Strong Confinement Conditions Research Articles

A Monte Carlo simulation study of a Janus discotic liquid crystal droplet

The study of discotic liquid crystals (DLCs) under spherical confinement has gained considerable significance due to its relevance in the design and optimization of advanced materials with tailored properties. The unique characteristics of DLC fluids, coupled with confinement within a spherical Janus surface, offer a compelling avenue for exploring novel behaviors and emergent phenomena. In this study, Monte Carlo simulations within the NpT ensemble are employed to investigate the behavior of a DLC fluid confined by a spherical Janus surface. The Janus surface is characterized by distinct hemispheres, with one promoting homeotropic (face-on) anchoring and the other planar (edge-on) anchoring. Our analysis reveals the emergence of two topological defects: one exclusively on the edge-anchoring hemisphere and the other at the boundary of both anchorings. Each topological defect possessing a topological charge of k = +1/2. We observe that as the temperature transitions the central region of the droplet into a nematic phase, a disclination line forms, linking the two surface defects. By investigating droplets of three different sizes, we confirm that the isotropic-nematic transition is first-order for the larger droplet studied. However, this transition becomes continuous under strong confinement conditions. In contrast, the nematic-columnar transition remains first order even for smaller systems.

Light-induced edge-excitation of two-dimensional surface polaritons in atomically thin crystals

We theoretically investigate the light-induced excitation of two-dimensional surface polaritons (2DSPs) at the edge of atomically thin crystals. Through our analytic model based on the coupled mode theory, we achieve a precise and quantitative description of light interaction with the crystal's edge. Our model unveils three notable properties associated with the edge-excitation of 2DSPs. Firstly, under strong confinement conditions (i.e., polariton momentum significantly surpassing free-space photon momentum), the amplitude of edge-excited 2DSPs exhibits a cosine dependency on the incident angle. Secondly, the amplitude reaches a saturation point at a specific value as polariton momentum increases, depending on the incident angle. Lastly, we demonstrate that the edge-excitation of 2DSPs entails the generation of induced unbounded waves near the edge, leading to peak oscillations due to the interference between the 2DSPs and the unbounded waves.

Anomalous Colloidal Motion under Strong Confinement.

Diffusion of biological macromolecules in the cytoplasm is a paradigm of colloidal diffusion in an environment characterized by a strong restriction of the accessible volume. This makes of the understanding of the physical rules governing colloidal diffusion under conditions mimicking the reduction in accessible volume occurring in the cell cytoplasm, a problem of a paramount importance. This work aims to study how the thermal motion of spherical colloidal beads in the inner cavity of giant unilamellar vesicles (GUVs) is modified by strong confinement conditions, and the viscoelastic character of the medium. Using single particle tracking, it is found that both the confinement and the environmental viscoelasticity lead to the emergence of anomalous motion pathways for colloidal microbeads encapsulated in the aqueous inner cavity of GUVs. This anomalous diffusion is strongly dependent on the ratio between the volume of the colloidal particle and that of the GUV under consideration as well as on the viscosity of the particle's liquid environment. Therefore, the results evidence that the reduction of the free volume accessible to colloidal motion pushes the diffusion far from a standard Brownian pathway as a result of the change in the hydrodynamic boundary conditions driving the particle motion.

Quantitative prediction of elongation deformation and shape relaxation of a red blood cell under tensile and shear stresses

The characteristic time of stress relaxation is a key viscoelastic property of cell membrane that controls time-dependent processes such as shape recovery. Although many experimental studies have been devoted to the measurement of characteristic relaxation time, considerable uncertainty still stands because existing methods rely on different experimental designs and analyses. Here, we present a mesoscopic computational study to investigate the elastic deformation and relaxation characteristics of an isolated red blood cell (RBC) under both tensile and shear stresses. We examine the elastic response and relaxation behavior of the RBC under static tensile stretching and dynamic shear stress. When the cell deformation index responding fluid shear stress is equivalent to the one responding external tensile stretching, we find that the characteristic relaxation time for the RBC in planar flows is longer than that for the RBC under tensile stretching. We also subject the RBC to confined tube/channel flows to probe the effect of geometric confinement on its elastic deformation and relaxation dynamics. Our simulations show that the computed characteristic relaxation time is further increased when compared to those obtained under tensile stretching or planar flows, indicating that the confinement would slow down the cell relaxation process, especially under strong confinement conditions. These findings may facilitate a better understanding of variable relaxation time observed in different experiments.

