Light-Scattering Analysis of Drying Behavior in Suspension Droplets with Silica and Polystyrene Particles and a Hydrosoluble Polymer.

The coffee-ring structure, which is the final drying pattern of a sessile suspension droplet, is a key factor in controlling the uniformity of the particulate deposits in various coatings. Two light-scattering methods, diffusing wave spectroscopy (DWS) and multispeckle DWS (MSDWS), were used to quantitatively distinguish temporal changes in particle mobility in evaporating suspension droplets containing micrometer-sized silica and polystyrene (PS) particles. The characteristic particle mobility was measured in terms of the mean square displacement in the early stage of drying, and the local particle dynamics around the edge and center regimes of the droplets during drying were analyzed using MSDWS. Hydroxyethyl cellulose (HEC), a hydrosoluble polymer, was added to the silica and PS suspensions to further investigate its role in suppressing or enhancing coffee-ring patterns based on particle-polymer interactions. Consequently, dried microstructures can be directly correlated with real-time drying dynamics, as well as the interactions between solutes by comprehensive light-scattering methods.

Optoelectrical Properties of Transparent Conductive Films Fabricated with Ag Nanoparticle-Suspended Emulsion under Various Formulations and Coating Conditions.

Transparent conductive films (TCFs) were fabricated through bar-coating with a water-in-toluene emulsion containing Ag nanoparticles (AgNPs). Morphological changes in the self-assembled TCF networks under different emulsion formulations and coating conditions and the corresponding optoelectrical properties were investigated. In preparing various emulsions, the concentration of AgNPs and the water weight fraction were important factors for determining the size of the water droplets, which plays a decisive role in controlling the optoelectrical properties of the TCFs affected by open cells and conductive lines. An increased concentration of AgNPs and decreased water weight fraction resulted in a decreased droplet size, thus altering the optoelectrical properties. The coating conditions, such as coating thickness and drying temperature, changed the degree of water droplet coalescence due to different emulsion drying rates, which also affected the final self-assembled network structure and optoelectrical properties of the TCFs. Systematically controlling various material and process conditions, we explored a coating strategy to enhance the optoelectrical properties of TCFs, resulting in an achieved transmittance of 86 ± 0.2%, a haze of 4 ± 0.2%, and a sheet resistance of 35 ± 2.8 Ω/□. TCFs with such optimal properties can be applied to touch screen fields.

Mechanisms of the Device Property Alteration Generated by the Proton Irradiation in GaN-Based MIS-HEMTs Using Extremely Thin Gate Insulator.

Recently, we reported that device performance degradation mechanisms, which are generated by the γ-ray irradiation in GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs), use extremely thin gate insulators. When the γ-ray was radiated, the total ionizing dose (TID) effects were generated and the device performance deteriorated. In this work, we investigated the device property alteration and its mechanisms, which were caused by the proton irradiation in GaN-based MIS-HEMTs for the 5 nm-thick Si3N4 and HfO2 gate insulator. The device property, such as threshold voltage, drain current, and transconductance varied by the proton irradiation. When the 5 nm-thick HfO2 layer was employed for the gate insulator, the threshold voltage shift was larger than that of the 5 nm-thick Si3N4 gate insulator, despite the HfO2 gate insulator exhibiting better radiation resistance compared to the Si3N4 gate insulator. On the other hand, the drain current and transconductance degradation were less for the 5 nm-thick HfO2 gate insulator. Unlike the γ-ray irradiation, our systematic research included pulse-mode stress measurements and carrier mobility extraction and revealed that the TID and displacement damage (DD) effects were simultaneously generated by the proton irradiation in GaN-based MIS-HEMTs. The degree of the device property alteration was determined by the competition or superposition of the TID and DD effects for the threshold voltage shift and drain current and transconductance deterioration, respectively. The device property alteration was diminished due to the reduction of the linear energy transfer with increasing irradiated proton energy. We also studied the frequency performance degradation that corresponded to the irradiated proton energy in GaN-based MIS-HEMTs using an extremely thin gate insulator.

