Enhanced layering is observed when molecularly thin films of a simple globular molecular fluid (octamethylcyclotetrasiloxane) are confined between step-free sheets of mica cleaved to be free of ...nanoparticulates produced by using a hot platinum wire. In addition, the linear shear responses depend on history. Films formed by rapid compression or under the action of shear display a large effective viscosity, but films formed by quasistatic compression display unprecedented low friction.
Dielectrophoretic behaviors and assembly of a binary suspension in aqueous media are examined in the presence of nonuniform alternating current (AC) electric field. A peculiar low-frequency threshold ...and dielectrophoresis (DEP) crossover frequency determine the applicable frequency window for binary assembly under positive DEP, which can be effectively tuned by medium conductivity and particle size, suggesting that the dynamic double-layer effect is responsible for the interfacial polarization of micrometer to submicrometer-sized particles in aqueous suspensions. Strong effects of AC-field frequency, medium conductivity, and size ratio on binary assembly morphology have been observed. A frequency-medium conductivity phase diagram is obtained to illustrate the morphological transition of assembled colloidal aggregates from segregated, ordered assemblies to inverted segregation with the appearance of amorphous phases upon increasing frequency and/or medium conductivity, which is a direct consequence of the competition between DEP and hydrodynamic mobility. Significantly, our results demonstrate a rapid method to form hybrid nanostructured materials.
Evaporation-induced pattern formation has attracted considerable attention as a simple yet versatile method for generating self-assembled structures that have broad applications from photonic devices ...to biomacromolecular recognition. Previous study of evaporative self-assembly has mainly focused on single nonvolatile component systems, and the driving mechanisms have been extensively investigated. In contrast, pattern formation from evaporating multicomponent systems, despite its wide existence in nature and numerous engineering applications, has been rarely explored. In this work, we examine a DNA−colloid binary suspension as a model system to understand the evaporation-induced interfacial hydrodynamics and self-assembled morphology in multicomponent systems involving complex competing intermolecular and interfacial interactions. Direct microscopic observations show that the composition of the binary system plays a critical role in the multiple-ring formation upon evaporation: (1) suspensions with high DNA concentrations and low colloidal concentrations favor the formation of the multiple-ring pattern; (2) the size of colloidal particles added into DNA aqueous droplets can significantly disrupt smooth multiple rings to form rippled rings and curtain-like periodic patterns with a curious spoke-like structure as the size of colloidal particles increases; and (3) the enhancement of DNA−colloid interaction by oppositely charged colloidal particles results in considerably high irregularity of DNA stain ring spacing. We examine the disruption of the multiring morphology under varied conditions and attribute it to local hydrodynamics governed by colloid aggregation and sedimentation. Our results demonstrate the feasibility of fabricating periodic self-assembled hybrid structures via one-step evaporation of droplets consisting of multiple components.
Hydrophobicity at a Janus Interface Zhang, Xueyan; Zhu, Yingxi; Granick, Steve
Science (American Association for the Advancement of Science),
01/2002, Letnik:
295, Številka:
5555
Journal Article
Recenzirano
Water confined between adjoining hydrophobic and hydrophilic surfaces (a Janus interface) is found to form stable films of nanometer thickness whose responses to shear deformations are ...extraordinarily noisy. The power spectrum of this noise is quantified. In addition, the frequency dependence of the complex shear modulus is a power law with slope one-half, indicating a distribution of relaxation processes rather than any dominant one. The physical picture emerges that whereas surface energetics encourage water to dewet the hydrophobic side of the interface, the hydrophilic side constrains water to be present, resulting in a flickering, fluctuating complex.
The structure of a hydrated poly(N-isopropylacrylamide) brush loaded with 5 vol % Isoniazid is studied as a function of temperature using neutron reflectometry (NR) and atomic force microscopy (AFM). ...NR measurements show that Isoniazid increases the thickness of the brush before, during and after the polymer collapse, and it is retained inside the brush at all measured temperatures. The Isoniazid concentration in the expanded brush is ∼14% higher than in the bulk solution, and the concentration nearly doubles in the collapsed polymer, suggesting stronger binding between Isoniazid and the polymer compared to water, even at temperatures below the lower critical solution temperature (LCST) where the polymer is hydrophilic. Typically, additives that bind strongly to the polymer backbone and increase the hydrophilicity of the polymer will delay the onset of the LCST, which is suggested by AFM and NR measurements. The extent of small-molecule loading and distribution throughout a thermo-responsive polymer brush, such as pNIPAAm, will have important consequences for applications such as drug delivery and gating.
The time-dependent nucleation phase is critical to amyloid fibrillation and related to many pathologies, in which the conversion from natively folded amyloidogenic proteins to oligomers via ...nucleation is often hypothesized as a possible underlying mechanism. In this work, non-uniform AC-electric fields across two asymmetric electrodes were explored to control and examine the aggregation of insulin, a model amyloid protein, in aqueous buffer solution at constant temperature (20 °C) by fluorescence correlation spectroscopy and fluorescence microscopy. Insulin was rapidly concentrated in a strong AC-field by imposed AC-electroosmosis flow over an optimal frequency range of 0.5-2 kHz. In the presence of an AC-field, direct fibrillation from insulin monomers without the formation of oligomer precursors was observed. Once the insulin concentration had nearly doubled its initial concentration, insulin aggregates were observed in solution. The measured lag time for the onset of insulin aggregation, determined from the abrupt reduction in insulin concentration in solution, was significantly shortened from months or years in the absence of AC-fields to 1 min-3 h under AC-fields. The ability of external fields to alter amyloid nucleation kinetics provides insights into the onset of amyloid fibrillation.
Ionic liquids (ILs) have been widely considered and used as “green solvents” for more than two decades. However, their ecotoxicity results have contradicted this view, as ILs, particularly ...hydrophobic ones, are reported to exhibit high toxicity. Yet the origin of their toxicology remains unclear. In this work, we have investigated the interaction of amphiphilic ILs with a lipid bilayer as a model cell membrane to understand their cytotoxicity at a molecular level. By employing fluorescence imaging and light and X-ray scattering techniques, we have found that amphiphilic ILs could disrupt the lipid bilayer by IL insertion, end-capping the hydrophobic edge of the lipid bilayer, and eventually disintegrating the lipid bilayer at high IL concentration. The insertion of ILs to cause the swelling of the lipid bilayer shows strong dependence on the hydrophobicity of IL cationic alky chain and anions and is strongly correlated with the reported IL cytotoxicity.
The application of ionic liquids (ILs) in many industrially relevant processes provides an urgent need to better understand their molecular interactions with biological systems. A detailed ...understanding of the cytotoxicity mechanism of ILs can be helpful in facilitating the molecular design of nontoxic ILs. Using coarse-grained molecular dynamics (MD) simulations, we investigate the effects of imidazolium-based ILs on several lipid bilayer morphologies. Our results demonstrate that the asymmetric insertion of IL cations into one side of a lipid bilayer leaflet enhances the leaflet strain, which upon reaching a critical value triggers a morphological disruption in the bilayer. Consistently, the bending modulus of the bilayer is reduced by 1 to 2 orders of magnitude relative to that of an IL-free planar bilayer prior to the disruption event. Our results suggest that ILs that can easily insert into the lipid bilayer without diffusing across or inducing lipid flip-flop can be more disruptive to a lipid biomembrane.
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