Effects Of Surface Confinement Research Articles

Nonmonotonic polymer translocation kinetics through nanopores under changing surface-polymer interactions.

Understanding the dynamics of polymers in confined environments is pivotal for diverse applications ranging from polymer upcycling to bioseparations. In this study, we develop an entropic barrier model using self-consistent field theory that considers the effect of attractive surface interactions, solvation, and confinement on polymer kinetics. In this model, we consider the translocation of a polymer from one cavity into a second cavity through a single-segment-width nanopore. We find that, for a polymer in a good solvent (i.e., excluded volume, u0 > 0), there is a nonmonotonic dependence of mean translocation time (τ) on surface interaction strength, ɛ. At low ɛ, excluded volume interactions lead to an energetic penalty and longer translocation times. As ɛ increases, the surface interactions counteract the energetic penalty imposed by excluded volume and the polymer translocates faster through the nanopore. However, as ɛ continues to increase, an adsorption transition occurs, which leads to significantly slower kinetics due to the penalty of desorption from the first cavity. The ɛ at which this adsorption transition occurs is a function of the excluded volume, with higher u0 leading to an adsorption transition at higher ɛ. Finally, we consider the effect of translocation across different size cavities. We find that the kinetics for translocation into a smaller cavity speeds up while translocation to a larger cavity slows down with increasing ɛ due to higher surface contact under stronger confinement.

Interplay of wettability and confinement enhancing the performance of heat sinks

• Tapered Double layer heat sink performance is enhanced by modifying surfaces (slip). • Value of slip length has been obtained using Molecular Dynamics simulation. • Analysis has been done based on both the first law and the second law of thermodynamics. • Entropy generation rate due to changes in kinetic energy has been introduced. • The optimal design of a double-layer microchannel heat sink has been obtained. Engineering microchannel heat sinks can help to develop compact and highly efficient cooling systems for electronic devices. In the present study, we demonstrate the coupled effect of surface modification and confinement on the performance of Double Layered Microchannel Heat Sink (DL-MCHS). The effect of wettability is incorporated using interfacial slip obtained from Molecular Dynamics (MD) simulations performed for various wettabilities, and the effect of confinement, through varying tapering degrees, is directly embodied in the geometry. The coupled transport equations are solved using the Finite Element Method (FEM) to depict the thermo-hydraulic characteristics of DL-MCHS. We quantified the enhancement in hydraulic performance and thermal performance of DL-MCHS with varying degrees of surface modification and varying tapering. For the first time, we demonstrate a regime where thermal and hydraulic performances can be enhanced simultaneously in tapered DL-MCHS thereby, making the configuration a viable heat sink system. Also, for the first time, we perform an entropy generation rate analysis in tapered DL-MCHS and demonstrated ways to reduce irreversibility making it a thermodynamically advantageous system, and following that perform an all parameter optimization study. We show that by tuning the tapering factor and interfacial characteristics, the performance of the heat sink can be adjusted for the best performance. The present study will provide helpful guidelines in designing viable heat sinks or exchangers with smart surfaces for electronics cooling and associated applications.

Role of surface silanols and confinements of siliceous MFI supports on stability of active Ga species for ethane dehydrogenation

Effect of surface silanol and confinement of siliceous MFI supports on the anchoring stability of active Ga species was demonstrated for ethane dehydrogenation. The catalysts were prepared by impregnation of Ga(NO3)3 solution on siliceous MFI (Si/Al >500) and amorphous SiO2, and characterized by XRD, XRF, SEM-EDX, H2-TPR, NH3-TPD, in situ XANES, and EXAFS. Extra-framework Ga3+ species were present with different dispersions and reducibility, depending on the surface silanols. Proximity of the silanols within the surface confinement played an essential role on anchoring stability of the extra-framework Ga3+ species. All Ga catalysts provided > 93% ethylene selectivity with appreciable TOF ~60 h−1 for ethane dehydrogenation at 650°C. In situ XANES, EXAFS, and H2-TPR suggested that the highly dispersed extra-framework Ga3+ species could exist as dimeric Ga oxide [Ga2O2]2+ species. This active site could be reversibly interconverted with the less active [HGaOH]+ species under the H2 flow.