Combinatorial Metabolic Engineering Strategies for the Enhanced Production of Free Fatty Acids in Escherichia coli.

In this study, we evaluated the effects of several metabolic engineering strategies in a systematic and combinatorial manner to enhance the free fatty acid (FFA) production in Escherichia coli. The strategies included (i) overexpression of mutant thioesterase I ('TesAR64C) to efficiently release the FFAs from fatty acyl-ACP; (ii) coexpression of global regulatory protein FadR; (iii) heterologous expression of methylmalonyl-CoA carboxyltransferase and phosphoenolpyruvate carboxylase to synthesize fatty acid precursor molecule malonyl-CoA; and (iv) disruption of genes associated with membrane proteins (GusC, MdlA, and EnvR) to improve the cellular state and export the FFAs outside the cell. The synergistic effects of these genetic modifications in strain SBF50 yielded 7.2 ± 0.11 g/L FFAs at the shake flask level. In fed-batch cultivation under nitrogen-limiting conditions, strain SBF50 produced 33.6 ± 0.02 g/L FFAs with a productivity of 0.7 g/L/h from glucose, which is the maximum titer reported in E. coli to date. Combinatorial metabolic engineering approaches can prove to be highly useful for the large-scale production of FA-derived chemicals and fuels.

Rheological Properties and Melt Spinning Application of Controlled-Rheology Polypropylenes via Pilot-Scale Reactive Extrusion.

Based on pilot-scale twin-screw reactive extrusion, the structural and rheological properties of controlled-rheology polypropylenes (CR-PPs) are investigated, where the effects of peroxide content and extrusion conditions such as screw configuration, extrusion temperature, and screw speed are prioritized. The active chain cleavage reaction by a small peroxide content of less than 600 ppm inside the extruder gradually increases the melt index and narrows the molecular weight distribution of CR-PPs, thereby affording favorable properties that are applicable to the fiber spinning process. The mechanical properties of CR-PPs are slightly degraded owing to the generation of unsaturated chain ends during the reactive extrusion, which suppresses crystal growth. Under all extrusion conditions, the chain scission and thermal degradation of polypropylene samples occur actively in the harsh twin-screw extruder compared with those in the mild twin-screw extruder. Finally, it is confirmed that CR-PPs can be suitably applied to the melt-spinning process for staple fiber production, thereby guaranteeing a more stable spinning process window against draw resonance instability.

Mild stratification in drying films of colloidal mixtures.

Size stratification of bidisperse colloidal mixtures during vertical drying was investigated using the implicit solvent Langevin dynamics (LD) simulation and the explicit solvent lattice Boltzmann (LB) method. Simulations were performed for the Péclet number (Pe) over a wide range of 1-1000. In the case of a low size ratio of 2, mild stratification was observed in both simulation methods, in contrast to distinct stratification with thick "small-on-top" or "large-on-top" layers. The LD simulations exhibited a "small-on-top" stratification or mixed state. In contrast, the LB simulations exhibited a "large-on-top" or mixed state, according to the variation in Pe. The results demonstrated that the explicit solvent reduced the collective diffusion under moderate Pe conditions. This suppressed the steep concentration gradient of small particles in the packed region of particles near the air-solvent interface. Thus, distinguishable stratification patterns were obtained for the implicit and explicit solvent models.

Dynamic and Reprocessable Fluorinated Poly(hindered urea) Network Materials Containing Ionic Liquids to Enhance Triboelectric Performance.