Anisotropic reduced diffusion in dilute liquid 3He–4He mixture in ordered aerogel

We report on the first observation of 3He diffusion anisotropy in 3He–4He liquid mixture confined in ordered aerogels at 1.5–4.2 K temperatures. The used aerogels are arrays of long Al2O3 parallel 8 nm strands. The possible origins of diffusion anisotropy are considered and the changes of roton properties introduced by parallel aerogel strands are discussed. Among the responsible mechanisms we account for Knudsen diffusion, potential anisotropy of layer mode excitations or of bulklike excitations, and helium vortices. The observed reduced 3He diffusion in aerogels is discussed and suggested to appear due to helium excitations at strong confinement conditions. These observations pave the way for future experiments to gain insight into the crossover regime expected at lower temperatures (below 1 K) for which roton density is lower and 3He collisions with strands play significant role.

Thermo-mechanical modeling of a filled elastomer based on the physics of mobility reduction

The addition of rigid fillers to an elastomeric matrix enhances its mechanical properties. This reinforcement effect is primarily due to a filler network structure in which polymer regions between aggregates play the principal role. In this study, a continuum constitutive equation is formulated for polymer behavior under strong confinement conditions. This behavior can be accounted for by a local glass transition temperature that combines the effects of physical interaction and stress softening in a unique viscoelastic formulation. The model reproduces, at a microscopic scale, the processes governing the Payne effect, including the temperature dependence of the viscoelastic behavior of the filled elastomer reinforcement.

Liquid Crystals under Confinement in Submicrometer Capsules.

Liquid crystal (LC) ordering and phase transition behavior under confined conditions have attracted extensive attention and enabled many applications. However, the ordering and phase transition behavior of LCs in submicrometer capsules have seldom been studied, primarily due to the lack of proper capsulizing and visualization approaches to such small LC microcapsules. Herein, we achieve submicrometer LC capsules with the sizes down to 100 nm by using emulsion-based interfacial sol-gel reaction. The behavior of LCs under the submicrometer confinement conditions is investigated while the sizes and chemical composition of the microcapsule shell surface are tuned in a controllable way. The phase transition temperatures of LCs in the submicrometer capsules shift from those of bulk LCs due to the surface-induced ordering of LCs under the strong confinement conditions, which causes formation of topological defects and alters the order parameter. Using nonlinear optical imaging technology, we explore the structures of director field of LCs that arise as a result of the competition between the surface boundary conditions and LC elasticity. The results show that the nanoscale encapsulation can significantly influence the structural configurations of the director and phase transitions of LCs under various confinement conditions.

Ion-ion correlations across and between electrified graphene layers.

When an ionic liquid adsorbs onto a porous electrode, its ionic arrangement is deeply modified due to a screening of the Coulombic interactions by the metallic surface and by the confinement imposed upon it by the electrode's morphology. In particular, ions of the same charge can approach at close contact, leading to the formation of a superionic state. The impact of an electrified surface placed between two liquid phases is much less understood. Here we simulate a full supercapacitor made of the 1-butyl-3-methylimidazolium hexafluorophosphate and nanoporous graphene electrodes, with varying distances between the graphene sheets. The electrodes are held at constant potential by allowing the carbon charges to fluctuate. Under strong confinement conditions, we show that ions of the same charge tend to adsorb in front of each other across the graphene plane. These correlations are allowed by the formation of a highly localized image charge on the carbon atoms between the ions. They are suppressed in larger pores, when the liquid adopts a bilayer structure between the graphene sheets. These effects are qualitatively similar to the recent templating effects which have been reported during the growth of nanocrystals on a graphene substrate.

