Conditions Of Spatial Confinement Research Articles

Kinetic simulations of laser-induced plume expansion into a background gas under conditions of spatial confinement

Expansion of vapor plume induced by short-pulse laser irradiation of a bottom of a cylindrical cavity or planar trench in a copper target into helium, argon, and xenon background gases at pressure varying from zero to 1 bar is studied numerically. The simulations are performed based on a hybrid computational model that includes an unsteady heat conduction equation, Hertz-Knudsen model of evaporation, and a kinetic model of two-component gas flow implemented in the form of the Direct Simulation Monte Carlo method. It is found that increasing background pressure results in the formation of a complex structure of shock waves, which drastically changes the flow compared to the case of expansion into a vacuum. The moving shocks strongly affect the deposition of the ablated material inside the cavity. In particular, they induce the formation of a near-surface layer with a reduced fraction of vapor, which protects the cavity wall from vapor redeposition. The overall effect of the increasing background gas pressure on the flow structure and redeposition of vapor inside the cavity can be described as a trade-off between the confinement effect, i.e. a reduction in the propagation speed of the primary shock and overall rate of the plume expansion, and the focusing effect of moving shock waves that induce the radial flow towards the cavity axis, increase vapor density around the axis of symmetry, and reduce vapor density at the cavity wall. The efficiency of vapor removal out of the cavity can increase or decrease with increasing molar mass of the background gas depending on whether the flow is dominated by the focusing or confinement effect. These findings suggest that tuning the background gas parameters and geometrical parameters of spatial confinement can be used to effectively control the laser-induced plume expansion process in applications of laser ablation.

Generation of nanocrystalline surface layer in short pulse laser processing of metal targets under conditions of spatial confinement by solid or liquid overlayer

The effect of spatial confinement by a solid or liquid overlayer on short pulse laser-induced surface microstructure modification is investigated in a series of large-scale atomistic simulations performed for Ag targets irradiated in the regime of melting and resolidification, below the thresholds for laser spallation and ablation. For Ag targets with free surfaces, the formation of a nanocrystalline region with random crystallographic grain orientation is observed under irradiation conditions leading to the generation of numerous sub-surface voids that slow down the solidification process. When no voids are generated, the resolidification produces grains misoriented with respect to the bulk of the target by just several degrees and separated from each other by low angle grain boundaries or dislocation walls. The presence of a liquid or solid overlayer suppresses nucleation of sub-surface voids, provides an additional pathway for cooling through the heat conduction to the overlayer, and facilitates the formation of nanocrystalline structure in a region of the metal target adjacent to the overlayer. Moreover, the stabilizing effect of the solid overlayer may result in an incomplete melting of metal in the vicinity of the interface, making it possible for grains growing from the interface to retain “memory” of the target orientation and to produce nanocrystalline interfacial region with small misorientation of grains with respect to the bulk of the target. In all simulations, the nanocrystalline layers generated by laser processing of single crystal Ag targets are characterized by a high density of stacking faults, twin boundaries, and point defects produced in the course of the rapid resolidification.

Intercalation and Confinement of Poly(ethylene oxide) in Porous Carbon Nanoparticles with Controlled Morphologies

Polymers confined at the nanometer scale often exhibit a distinct structural and dynamical response compared to their bulk counterparts. In this study, we observe that the confinement of poly(ethylene oxide) (PEO) in the nanopores of carbon nanoparticles (CNPs) leads to the suppression of crystallization and to a significant reduction of the ΔCp at the glass transition. We ask whether these changes are dominated by interfacial interactions (van der Waals type) or by geometrical constraints. For pore diameters below 2 nm (micropores following IUPAC nomenclature), we find that the larger the pore surface, the higher the amount of PEO intercalated in the micropores and, consequently, the larger the reduction of the ΔCp at the glass transition (up to 50%). For pore diameters in the range 2–50 nm (mesopores), larger pore surfaces lead to a higher amount of PEO adsorbed on the mesopore walls and the smaller the reduction of the ΔCp at the glass transition. Under these conditions of spatial confinement at the na...

