A method for direct simulation of flexible fiber suspensions using lattice Boltzmann equation with external boundary force

The computational method presented here can be used to study the effect of volume fraction and particle deformation on the rheology and microstructure of deformable fibers suspended in Newtonian fluid. In this method, the flow is computed on a fixed regular ‘lattice’ using the lattice Boltzmann method, where each solid particle is mapped onto a Lagrangian frame moving continuously through the domain. Instead of the standard bounce-back method, an external boundary force is used to impose the no-slip boundary condition at the fluid–solid interface for stationary or moving boundaries. The motion and orientation of the fiber are obtained from Newtonian dynamics equations. Although the external boundary force method is general, in this application it is used in conjunction with a flexible fiber model, which calculates the flexible fiber deformation by the real material properties. The methodology is validated by comparing with experimental and theoretical results.     

Liquid crystal assemblies in biologically inspired systems

In this paper, which is part of a collection in honour of Noel Clark’s remarkable career on liquid crystal (LC) and soft matter research, we present examples of biologically inspired systems, which form LC phases with their LC nature impacting biological function in cells or being important in biomedical applications. One area focuses on understanding network and bundle formation of cytoskeletal polyampholytes (filamentous actin, microtubules and neurofilaments (NFs)). Here, we describe studies on NFs, the intermediate filaments of neurons, which form open network nematic LC hydrogels in axons. Synchrotron small-angle-X-ray scattering studies of NF protein dilution experiments and NF hydrogels subjected to osmotic stress show that NF networks are stabilised by competing long-range repulsion and attractions mediated by the NF’s polyampholytic sidearms. The attractions are present both at very large inter-filament spacings, in the weak sidearm-interpenetrating regime, and at smaller inter-filament spacings, in the strong sidearm-interpenetrating regime. A second series of experiments will describe the structure and properties of cationic liposomes (CLs) complexed with nucleic acids (NAs). CL-NA complexes form liquid crystalline phases, which interact in a structure-dependent manner with cellular membranes enabling the design of complexes for efficient delivery of NA (DNA and RNA) in therapeutic applications.     

Structural evolution of environmentally responsive cationic liposome-DNA complexes with a reducible lipid linker

Environmentally responsive materials (i.e., materials that respond to changes in their environment with a change in their properties or structure) are attracting increasing amounts of interest. We recently designed and synthesized a series of cleavable multivalent lipids (CMVLn, with n = 2-5 being the number of positive headgroup charges at full protonation) with a disulfide bond in the linker between their cationic headgroup and hydrophobic tails. The self-assembled complexes of the CMVLs and DNA are a prototypical environmentally responsive material, undergoing extensive structural rearrangement when exposed to reducing agents. We investigated the structural evolution of CMVL-DNA complexes at varied complex composition, temperature, and incubation time using small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). A related lipid with a stable linker, TMVL4, was used as a control. In a nonreducing environment, CMVL-DNA complexes form the lamellar (L(α)(C)) phase, with DNA rods sandwiched between lipid bilayers. However, new self-assembled phases form when the disulfide linker is cleaved by dithiothreitol or the biologically relevant reducing agent glutathione. The released DNA and cleaved CMVL headgroups form a loosely organized phase, giving rise to a characteristic broad SAXS correlation profile. CMVLs with high headgroup charge also form condensed DNA bundles. Intriguingly, the cleaved hydrophobic tails of the CMVLs reassemble into tilted chain-ordered L(β') phases upon incubation at physiological temperature (37 °C), as indicated by characteristic WAXS peaks. X-ray scattering further reveals that two of the three phases (L(βF), L(βL), and L(βI)) constituting the L(β') phase coexist in these samples. The described system may have applications in lipid-based nanotechnologies.     