Complex dynamics of capillary imbibition of poly(ethylene oxide) melts in nanoporous alumina

Capillary penetration of a series of entangled poly(ethylene oxide) melts within nanopores of self-ordered alumina follows an approximate t1/2 behavior according to the Lucas-Washburn equation; t is the time. However, the dependence on the capillary diameter deviates from the predicted proportionality to d1/2; d is the pore diameter. We observed a reversal in the dynamics of capillary rise with polymer molecular weight. Chains with 50 entanglements (Mw ≤ 100 kg/mol) or less show a slower capillary rise than theoretically predicted as opposed to chains with more entanglements (Mw ≥ 500 kg/mol) that display a faster capillary rise. Although a faster capillary rise has been predicted by theory and observed experimentally, it is the first time to our knowledge that a slower capillary rise is observed for an entangled polymer melt under conditions of strong confinement (with 2Rg/d = 1). These results are discussed in the light of theoretical predictions for the existence of a critical length scale that depends on the molecular weight and separates the microscopic (d < d*) from the macroscopic (d > d*) regime.

Finite particle size drives defect-mediated domain structures in strongly confined colloidal liquid crystals.

When liquid crystals are confined to finite volumes, the competition between the surface anchoring imposed by the boundaries and the intrinsic orientational symmetry-breaking of these materials gives rise to a host of intriguing phenomena involving topological defect structures. For synthetic molecular mesogens, like the ones used in liquid-crystal displays, these defect structures are independent of the size of the molecules and well described by continuum theories. In contrast, colloidal systems such as carbon nanotubes and biopolymers have micron-sized lengths, so continuum descriptions are expected to break down under strong confinement conditions. Here, we show, by a combination of computer simulations and experiments with virus particles in tailor-made disk- and annulus-shaped microchambers, that strong confinement of colloidal liquid crystals leads to novel defect-stabilized symmetrical domain structures. These finite-size effects point to a potential for designing optically active microstructures, exploiting the as yet unexplored regime of highly confined liquid crystals.

Spherically confined H2+: and states

The molecular ion H2+ is studied under strong confinement conditions produced by a spherical barrier centered in the gravity center of the molecule. Results for the potential curves are obtained by diffusion Monte Carlo methods for the ground state (X) and the first excited state (A), and reported as functions of the internuclear distance d for different values of the confinement radius. Results show that the compressed states corresponding to both and present deep minima in their potential curves, due to the increased space for electron wave-functions when the protons are displaced from the barrier surface.

Polymer dynamics in nanoconfinement: Interfaces and interphases

The dynamics of polymers in nanoconfinement was studied by using neutron spectroscopy. A number of pronounced effects on different time and length scales for the polymers confined in nanopores of anodic aluminium oxide were observed. Local segmental dynamics was found to be dependent on the type of the interaction between the solid pore wall and polymer: attractive interactions lead to the formation of a surface layer with the dynamics slowed down as compared to the dynamics of pure polymer; neutral/repulsive interaction do not change the local dynamics. Attractive interactions cause anchoring of polymer segments on the surface creating an interphase between the polymer in close vicinity to the solid surface and pure polymer. In addition, at strong confinement conditions the dilution of the entanglement network is observed.

Hydrodynamic modeling of femtosecond laser ablation of metals in vacuum and in liquid

We numerically examine the mechanisms involved in nanoparticle formation by laser ablation of metallic targets in vacuum and in liquid. We consider the very early ablation stage providing initial conditions for much longer plume expansion processes. In the case of ultrashort laser ablation, the initial population of primary nanoparticles is formed at this stage. When a liquid is present, the dynamics of the laser plume expansion differs from that in vacuum. Low compressibility of the ambient liquid results in strong confinement conditions. As a result, ablation threshold rises drastically, the ablated material is compressed, part of it becomes supersaturated and the backscattered material additionally heats the target. The extension of a molten layer leads to the additional ablation at a later stage also favoring nanoparticle formation. The obtained results thus explain recent experimental findings and help to predict the role of the experimental parameters. The performed analysis indicates ways of a control over nanoparticle synthesis.