Surface-Confined Macrocyclization via Dynamic Covalent Chemistry.

Surface-confined synthesis is a promising approach to build complex molecular nanostructures including macrocycles. However, despite the recent advances in on-surface macrocyclization under ultrahigh vacuum, selective synthesis of monodisperse and multicomponent macrocycles remains a challenge. Here, we report on an on-surface formation of [6 + 6] Schiff-base macrocycles via dynamic covalent chemistry. The macrocycles form two-dimensional crystalline domains on the micrometer scale, enabled by dynamic conversion of open-chain oligomers into well-defined ∼3.0 nm hexagonal macrocycles. We further show that by tailoring the length of the alkyl substituents, it is possible to control which of three possible products-oligomers, macrocycles, or polymers-will form at the surface. In situ scanning tunneling microscopy imaging combined with density functional theory calculations and molecular dynamics simulations unravel the synergistic effect of surface confinement and solvent in leading to preferential on-surface macrocyclization.

Comparison of the effects of surface roughness and confinement on rotor–stator cavity flow

ABSTRACTResults of a computational study are discussed which investigate roughness-induced and geometry-induced (confinement) effects on the steady-state velocity components in 3-D boundary-layer flow over the rotor disc in a rotor–stator flow configuration. It is found that, for the rotor–stator flow investigated, the roughness-induced effects are very similar to geometry-induced effects, both in nature and magnitude. The overall aim was to compare these two types of effects with corresponding roughness-induced effects in the von Kármán boundary-layer flow over a disc spinning freely in an unrestricted fluid environment. The research was conducted in the context of a programme investigating surface roughness as a means of laminar flow control for the development of new passive drag-reduction techniques. The goal was to establish whether it was possible unequivocally to distinguish between roughness-induced and geometry-induced effects on the boundary-layer flow above the rotor disc. The results obtained suggest that, for the type of system discussed here, it must be expected to be difficult to distinguish between these effects in experiments. The similarities regarding the nature and magnitude of results obtained from comparing predictions for three different computational modelling approaches indicate the required sensitivity of measurement technologies aiming to resolve the investigated effects in experimental studies.

Ferromagnetic Switching of Knotted Vector Fields in Liquid Crystal Colloids.

We experimentally realize polydomain and monodomain chiral ferromagnetic liquid crystal colloids that exhibit solitonic and knotted vector field configurations. Formed by dispersions of ferromagnetic nanoplatelets in chiral nematic liquid crystals, these colloidal ferromagnets exhibit spontaneous long-range alignment of magnetic dipole moments of individual platelets, giving rise to a continuum of the magnetization field M(r). Competing effects of surface confinement and chirality prompt spontaneous formation and enable the optical generation of localized twisted solitonic structures with double-twist tubes and torus knots of M(r), which exhibit a strong sensitivity to the direction of weak magnetic fields ∼1 mT. Numerical modeling, implemented through free energy minimization to arrive at a field-dependent three-dimensional M(r), shows a good agreement with experiments and provides insights into the torus knot topology of observed field configurations and the corresponding physical underpinnings.

Hydraulic characteristics of turbulent circular jets under surface confinement

Experimental study is carried out to investigate the hydraulics of turbulent circular jet under surface confinement. The effect of surface confinement on the jet centerline velocity and axial velocity profiles under different surface confinements and jet exit velocities has been studied. The decay of jet centerline velocity is high under surface confinement compared to the free jet. The effect of surface confinement on the velocity profile in the vertical axis is negligible near the jet exit; however, it increases downstream of the jet exit. Flux entrainment increases linearly along the jet axis and decreases as surface confinement increases, while the kinetic energy decreases linearly along the jet axis and increases as surface confinement increases.

