous medium. In the saturated solution, the dependence of pattern geometry and oil viscosity on transported volume and the flow rate in the underwater oil transport process was investigated for dichloromethane and hexadecane. Experimental results were in good agreement with a simple model for Laplace pressure-driven flow. Depending on droplet curvatures, oil can be transported from large to small reservoirs or vice versa. The model predictions enable the design of reservoir and channel dimensions to control liquid transport in the water-solid surface-oil system. The patterning technique was extended to more complex patterns with multiple reservoirs for smart oil separation and mixing processes. The concepts demonstrated in this study can be employed to seed droplet arrays with specific initial drop volumes and achieve subsequent droplet mixing at controlled flow rates for potential lab-on-a-chip applications ranging from oil-droplet-based miniature reactors and sensors to high-throughput assays.

Inhibition of Bacterial Adhesion on Nanotextured Stainless Steel 316L by Electrochemical Etching.

Bacterial adhesion to stainless steel 316L (SS316L), which is an alloy typically used in many medical devices and food processing equipment, can cause serious infections along with substantial healthcare costs. This work demonstrates that nanotextured SS316L surfaces produced by electrochemical etching effectively inhibit bacterial adhesion of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, but exhibit cytocompatibility and no toxicity toward mammalian cells in vitro. Additionally, the electrochemical surface modification on SS316L results in formation of superior passive layer at the surface, improving corrosion resistance. The nanotextured SS316L offers significant potential for medical applications based on the surface structure-induced reduction of bacterial adhesion without use of antibiotic or chemical modifications while providing cytocompatibility and corrosion resistance in physiological conditions.

Phase behavior in highly concentrated assemblies of microgels with soft repulsive interaction potentials.

Microgel particles with a soft repulsive interaction potential are investigated with particle tracking methods to study the phase behavior of soft-sphere systems. The use of poly(N-isopropylacrylamide) particles allows the effective volume fraction of a sample to be tuned via thermal modulation without altering the particle number density. This allows for investigation of the phase behavior of an assembly as a function of its initial packing density. In particular, we have elucidated the influence of soft colloid "overpacking" on the freezing effective volume fraction (phieff,f). These studies thereby illustrate the interplay between energetics/packing forces occurring at the colloidal and polymer chain length scales.

Crystallization behavior of soft, attractive microgels.

The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N'-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH = 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.

Two-Step Process To Create "Roll-Off" Superamphiphobic Paper Surfaces.

Surface modification of cellulose-based paper, which displays roll-off properties for water and oils (surface tension ≥23.8 mN·m-1) and good repellency toward n-heptane (20.1 mN·m-1), is reported. Droplets of water, diiodomethane, motor oil, hexadecane, and decane all "bead up", i.e., exhibit high contact angles, and roll off the treated surface under the influence of gravity. Unlike widely used approaches that rely on the deposition of nanoparticles or electrospun nanofibers to create superamphiphobic surfaces, our method generates a hierarchical structure as an inherent property of the substrate and displays good adhesion between the film and substrate. The two-step combination of plasma etching and vapor deposition used in this study enables fine-tuning of the nanoscale roughness and thereby facilitates enhanced fundamental understanding of the effect of micro- and nanoscale roughness on the paper wetting properties. The surfaces maintain their "roll-off" properties after dynamic impact tests, demonstrating their mechanical robustness. Furthermore, the superamphiphobic paper has high gas permeability due to pore-volume enhancement by plasma etching but maintains the mechanical flexibility and strength of untreated paper, despite the presence of nanostructures. The unique combination of the chemical and physical properties of the resulting superamphiphobic paper is of practical interest for a range of applications such as breathable and disposable medical apparel, antifouling biomedical devices, antifingerprint paper, liquid packaging, microfluidic devices, and medical testing strips through a simple surface etching plus coating process.

Process Principles for Large-Scale Nanomanufacturing.

Nanomanufacturing-the fabrication of macroscopic products from well-defined nanoscale building blocks-in a truly scalable and versatile manner is still far from our current reality. Here, we describe the barriers to large-scale nanomanufacturing and identify routes to overcome them. We argue for nanomanufacturing systems consisting of an iterative sequence of synthesis/assembly and separation/sorting unit operations, analogous to those used in chemicals manufacturing. In addition to performance and economic considerations, phenomena unique to the nanoscale must guide the design of each unit operation and the overall process flow. We identify and discuss four key nanomanufacturing process design needs: (a) appropriately selected process break points, (b) synthesis techniques appropriate for large-scale manufacturing,

Microrheological detection of protein unfolding.

We apply passive probes to protein solutions and evaluate the viscous response to folding and unfolding, allowing us to accurately quantify both the thermodynamics of protein folding and the structural dimensions of the protein molecules with subnanometer resolution. Hard-sphere approximations predict a measurable change in relative viscosity as the hydrodynamic volume fraction of protein molecules increases during unfolding. Microrheology measures these changes to unambiguously evaluate the ensemble average characteristics of the unfolded state in a denaturant, urea, while minimizing the shear-induced unfolding and alignment associated with conventional rheometry.