Triboelectric nanogenerators (TENGs), a newly developed energy harvesting device that converts surrounding environmental mechanical stimuli into electricity, have been significantly explored as an ideal long-term power source for electrical devices. Despite recent advances, the development of advanced TENG devices with sufficient outputs to sustainably power electronic devices and rapid self-healability under mild conditions to improve their lifetime and function is highly demanded. Here, we report a robust self-healable and reprocessable TENG fabricated with a covalent adaptive network based on mechanically strong fluorinated poly(hindered urea) (F-PHU) integrated with ionic liquid as an efficient dielectric material to improve its triboelectric efficiency and self-healing capability simultaneously. The synthesis and integration of a well-defined reactive copolymer having both pendant fluorinated and t-butylamino bulky groups are the key to fabricate robust F-PHU networks containing fluorinated dangling chains that can interact with ionic liquids to induce ionic polarization, which raises the dielectric constant and thus increases triboelectric performance. They also are cross-linked with dynamic bulky urea linkages for rapid self-healability and high reprocessability through their reversible exchange reactions at moderate temperatures. The developed ionic F-PHU materials exhibit a high TENG output performance (power density of 173.0 mW/m2) as well as high TENG output recovery upon repairing their surface damages. This work demonstrates that such a synergistic design of triboelectric ionic F-PHU materials could have great potential for applications requiring high-performance and long-lasting energy harvesting.

Enhanced production of nonanedioic acid from nonanoic acid by engineered Escherichia coli.

In this study, whole-cell biotransformation was conducted to produce nonanedioic acid from nonanoic acid by expressing the alkane hydroxylating system (AlkBGT) from Pseudomonas putida GPo1 in Escherichia coli. Following adaptive laboratory evolution, an efficient E. coli mutant strain, designated as MRE, was successfully obtained, demonstrating the fastest growth (27-fold higher) on nonanoic acid as the sole carbon source compared to the wild-type strain. Additionally, the MRE strain was engineered to block nonanoic acid degradation by deleting fadE. The resulting strain exhibited a 12.8-fold increase in nonanedioic acid production compared to the wild-type strain. Six mutations in acrR, Pcrp , dppA, PfadD , e14, and yeaR were identified in the mutant MRE strain, which was characterized using genomic modifications and RNA-sequencing. The acquired mutations were found to be beneficial for rapid growth and nonanedioic acid production.

Ultrafast and Ultrastable Heteroarchitectured Porous Nanocube Anode Composed of CuS/FeS2 Embedded in Nitrogen-Doped Carbon for Use in Sodium-Ion Batteries.

The enhancement of the structural stability of conversion-based metal sulfides at high current densities remains a major challenge in realizing the practical application of sodium-ion batteries (SIBs). The instability of metal sulfides is caused by the large volume variation and sluggish reaction kinetics upon sodiation/desodiation. To overcome this, herein, a heterostructured nanocube anode composed of CuS/FeS2 embedded in nitrogen-doped carbon (CuS/FeS2 @NC) is synthesized. Size- and shape-controlled porous carbon nanocubes containing metallic nanoparticles are synthesized by the two-step pyrolysis of a bimetallic Prussian blue analog (PBA) precursor. The simple sulfurization-induced formation of highly conductive CuS along with FeS2 facilitates sodium-ion diffusion and enhances the redox reversibility upon cycling. The mesoporous carbon structure provides excellent electrolyte impregnation, efficient charge transport pathways, and good volume expansion buffering. The CuS/FeS2 @NC nanocube anode exhibits excellent sodium storage characteristics including high desodiation capacity (608 mAh g-1 at 0.2 A g-1 ), remarkable long-term cycle life (99.1% capacity retention after 300 cycles at 5 A g-1 ), and good rate capability up to 5 A g-1 . The simple, facile synthetic route combined with the rational design of bimetallic PBA nanostructures can be widely utilized in the development of conversion-based metal sulfides and other high-capacity anode materials for high-performance SIBs.

Van der Waals Heterostructure of Hexagonal Boron Nitride with an AlGaN/GaN Epitaxial Wafer for High-Performance Radio Frequency Applications.