Effects of Confinement on Molecular Motor-Driven Self-Assembly of Ring Structures

Active transport by kinesin molecular motors has been used to assemble ring nanocomposites comprised of biotinylated microtubules and streptavidin-coated quantum dots. Here we studied the effects of two-dimensional surface confinement on ring self-assembly using substrates of microfluidic channels or periodic post arrays. Microfabricated devices were composed of gold–silicon oxide surfaces where the gold surfaces were functionalized with thiol-based self-assembled monolayers, which enabled selective adsorption of kinesin to silicon surfaces. Confinement of ring self-assembly within microfluidic channels was observed as a change in the distribution of ring diameters, specifically by placing an upper limit on the diameter capable of forming in the channels. Confining assembly using periodic post arrays where the edge-to-edge spacing was 2 μm resulted in a significantly smaller average diameter when compared against those formed in arrays with 5 and 10 μm spacing. Differences in diameters formed in 5 and 10 μm arrays were not observed. Observations of ring composite assembly along channel edges on the top surface, as well as around posts in the arrays confirm the fundamental role of active transport-induced mechanical strain in initiating the self-assembly process.

Molecular Machines Drive Smart Drug Delivery

mechanical stress. The diameters of the pores can be tuned between 2 and 10 nm to allow for different drug loadings. Compared with traditional oral or injectable drugs, these meso­ porous nanoparticles can enhance bioavailabil­ ity, improve therapeutic efficacy and minimize side effects [4]. At present, the key challenge in developing mesoporous nanoparticle­based DDS is designing the structure and surface properties of these nanoparticles to enable the targeted, on­demand release of drug molecules. Molecular machines (e.g., rotaxanes, pseu­ dorotaxanes and azobenzenes) are capable of delivering efficient actuations at dramatically reduced length scales when compared with tra­ ditional microscale actuators [5–10]. Their ability to precisely and cooperatively control mechani­ cal motions at the molecular level, in addition to their comparable sizes to the mesopores of nanoparticles, makes molecular machines prime candidates for controlled release systems in mesoporous nanoparticle­based drug delivery. Molecular machines are enabling timed and controlled release of therapeutic compounds from the mesoporous nanoparticles upon vari­ ous forms of external stimuli, such as chemi­ cal reduction/oxidation, light, magnetic fields, enzymes and changes in the surrounding pH [11]. This area of research is swiftly maturing with the progress in synthesis, surface function­ alization, operation and characterizations of molecular machines. To fulfill their potential in DDS, molecular machines must retain the switching capabil­ ity and full range of mechanical motion when transferred from solution to nanoparticle sur­ faces. Extensive research on molecular machines assembled on metallic nanoparticles reveals multiple effects of surface confinement on the switching and motions of molecular machines, In his historic lecture to the American Physical Society in 1959, ‘There is plenty of room at the bottom’, Richard Feynman, one of the greatest scientists of the 20th Century, shared a futur­ istic vision with his audience using molecular machines in medicine. He contemplated, “What would be the utility of such (molecular­scale) machines? ... Although it is a very wild idea, it would be interesting in surgery if you could swallow the surgeon. You put the mechanical surgeon inside the blood vessel and it goes into the heart and ‘looks’ around. It finds out which valve is the faulty one and takes a little knife and slices it out. Other small machines might be per­ manently incorporated in the body to assist some inadequately functioning organ” [1]. As of today, the small machine­powered surgeon predicted by Feynman has yet to be realized; however, the idea of using molecular­level machines in nanomedi­ cine is burgeoning just as predicted [2,3]. Thus far, scientists have made significant progress in devel­ oping molecular­machine­driven, mesoporous nanoparticle­based smart drug delivery systems (DDS) that are capable of timely, on­demand drug release. By combining the robustness of mesoporous nanoparticles and the preciseness of molecular machines, these smart DDS hold great promise for future nanotechnology­based medical therapy and treatment.

Path of ordering under confinement in alloy films

The effects of surface confinement on the kinetics of ordering in fcc alloy films are studied by the Monte Carlo method. Under weak surface confinement, there are two kinds of kinetic path for alloy films with even layer. One is the films directly relaxing to the equilibrium state, the other is the films relaxing to a metastable state first. Even for the same initial atomic configuration, the kinetic path of ordering is randomized, and the possibility that the films relax to a metastable state is independent with the initial atomic configurations. We attribute this randomicity to the competition between surface confinement and chemical ordering. The evolution of the metastable state to the equilibrium state is also simulated. The results indicate that an alloy film with weak surface segregation can easily be trapped into metastable state.