Linear and nonlinear optical properties of a macrocyclic trichromophore bundle with parallel-aligned dipole moments

A macrocyclic trichromophore bundle 1 with parallel-aligned dipole moments has been synthesized to study the influence of aggregation and orientation of a nonlinear optical (NLO) chromophore on its optical properties. The linear and nonlinear optical properties of 1 and a single chromophore standard 2 have been studied by UV-vis absorption, fluorescence, solvatochromic spectrometry, and hyper-Rayleigh scattering (HRS). Reduced first-order hyperpolarizability beta, hypsochromic shift, enhanced solvatochromic shifts, and fluorescence quenching for individual chromophores were observed when 1 was compared with 2. Analysis of the data showed that the transition dipole moment changes only slightly when the chromophores are parallel aligned in the bundle architecture. However, the apparent hyperpolarizability of the individual chromophores decreased significantly by about 20%. The reduction in beta for the individual chromophores in 1 is largely due to the hypsochromic shift, i.e., excitation energy increase of the interband (charge-transfer) energy gap and the reduced difference between the ground-state and excited-state dipole moments. The hypsochromic shift and fluorescence quenching are consistent with exciton theory. Possible reasons for the enhanced solvatochromic shift are discussed.     

Experimental Investigation of Rheological and Morphological Properties of Water in Crude Oil Emulsions Stabilized by a Lipophilic Surfactant

Rheological behavior of two crude oils and their surfactant-stabilized emulsions with initial droplet sizes ranging from 0.5 to 75 µm were investigated at various temperatures under steady and dynamic shear testing conditions. In order to evaluate the morphology and Stability of emulsions, microscopic analysis was carried out over three months and average diameter and size distribution of dispersed droplets were determined. The water content and surfactant concentration ranged from 10 to 60% vol/vol and 0.1 to 10% wt/vol, respectively. The results indicated that the rheological properties and the physical structure and stability of emulsions were significantly influenced by the water content and surfactant concentration. The crude oils behaved as Newtonian fluids over a wide range of shear rates, whereas the emulsions behaved as non-Newtonian fluids, indicating shear-thinning effects over the entire range of shear rates. The viscosity, storage modulus and degree of elasticity were found to be significantly increased with the increase in water content and surfactant concentration. The maximum viscosity was observed at the point close to the phase inversion point where the emulsion system changes from water-in-oil emulsion to oil-in-water emulsion. The results also indicated that the rheological properties of crude oils and their emulsions are significantly temperature-dependent.     

Controlled‐Potential Electromechanical Reshaping of Cartilage

An alternative to conventional “cut‐and‐sew” cartilage surgery, electromechanical reshaping (EMR) is a molecular‐based modality in which an array of needle electrodes is inserted into cartilage held under mechanical deformation by a jig. Brief (ca. 2 min) application of an electrochemical potential at the water‐oxidation limit results in permanent reshaping of the specimen. Highly sulfated glycosaminoglycans within the cartilage matrix provide structural rigidity to the tissue through extensive ionic‐bonding networks; this matrix is highly permselective for cations. Our studies indicate that EMR results from electrochemical generation of localized, low‐pH gradients within the tissue: fixed negative charges in the proteoglycan matrix are protonated, resulting in chemically induced stress relaxation of the tissue. Re‐equilibration to physiological pH restores the fixed negative charges, and yields remodeled cartilage that retains a new shape approximated by the geometry of the reshaping jig.     

Toward a comprehensive microextraction/determination unit: A chip silicon rubber polyaniline‐based system and its direct coupling with gas chromatography and mass spectrometry

An inexpensive silicon rubber‐based chip was constructed by fabricating a triangle‐shaped microcanal with a 135 μm width and 234 μm depth by laser ablation technique. The fabricated groove was sealed by a thin glass cover while two pieces of stainless‐steel tubing were connected to each side of the canal. Then, a thin polyaniline film was synthesized on the walls of the canal by chemical oxidation using a syringe pump to deliver the relevant reagents. The microfluidic system was eventually connected to a gas chromatography‐mass spectrometry. To evaluate the capability of the constructed microfluidic system, it was implemented to the analysis of submilliliter volumes of environmental samples spiked with the trace amounts of some pesticide residues. To show the applicability of the hyphenated system, the extraction/determination of triazines was implemented while only 500 μL sample with the limits of detection ranged from 0.2 to 0.5 ng/mL could be easily achieved. In addition, the influential extraction parameters such as sample volume, flow rate, and sample pH were optimized. Under the optimized conditions, the relative standard deviation values for double‐distillated water sample spiked with the selected triazines at 250 ng/mL were 6.5–12.5% (n = 3).