While two-dimensional (2D) hexagonal boron nitride (h-BN) is emerging as an atomically thin and dangling bond-free insulating layer for next-generation electronics and optoelectronics, its practical implementation into miniaturized integrated circuits has been significantly limited due to difficulties in large-scale growth directly on epitaxial semiconductor wafers. Herein, the realization of a wafer-scale h-BN van der Waals heterostructure with a 2 in. AlGaN/GaN high-electron mobility transistor (HEMT) wafer using metal-organic chemical vapor deposition is presented. The combination of state-of-the-art microscopic and spectroscopic analyses and theoretical calculations reveals that the heterointerface between ∼2.5 nm-thick h-BN and AlGaN layers is atomically sharp and exhibits a very weak van der Waals interaction without formation of a ternary or quaternary alloy that can induce undesired degradation of device performance. The fabricated AlGaN/GaN HEMT with h-BN shows very promising performance including a cutoff frequency (fT) and maximum oscillation frequency (fMAX) as high as 28 and 88 GHz, respectively, enabled by an effective passivation of surface defects on the HEMT wafer to deliver accurate information with minimized power loss. These findings pave the way for practical implementation of 2D materials integrated with conventional microelectronic devices and the realization of future all-2D electronics.

Effect of Silica Nanoparticles Blocked with Epoxy Groups on the Crosslinking and Surface Properties of PEG Hydrogel Films.

Silica nanoparticles (G-SiNPs) blocked with 3-glycidoxypropyl trimethoxysilane (GPTS) were newly applied to hydrogel films for improving film coating properties and to distribute the epoxy groups on the film surface. The effects of the content of epoxy-functionalized G-SiNPs on the crosslinking features by photo-induced radical polymerization and the surface mechanical properties of the hydrogel films containing poly(ethylene glycol) dimethacrylate (PEGDMA) and glycidyl methacrylate (GMA) were investigated. The real-time elastic modulus of various PEG hydrogel mixtures with prepared particles was monitored using a rotational rheometer. The distribution of epoxy groups on the crosslinked film surface was directly and indirectly estimated by the elemental analysis of Si and Br. The surface mechanical properties of various hydrogel films were measured by nano-indentation and nano-scratch tests. The relationship between the rheological and surface properties of PEG-based hydrogel films suggests that the use of small amounts of G-SiNPs enhances the surface hardness and crosslinked network of the film and uniformly distributes sufficient epoxy groups on the film surface for further coating applications.

A Study on the Oxygen Permeability Behavior of Nanoclay in a Polypropylene/Nanoclay Nanocomposite by Biaxial Stretching.

Polypropylene (PP) has poor oxygen barrier properties, therefore it is manufactured in a multi-layer structure with other plastics and metals, and has been widely used as a packaging material in various industries from food and beverage to pharmaceuticals. However, multi-layered packaging materials are generally low in recyclability and cause serious environmental pollution, therefore we have faced the challenge of improving the oxygen barrier performance as a uni-material. In this work, PP/nanoclay nanocomposites were prepared at nanoclay contents ranging from 0.8 to 6.4 wt% by the biaxial stretching method, performed through a sequential stretching method. It was observed that, as the draw ratio increased, the behavior of the agglomerates of the nanoclay located in the PP matrix changed and the nanoclay was dispersed along the second stretching direction. Oxygen barrier properties of PP/nanoclay nanocomposites are clearly improved due to this dispersion effect. As the biaxial stretching ratio and the content of nanoclay increased, the oxygen permeability value of the PP/nanoclay nanocomposite decreased to 43.5 cc·mm/m2·day·atm, which was reduced by about 64% compared to PP. Moreover, even when the relative humidity was increased from 0% to 90%, the oxygen permeability values remained almost the same without quality deterioration. Besides these properties, we also found that the mechanical and thermal properties were also improved. The biaxially-stretched PP/nanoclay nanocomposite fabricated in this study is a potential candidate for the replacement of the multi-layered packaging material used in the packaging fields.

Comprehensive Research of Total Ionizing Dose Effects in GaN-Based MIS-HEMTs Using Extremely Thin Gate Dielectric Layer.