Tailwater effects on the characteristics of a square jet near a free-surface

Characteristics of a square jet near a free surface were experimentally studied at four different tailwater depths. Velocity measurements were made with a two-component laser Doppler anemometer at five stations along the jet streamwise axis. A qualitative assessment was made of the location of jet impingement on the free surface. The results reveal that the tailwater depth has little influence on the mean velocity distribution beyond a certain depth. Turbulence intensities and shear stress profiles indicate jet anisotropy and higher spread rates than an axi-symmetric free jet. The analysis of third-order moments divulged a marked increase in the turbulent ejection and entrainment events for the jet as the tailwater depth increased and the effect of free surface confinement was reduced. A quadrant analysis confirmed that the confinement effect due to the proximity of the free surface acted to curtail the ability of the jet to eject or entrain fluid.

Salt Permeation and Exclusion in Hydroxylated and Functionalized Silica Pores

We use combined ab initio molecular dynamics (AIMD), grand canonical Monte Carlo, and molecular dynamics techniques to study the effect of pore surface chemistry and confinement on the permeation of salt into silica nanopore arrays filled with water. AIMD shows that 11.6 A diameter hydroxylated silica pores are relatively stable in water, whereas amine groups on functionalized pore surfaces abstract silanol protons, turning into NH3+. Free energy calculations using an ab initio parametrized force field show that the hydroxylated pores strongly attract Na+ and repel Cl- ions. Pores lined with NH3+ have the reverse surface charge polarity. Finally, studies of ions in carbon nanotubes suggest that hydration of Cl- is more strongly frustrated by pure confinement effects than Na+.

Effect of Surface Immobilization on the Electrochemiluminescence of Ruthenium-Containing Metallopolymers

The effect of surface confinement on the electrochemiluminescence (ECL) properties of metallopolymer [Ru(bpy)2(PVP)10]2+, where bpy is 2,2'-bipyridyl and PVP is poly(4-vinylpyridine), is reported. Immobilizing a luminescent material on an electrode surface can substantially modulate its photophysical properties. Significantly, our study revealed that the overall efficiency of the ECL reaction for the metallopolymer film is almost four times higher, at 0.15%, than the highest value obtained for [Ru(bpy)2(PVP)10]2+ dissolved in solution, (phi(ECL) = 0.04%). Electrochemistry has been used to create well-defined concentrations of the quencher Ru3+ within the film. Analysis of both the steady-state luminescence and lifetimes of the film reveals that static quenching by electron transfer between the photoexcited Ru2+ and the Ru3+ centers is the dominant quenching mechanism. The bimolecular rate of electron transfer is (2.5 +/- 0.4) x 10(6) M(-1) s(-1). The implications of these findings for ECL-based sensors, in terms of optimum luminophore loading, is considered.

Modifications of the equation for gas bubble dynamics in a soft elastic medium

A model equation for the oscillation of a pressurized gas bubble in a nonlinear incompressible elastic medium [Emelianov et al., J. Acoust. Soc. Am. 115, 581 (2004)] is extended to include effects of surface tension, viscosity, weak compressibility, and confinement by an elastic shell. The significance of this work is that starting from first principles, the full nonlinearity of the incompressible elastic medium surrounding the bubble and forming its shell is taken into account. Measurements of equilibrium radius as a function of external pressure for a gas bubble in a tissue-like gel are also presented. A general approach to including hysteresis is also discussed.