Poly(3-hexylthiophene) nanowires in porous alumina: internal structure under confinement

We study the structure of poly(3-hexylthiophene) (P3HT) subjected to nanoscale confinement in two dimensions (2D) as imposed by the rigid walls of nanopore anodic aluminum oxide (AAO) templates. P3HT nanowires with aspect ratios (length-to-diameter) above 1000 and diameters ranging between 15 nm and 350 nm are produced in the pores of the AAO templates via two processing routes. These are, namely, drying a solution or cooling from the melt. Our study focuses on the effects of nanoconfinement on the semicrystalline nature of the nanowires, the orientation of crystals, and the evolution of the structures that P3HT might develop under confinement, which we investigate by combining imaging (SEM), spectroscopic (FTIR, photoluminescence) and structural characterization (WAXS, DSC) techniques. Solution-processed P3HT nanowires are essentially amorphous and porous, whereas melt-processed nanowires are semicrystalline, and present a more compact morphology and smoother surfaces. In the latter case, the orientation of crystals was found to strongly depend on the pore diameter. In large diameter nanowires (250 nm and 120 nm), crystals are oriented laying the π-π stacking direction parallel to the nanowire axis. In contrast, in small diameter nanowires, the π-π stacking direction is mainly perpendicular to the nanowires, as crystals are likely to nucleate at pore walls. The structural evolution of P3HT upon heating into weakly (250 nm in diameter) and strongly (15 nm in diameter) confining pores has been studied. A complex set of structures is observed, i.e., crystals, a solid layered mesophase, a nematic/smectic mesophase, and the isotropic melt. Interestingly, a rare crystal polymorph (form II) is also observed under strong confinement conditions together with the usual lamellar crystal form I. Furthermore, we show that nanoconfinement stabilizes form II: such crystals are still present at 210 °C while in the bulk they get converted to form I crystals at around 50 °C.

Vortices at nanoscale: Still some room at the bottom

Vortices as topological defects existon all scales of Nature from cosmicstrings to a dense quark matter.In macroscopic quantum systemslike superconductors, superfluidsand Bose-Einstein condensatesthe vortices are of special interest.There, they are quantized having“normal” cores where the superfluidorder parameter is zero and itsphase usually changes by 2

Macromolecular Diffusion in a Crowded Polymer Nanocomposite

Macromolecular motion is reduced in crowded polymer nanocomposites. Tracer diffusion is measured for deuterated polystyrene (dPS) into a polystyrene (PS):silica nanoparticle (NP) matrix using elastic recoil detection. This nanocomposite is ideal for studying diffusion in a crowded system because the interparticle distance (ID) that defines confinement can be varied from much greater than to much less than the size of the dPS chain, which is described by 2Rg, the radius of gyration, and varies from 10 to 40 nm in this study. Diffusion is observed to be significantly slower than that predicted by the Maxwell model. The tracer diffusion coefficient of dPS in the nanocomposite relative to the pure PS matrix (D/D0) plotted against the NP separation relative to probe size (i.e., ID/2Rg) falls on a master curve, indicating that crowding is a property of both the dPS size and confinement in the nanocomposite. Moreover, the normalized diffusion coefficient decreases more rapidly when ID/2Rg is less than ∼1, suggesting strong confinement conditions. The scaling of the diffusion coefficient with chain length is in excellent agreement with the entropic barrier model that accounts for the slowing down associated with the loss of chain entropy due to constrictive bottlenecks.

