Stagnation Point of Surface Flow during Drop Evaporation

Capillary flow and Marangoni flow influence flow patterns of an evaporating liquid drop. While it is obvious that Marangoni stress on the drop surface affects the surface flow direction, we found that capillary flow also has an impact. The numerical results of this study showed a stagnation point near the contact line, which was further explained by the lubrication theory. The stagnation point is produced by the competing effects of Marangoni flow and capillary flow and emerges when the contact angle is small because the divergence of the capillary flow near the contact line increases as the contact angle decreases. The radial position of the stagnation point from the numerical results (rnumerical ≈ 0.995) agreed with the experimentally observed stagnation point (rexperimental > 0.992)

Structural Insights into Self-Assembled Aerosol-OT Aggregates in Aqueous Media Using Atomistic Molecular Dynamics

In water, the surfactant dioctyl sulfosuccinate (Aerosol-OT or AOT) exhibits diverse aggregate structures, ranging from micelles to lamella. An atomic-level understanding, however, of the formation and structure of these aggregates is lacking. Herein, using atomistic molecular dynamics (MD) with microsecond-long simulations, self-assembly of AOT in water is studied for concentrations of 1, 7.2, and 20 wt % at 293 K and for 7.2 wt % at 353 K. Assembly proceeds through stepwise association and dissociation of single AOT molecules, and the fusion and fission of AOT clusters. At 293 K, AOT self-assembles into either (i) spherical micelles (1 wt %), (ii) biphasic systems consisting of rod-like and prolate spheroidal micelles (7.2 wt %), or (iii) bilayers (20 wt %). We hypothesize that the observed rod-like structure is a precursor to lamellar microdomains found experimentally in biphasic dispersions. Increasing temperature to 353 K at 7.2 wt % results in a system consisting of prolate micelles but no rod-like micelles. Simulated phase behavior agrees with previously published experimental observations. Individual aggregates formed during self-assembly are identified using graph theory. Structural metrics of these aggregates like the radius of gyration, shape anisotropy, and prolateness are presented. Trends in structural metrics quantitatively reflect how shapes and sizes of AOT aggregates vary with surfactant concentration and temperature. These simulations provide deeper insight into open questions in the scientific community and demonstrate a method to generate physics-based micelle structures that can be used to rationalize experimental observations

Modification of Crystallization Behavior in Drug/Polyethylene Glycol Solid Dispersions

The crystallization kinetics of various active pharmaceutical ingredient/polyethylene glycol (API/PEG) solid dispersions has been investigated using wide-angle X-ray diffraction (XRD) and Raman spectroscopy. APIs with different physicochemical properties and crystallization tendency were employed to form solid dispersions with PEG. The crystallization rate of benzocaine (BZC) in BZC/PEG (20/80 wt %) solid dispersions was decreased substantially in comparison to that of the pure API, while the PEG matrix did not affect the crystallization behavior of haloperidol (HLP). The induction time for crystallization of ibuprofen (IBP) and fenofibrate (FNB) in a PEG matrix was decreased relative to the induction times for pure IBP and FNB. For the latter systems, it appears that crystalline PEG acted as a favorable heterogeneous nucleation site. The crystallization behavior of PEG in the API/PEG systems was also affected to different extents, depending on the API studied. These results suggest that PEG can delay, promote or have no influence on the crystallization kinetics of different APIs, and that any effects on crystallization behavior should be investigated in order to be able to produce a solid dispersion with consistent properties

Time-Resolved SAXS/WAXS Study of the Phase Behavior and Microstructural Evolution of Drug/PEG Solid Dispersions

Simultaneous small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) was employed to elucidate the physical state and location of various small molecule drugs blended with polyethylene glycol (PEG), as well as the time dependent microstructural evolution of the systems. Samples were prepared by comelting physical mixtures of the drug and PEG, followed by solidification at 25 °C. The model drugs selected encompassed a wide variety of physicochemical properties in terms of crystallization tendency and potential for interaction with PEG. It was observed that compounds which crystallized rapidly and had weak interactions with PEG tended to be excluded from the interlamellar region of the PEG matrix. In contrast, drugs which had favorable interactions with PEG were incorporated into the interlamellar regions of the polymer up until the point at which the drug crystallized whereby phase separation occurred. These factors are likely to impact the effectiveness of drug/PEG systems as drug delivery systems

Preparation of Calcium Alginate Microgel Beads in an Electrodispersion Reactor Using an Internal Source of Calcium Carbonate Nanoparticles

An electrodispersion reactor has been used to prepare calcium alginate (Ca-alginate) microgel beads in this study. In the electrodispersion reactor, pulsed electric fields are utilized to atomize aqueous mixtures of sodium alginate and CaCO3 nanoparticles (dispersed phase) from a nozzle into an immiscible, insulating second liquid (continuous phase) containing a soluble organic acid. This technique combines the features of the electrohydrodynamic force driven emulsion processes and externally triggered gelations in microreactors (the droplets) ultimately to yield soft gel beads. The average particle size of the Ca-alginate gels generated by this method changed from 412 ± 90 to 10 ± 3 μm as the applied peak voltage was increased. A diagram depicting structural information for the Ca-alginate was constructed as a function of the concentrations of sodium alginate and CaCO3 nanoparticles. From this diagram, a critical concentration of sodium alginate required for sol−gel transformation was observed. The characteristic highly porous structure of Ca-alginate particles made by this technique appears suitable for microencapsulation applications. Finally, time scale analysis was performed for the electrodispersion processes that include reactions in the microreactor droplets to provide guidelines for the future employment of this technique. This electrodispersion reactor can be used potentially in the formation of many reaction-based microencapsulation systems