A surface forces platform for dielectric measurements

Methods are described to implement dielectric spectroscopy (frequency range 10−1–106 Hz) within a surface forces apparatus by using as electrodes silver sheets on the backside of mica. These methods are applied to study the competitive effects of surface alignment, confinement, and shear field on 5CB (5-cyanobiphenyl), a nematic liquid crystal at the experimental temperature of 25 °C. In the planar orientation, films could be squeezed to a minimum thickness of ≈5 Å, the molecule’s thickness. In the perpendicular (homeotropic) orientation, films could be squeezed to ≈25 Å, the expected thickness of the head-to-tail 5CB dimer. It was difficult to discuss responses at f>105 Hz quantitatively because the peak was not visible in the experimental frequency window. Nonetheless, the onset of the relaxation mode for the planar oriented molecules appears at higher frequency than for the homeotropic orientation. A slower relaxation mode, peaked at f≈10 Hz, was assigned to electrode polarization due to the mobility of trace ions within the 5CB samples although these samples were >99.7% pure. The peak frequency was a factor of 3 slower with homeotropic than planar alignment and, in both cases, independent of film thickness except when the film thickness exceeded 10 μm. This was explained using a simple model based on the assumption that trace ions move to oppositely charged electrodes to form electric double layers. A small influence of shear on the dielectric response was observed but only when the dielectric response was measured at the same frequency as the large-amplitude shear deformation. Also described is the use of capacitance to measure force–distance profiles.

Electron behavior and photoelectric gas-sensitive characters of nanocrystalline La1−xSrxFeO3

Abstract Electron transport types and photovoltage gas-sensitive characters of nano-La1−xSrxFeO3 were probed by the vacuum adsorptive surface photovoltage technique. The results show that the band distortion and the increased anion vacancy caused by doping with a little Sr2+ ions in the nanomaterial are of advantage to form new exciton levels and the localized photogenic free charge carriers, to promote the separation of exciton pairs with increasing sensor gas pressure, and to increase quantum-optical efficiency as result of the effect of surface and quantum confinement.

Spectral properties and persistent currents of small metallic clusters

The effects of surface roughness and confinement on the spectral properties of bidimensional metallic loops threaded by a magnetic flux are analyzed. At half filling, the existence of localized border states is shown. At zero flux, these states are degenerated with energy very close to zero. They belong to different conduction channels and as a consequence, a coherent contribution to the persistent current is obtained.

In situ X-ray diffraction studies of a multilayered membrane fluid under confinement and shear

The structure of a fluid membrane system composed of surfactant-co-surfactant-oil-water mixtures has been investigated under confinement and shear conditions. Small angle x-ray scattering (SAXS) was employed with a second generation x-ray surface forces apparatus (XSFA-II) to study the time evolution of the orientational structure of the lamellar fluid under oscillatory shear. In a regime of relatively big confinement gap (∼800 μm) and small shear amplitude (∼40 μm), direct evidence of an “orientational phase separation” behavior, where a surface boundary layer adopts different orientation and separates from the bulk region, was observed for the first time. Under continuous shearing, the surface boundary layer grows in thickness and aligns towards a shear-favored (low friction) state while the bulk orientation remains unchanged. To further investigate the effects of surface confinement, we spatially mapped, in ∼1 μm sections, the orientation structure of the lamellar fluid sample confined between two glass surfaces using a micro-focused x-ray beam produced by a linear Bragg–Fresnel lens at the Advanced Photon Source. The data confirmed the expected trend that the smectic domains align progressively better with respect to the surface as they approach the surface.

Surface electronic states and photovoltage gas-sensitive characters of nanocrystalline LaFeO 3

The electron transport types and photovoltage gas-sensitive characters of nano-LaFeO 3 surface adsorbed by the active gas are probed by surface photovoltage and electric field induced surface photovoltage techniques. The results show that nano-LaFeO 3 has the effects of surface and quantum confinement, which results in producing a new surface state band ca. 0.61 eV at the top of the valence band and the photogenic exciton pair in relevant transition. A plot of optimum phase of SPV response versus particle size of the nanocrystalling material is obtained.

Studies on photoelectric gas-sensitive characters of nanocrystalline LaFeO 3

The effects of surface and quantum confinement on nano-LaFeO 3 surface adsorbed by the sensor gas are probed by vacuum adsorptive surface photovoltage technique. The quantum confinement effect depends exponentially on the pressure of the sensor gas. There are two main ways of charge transfer transition on the adsorbed nano-LaFeO 3 surface.