The device performance deterioration mechanism caused by the total ionizing dose effect after the γ-ray irradiation was investigated in GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) for a 5 nm-thick SiN and HfO2 gate dielectric layer. The γ-ray radiation hardness according to the gate dielectric layer was also compared between the two different GaN-based MIS-HEMTs. Although HfO2 has exhibited strong tolerance to the total ionizing dose effect in Si-based devices, there is no detail report of the γ-ray radiation effects in GaN-based MIS-HEMTs employing a HfO2 gate dielectric layer. The pulsed-mode stress measurement results and carrier mobility behavior revealed that the device properties not only have direct current (DC) characteristics, but radio frequency (RF) performance has also been mostly degraded by the deterioration of the gate dielectric quality and the trapped charges inside the gate insulator. We also figured out that the immunity to the γ-ray radiation was improved when HfO2 was employed instead of SiN as a gate dielectric layer due to its stronger endurance to the γ-ray irradiation. Our results highlight that the application of a gate insulator that shows superior immunity to the γ-ray irradiation is a crucial factor for the improvement of the total ionizing dose effect in GaN-based MIS-HEMTs.

Synergistic Effect of Cellulose Nanofiber and Nanoclay as Distributed Phase in a Polypropylene Based Nanocomposite System.

Since the plastic-based multilayer films applied to food packaging are not recyclable, it is necessary to develop easily recyclable single materials. Herein, polypropylene (PP)-based cellulose nanofiber (CNF)/nanoclay nanocomposites were prepared by melt-mixing using a fixed CNF content of 1 wt %, while the nanoclay content varied from 1 to 5 wt %. The optimum nanoclay content in the PP matrix was found to be 3 wt % (PCN3), while they exhibited synergistic effects as a nucleating agent. PCN3 exhibited the best mechanical properties, and the tensile and flexural moduli were improved by 51% and 26%, respectively, compared to PP. In addition, the oxygen permeability was reduced by 28%, while maintaining the excellent water vapor permeability of PP. The improvement in the mechanical and barrier properties of PP through the production of PP/CNF/nanoclay hybrid nanocomposites suggested their possible application in the field of food packaging.

Charging Effect by Fluorine-Treatment and Recess Gate for Enhancement-Mode on AlGaN/GaN High Electron Mobility Transistors.

An enhancement-mode AlGaN/GaN metal-insulator-semiconductor high-electron- mobility-transistor was fabricated using a recess gate and CF4 plasma treatment to investigate its reliable applicability to high-power devices and circuits. The fluorinated-gate device showed hysteresis during the DC current-voltage measurement, and the polarity and magnitude of hysteresis depend on the drain voltage. The hysteresis phenomenon is due to the electron trapping at the Al2O3/AlGaN interface and charging times longer than milliseconds were obtained by pulse I-V measurement. In addition, the subthreshold slope of the fluorinated-gate device was increased after the positive gate bias stress because of the two-dimensional electron gas reduction by ionized fluorine. Our systematic observation revealed that the effect of fluorine ions should be considered for the design of AlGaN/GaN power circuits.

Effect of Immiscible Secondary Fluid on Particle Dynamics and Coffee Ring Characteristics during Suspension Drying.

Particle motion and coffee ring patterns in water-borne suspensions of polystyrene (PS) particle added with small amounts of secondary hydrophobic decalin are investigated during the drying of the suspension droplets, mainly employing light scattering methods. Very tiny secondary fluid insertions via high-speed agitation effectively link the particles through hydrophobic dissolution leading to the formation of multimodal particulate clusters, with resistance to the outward capillary flow and suppression of coffee ring formation after drying. The impact of decalin on particles is corroborated by actual images acquired from an optical profiler and a scanning electron microscope (SEM). The average particle motion inside the suspension changed by decalin was expressed in terms of mean square displacement (MSD) based on diffusing wave spectroscopy (DWS). Employing multispeckle diffusing wave spectroscopy (MSDWS), the rapid motion or ß-relaxation of particles in various suspensions with and without decalin is quantified in early lag time during the drying of droplets. The change in particle dynamics during suspension drop drying, when adding a small secondary fluid, plays a key role in tuning coffee ring patterns.

Self-Healable Reprocessable Triboelectric Nanogenerators Fabricated with Vitrimeric Poly(hindered Urea) Networks.