Self-Assembly of Virus-Structured High Surface Area Nanomaterials and Their Application as Battery Electrodes

High area nickel and cobalt surfaces were assembled using modified Tobacco mosaic virus (TMV) templates. Rod-shaped TMV templates (300 × 18 nm) engineered to encode unique cysteine residues were self-assembled onto gold patterned surfaces in a vertically oriented fashion, producing a >10-fold increase in surface area. Electroless deposition of ionic metals onto surface-assembled virus templates produced uniform metal coatings up to 40 nm in thickness. Within a nickel−zinc battery system, the incorporation of virus-assembled electrode surfaces more than doubled the total electrode capacity. When combined, these findings demonstrate that surface-assembled virus templates provide a robust platform for the fabrication of oriented high surface area materials

Biotemplated Aqueous-Phase Palladium Crystallization in the Absence of External Reducing Agents

A new synthetic strategy enabling highly controlled aqueous-phase palladium crystallization on the tobacco mosaic virus (TMV) is demonstrated without the addition of external reducing agents. This low cost, solution processing method yields continuous and uniform coatings of polycrystalline palladium on TMV, creating highly uniform palladium nanowires of tens of nanometers in thickness and hundreds of nanometers in length. Our approach utilizes a palladium chloride precursor to produce metallic Pd coatings on TMV without the need for an external reducing agent. X-ray photoelectron spectroscopy and in situ transmission electron microscopy were used to confirm the reduction of the surface palladium oxide layer on the palladium metal wires during room temperature hydrogenation. This leads to metallic palladium nanowires with surfaces free of residual organics, making these structures suitable for applications in nanoscale electronics

Minimizing the Formation of Polynuclear Pd(II) Hydroxo Complex Clusters in Biomineralization of Barley Stripe Mosaic Virus

The Barley stripe mosaic virus (BSMV) has been successfully adopted to synthesize metal–organic nanorods of high quality. The biomineralization of palladium takes place without the presence of a reducing agent. However, PdCl42–, the precursor used in mineralization, forms hydrolyzed chloropalladate species in aqueous media. More highly hydrolyzed chloropalladate species can cause the formation of clusters that can affect the coating quality of the palladium metal on the surface of the biotemplate. The aim of this research is to minimize the formation of hydrolyzed chloropalladate clusters. The stability constants of hydrolyzed chloropalladate species were studied and utilized to calculate the distribution of hydrolyzed chloropalladate species as a function of initial Na2PdCl4 concentration. The calculations show that high initial concentration of Na2PdCl4 favors the stability of the solution. Besides, dilute hydrochloric acid solutions were used in the preparation of the tetrachloropalladate precursor stock solution. The metal precursor stock solution was stable for up to 30 days. Preliminary study of the hydrolysis reaction was evaluated by dynamic light scattering. Additionally, the effect of preheating on the morphology of Pd-BSMV nanorods was compared and evaluated. Finally, the Pd-BSMV nanorods prepared from the HCl-added Na2PdCl4 stock solution contained minimized irregular-shaped palladium particles

Patterned Assembly of Genetically Modified Viral Nanotemplates via Nucleic Acid Hybridization

The patterning of nanoparticles represents a significant obstacle in the assembly of nanoscale materials and devices. In this report, cysteine residues were genetically engineered onto the virion surface of tobacco mosaic virus (TMV), providing attachment sites for fluorescent markers. To pattern these viruses, labeled virions were partially disassembled to expose 5‘ end RNA sequences and hybridized to virus-specific probe DNA linked to electrodeposited chitosan. Electron microscopy and RNAase treatments confirmed the patterned assembly of the virus templates onto the chitosan surface. These findings demonstrate that TMV nanotemplates can be dimensionally assembled via nucleic acid hybridization

BSMV as a Biotemplate for Palladium Nanomaterial Synthesis

The vast unexplored virus biodiversity makes the application of virus templates to nanomaterial synthesis especially promising. Here, a new biotemplate, Barley stripe mosaic virus (BSMV) was successfully used to synthesize organic-metal nanorods of similarly high quality to those produced with Tobacco mosaic virus (TMV). The mineralization behavior was characterized in terms of the reduction and adsorption of precursor and nanocrystal formation processes. The BSMV surface-mediated reduction of Pd⁽²⁺⁾ proceeded via first-order kinetics in both Pd⁽²⁺⁾ and BSMV. The adsorption equilibrium relationship of PdCl₃H₂O^– on the BSMV surface was described by a multistep Langmuir isotherm suggesting alternative adsorbate–adsorbent interactions when compared to those on TMV. It was deduced that the first local isotherm is governed by electrostatically driven adsorption, which is then followed by sorption driven by covalent affinity of metal precursor molecules for amino acid residues. Furthermore, the total adsorption capacity of palladium species on BSMV is more than double of that on TMV. Finally, study of the BSMV-Pd particles by combining USAXS and SAXS enabled the characterization of all length scales in the synthesized nanomaterials. Results confirm the presence of core–shell cylindrical particles with 1–2 nm grains. The nanorods were uniform and monodisperse, with controllable diameters and therefore, of similar quality to those synthesized with TMV. Overall, BSMV has been confirmed as a viable alternate biotemplate with unique biomineralization behavior. With these results, the biotemplate toolbox has been expanded for the synthesis of new materials and comparative study of biomineralization processes