General approach to polymer chains confined by interacting boundaries

Polymer chains, confined to cavities or polymer layers with dimensions less than the chain radius of gyration, appear in many phenomena, such as gel chromatography, rubber elasticity, viscolelasticity of high molar mass polymer melts, the translocation of polymers through nanopores and nanotubes, polymer adsorption, etc. Thus, the description of how the constraints alter polymer thermodynamic properties is a recurrent theoretical problem. A realistic treatment requires the incorporation of impenetrable interacting (attractive or repulsive) boundaries, a process that introduces significant mathematical complications. The standard approach involves developing the generalized diffusion equation description of the interaction of flexible polymers with impenetrable confining surfaces into a discrete eigenfunction expansion, where the solutions are normally truncated at the first mode (the "ground state dominance" approximation). This approximation is mathematically well justified under conditions of strong confinement, i.e., a confinement length scale much smaller than the chain radius of gyration, but becomes unreliable when the polymers are confined to dimensions comparable to their typically nanoscale size. We extend a general approach to describe polymers under conditions of weak to moderate confinement and apply this semianalytic method specifically to determine the thermodynamics and static structure factor for a flexible polymer confined between impenetrable interacting parallel plate boundaries. The method is first illustrated by analyzing chain partitioning between a pore and a large external reservoir, a model system with application to chromatography. Improved agreement is found for the partition coefficients of a polymer chain in the pore geometry. An expression is derived for the structure factor S(k) in a slit geometry to assist in more accurately estimating chain dimensions from scattering measurements for thin polymer films.

Thermoelectric Figure of Merit Calculations for Nanowires –The Moderate Confinement Regime

AbstractWe report room temperature ZT calculations for silicon and indium antimonide nanowires of varying radii. Interestingly, some systems deviate significantly from the anticipated trend of ZT vs. radius. The InSb results are particularly remarkable due to the non-monotonic relationship seen between n-type ZT and wire radius; where typically we expect to see only a decrease with increasing radius, for InSb ZT increases between 20 and 100 nm wire radii. This is thought to be due to the high level of degeneracy of subbands for larger nanowire radii. These results indicate that the monotonic relationship between ZT and wire radius observed under strong confinement conditions cannot be assumed, but must be tested on a case-by-case basis for each materials system.

Antiferroelectric and ferroelectric switching of a liquid crystal in Langmuir-Blodgett films under strong confinement conditions

An antiferroelectric liquid-crystal (LC) material formed of banana-shaped molecules in the sandwich metal-thin Langmuir-Blodgett film-metal geometry, which is typical of solid-state technology, is investigated. Upon heating the thin-film elements, the material passes to the high-temperature (68–127°C) LC B 2 phase, which, despite severe limitation on the film thickness (∼100 nm or less), exhibits antiferroelectric switching, which was previously observed only in bulk samples (10 μm thick) of the LC studied. At film thicknesses smaller than ∼40 nm, ferroelectric switching is observed, which is caused by the stabilization of the ferroelectric LC phase by the boundary surfaces. The largest values of the switched polarization of films (∼400 nC/cm2) are comparable with the polarization of bulk samples. The coercive field increases with decreasing film thickness and reaches ∼106 V/cm for the thinnest films. This value corresponds to the intrinsic coercive field of the ferroelectric under conditions of a strong surface effect, which suppresses domain processes.

Entropy-driven spatial organization of highly confined polymers: lessons for the bacterial chromosome.

Despite recent progress in visualization experiments, the mechanism underlying chromosome segregation in bacteria still remains elusive. Here we address a basic physical issue associated with bacterial chromosome segregation, namely the spatial organization of highly confined, self-avoiding polymers (of nontrivial topology) in a rod-shaped cell-like geometry. Through computer simulations, we present evidence that, under strong confinement conditions, topologically distinct domains of a polymer complex effectively repel each other to maximize their conformational entropy, suggesting that duplicated circular chromosomes could partition spontaneously. This mechanism not only is able to account for the spatial separation per se but also captures the major features of the spatiotemporal organization of the duplicating chromosomes observed in Escherichia coli and Caulobacter crescentus.