Atomistic simulation study of short pulse laser interactions with a metal target under conditions of spatial confinement by a transparent overlayer

The distinct characteristics of short pulse laser interactions with a metal target under conditions of spatial confinement by a solid transparent overlayer are investigated in a series of atomistic simulations. The simulations are performed with a computational model combining classical molecular dynamics (MD) technique with a continuum description of the laser excitation, electron-phonon equilibration, and electronic heat transfer based on two-temperature model (TTM). Two methods for incorporation of the description of a transparent overlayer into the TTM-MD model are designed and parameterized for Ag-silica system. The material response to the laser energy deposition is studied for a range of laser fluences that, in the absence of the transparent overlayer, covers the regimes of melting and resolidification, photomechanical spallation, and phase explosion of the overheated surface region. In contrast to the irradiation in vacuum, the spatial confinement by the overlayer facilitates generation of sustained high-temperature and high-pressure conditions near the metal-overlayer interface, suppresses the generation of unloading tensile wave, decreases the maximum depth of melting, and prevents the spallation and explosive disintegration of the surface region of the metal target. At high laser fluences, when the laser excitation brings the surface region of the metal target to supercritical conditions, the confinement prevents the expansion and phase decomposition characteristic for the vacuum conditions leading to a gradual cooling of the hot compressed supercritical fluid down to the liquid phase and eventual solidification. The target modification in this case is limited to the generation of crystal defects and the detachment of the metal target from the overlayer.

Advances in the preparation of porous polymer monoliths in capillaries and microfluidic chips with focus on morphological aspects

Porous polymer monoliths have emerged as unique materials for many applications, including liquid-chromatographic analyses at an unrivaled speed, solid-phase extraction, and enzyme immobilization in capillary and microfluidic chip format. This article reviews the state of the art in the preparation of monoliths in narrow-bore capillaries and microfluidic chips and their miniaturization under conditions of spatial confinement. New developments in their preparation mainly using free radical polymerization techniques with a focus on morphological aspects in view of homogeneous porous materials are described. The suitability of monoliths for analysis of both large and small molecules is also discussed.

Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores

We present a neutron and X-rays scattering study of the phase transitions of 4-n-octyl-4' -cyanobiphenyl (8CB) confined in unidirectional nanopores of porous alumina and porous silicon (PSi) membranes with an average diameter of 30 nm. Spatial confinement reveals a rich polymorphism, with at least four different low temperature phases in addition to the smectic A phase. The structural study as a function of thermal treatments and conditions of spatial confinement allows us to get insights into the formation of these phases and their relative stability. It gives the first description of the complete phase behavior of 8CB confined in PSi and provides a direct comparison with results obtained in bulk conditions and in similar geometric conditions of confinement but with reduced quenched disorder effects using alumina anopore membranes.

Direct observation of growth and collapse of a Bose-Einstein condensate with attractive interactions.

Quantum theory predicts that Bose-Einstein condensation of a spatially homogeneous gas with attractive interactions is precluded by a conventional phase transition into either a liquid or solid. When confined to a trap, however, such a condensate can form, provided that its occupation number does not exceed a limiting value. The stability limit is determined by a balance between the self-attractive forces and a repulsion that arises from position-momentum uncertainty under conditions of spatial confinement. Near the stability limit, self-attraction can overwhelm the repulsion, causing the condensate to collapse. Growth of the condensate is therefore punctuated by intermittent collapses that are triggered by either macroscopic quantum tunnelling or thermal fluctuation. Previous observations of growth and collapse dynamics have been hampered by the stochastic nature of these mechanisms. Here we report direct observations of the growth and subsequent collapse of a 7Li condensate with attractive interactions, using phase-contrast imaging. The success of the measurement lies in our ability to reduce the stochasticity in the dynamics by controlling the initial number of condensate atoms using a two-photon transition to a diatomic molecular state.