In recent years, the advent of highly deformable and healable electronics is exciting and promising for next-generation electronic devices. In particular, self-healable triboelectric nanogenerators (SH-TENGs) serve as promising candidates based on the combination of the triboelectric effect, electrostatic induction, and self-healing action. However, the majority of SH-TENGs have been devised with weak polymeric networks that are healed with reversible supramolecular interactions or disulfides, thus resulting in poor mechanical properties and low resistance to creeping. To address this issue, we demonstrate the integration of mechanically strong and self-healable poly(hindered urea) (PHU) network in the fabrication of effective TENGs. The designed PHU network is flexible but shows greater mechanical property of tensile strength as high as 1.7 MPa at break. The network is capable of self-healing quickly and repeatedly as well as being reprocessable under mild conditions, enabling the recovery of triboelectric performances after the complete healing of the damaged surfaces. Furthermore, the interfacial-polarization-induced enhancement of dielectric constant endows our SH-TENG with the highest triboelectric output performance (169.9 V/cm2) among the reported healable TENGs. This work presents an avenue to the development of mechanical energy-harvesting devices and self-powered sensors with excellent stretchability, high recoverability, and good mechanical strength.

Crosslinking Dynamics and Gelation Characteristics of Photo- and Thermally Polymerized Poly(Ethylene Glycol) Hydrogels.

The crosslinking behaviors and gelation features of poly(ethylene glycol) (PEG) hydrogels were scrutinized during the UV and thermal polymerizations of mixtures of poly(ethylene glycol) methacrylate (PEGMA, monomer) and poly(ethylene glycol) dimethacrylates (PEGDMAs, crosslinkers). The real-time crosslinking behavior of the PEG hydrogels was quantified as a function of the UV irradiation time and reaction temperature during the UV and thermal polymerization, respectively, using real-time FT-IR spectrometry and rotational rheometry. The gelation characteristics of UV- and thermally crosslinked hydrogels were compared through the analysis of the gel fraction, swelling ratio, surface hardness, and the loading and release of rhodamine-B. The gelation properties of the cured hydrogel films were suitably correlated with the real-time rheological properties and crosslinked network state of the PEG mixtures. The crosslinking and gelation properties of the cured hydrogels could be optimally tuned by not only the molecular weight of the crosslinker but also the UV or thermal polymerization conditions.

A Heterocyclic Polyurethane with Enhanced Self-Healing Efficiency and Outstanding Recovery of Mechanical Properties.

A functional polyurethane based on the heterocyclic group was synthesized and its self-healing and mechanical properties were examined. To synthesize a heterocyclic polyurethane, a polyol and a heterocyclic compound with di-hydroxyl groups at both ends were blended and the blended solution was reacted with a crosslinker containing multiple isocyanate groups. The heterocyclic polyurethane demonstrates better self-healing efficiency than the conventional polyurethane with no heterocyclic groups. Furthermore, unlike the conventional self-healing materials, the heterocyclic polyurethane examined in this study shows an outstanding recovery of the mechanical properties after the self-healing process. These results are attributed to the unique supramolecular network resulting from the strong hydrogen bonding interaction between the urethane group and the heterocyclic group in the heterocyclic polyurethane matrix.

Two-phase flow in microfluidic-chip design of hydrodynamic filtration for cell particle sorting.

As one of the flow-based passive sorting, the hydrodynamic filtration using a microfluidic-chip has shown to effectively separate into different sizes of subpopulations from cell or particle suspensions. Its model framework involving two-phase Newtonian or generalized Newtonian fluid (GNF) was developed, by performing the complete analysis of laminar flow and complicated networks of main and multiple branch channels. To predict rigorously what occurs in flow fields, we estimated pressure drop, velocity profile, and the ratio of the flow fraction at each branch point, in which the analytical model was validated with numerical flow simulations. As a model fluid of the GNF, polysaccharide solution based on Carreau type was examined. The objective parameters aiming practical channel design include the number of the branches and the length of narrow section of each branch for arbitrary conditions. The flow fraction and the number of branches are distinctly affected by the viscosity ratio between feed and side flows. As the side flow becomes more viscous, the flow fraction increases but the number of branches decreases, which enables a compact chip designed with fewer branches being operated under the same throughput. Hence, our rational design analysis indicates the significance of constitutive properties of each stream.