共查询到20条相似文献,搜索用时 15 毫秒
1.
Levitz PE 《Magnetic resonance imaging》2003,21(3-4):177-184
Mesoscopic media such as porous materials or colloidal pastes develop large specific surface area which strongly influence the dynamics of the embedded fluid. This fluid confinement can be used either to probe the interfacial geometry (frozen porous media) or the particle dynamics (paste and colloidal glass). In the strong adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long time pathology (Lévy walks). This phenomena is directly related to the time and space properties of loop trajectories appearing in the bulk between a desorption and a readsorption step. The Lévy statistics extends the time domain of the embedded fluid dynamics toward the low frequency regime. An interesting way to probe such a slow interfacial process is to use field cycling NMR relaxometry. In the first part of this paper, we propose a simple theoretical model of NMR dispersion which only involves elementary time steps of the solvent dynamics near an interface (loops, trains, tails in relation with the confining geometry). In the second part, field cycling NMR relaxometry is used to probe the slow solvent dynamics in two type of interfacial systems: (i) a colloidal glass made of thin and flat particles (ii) two fully saturated porous media, the Vycor glass and MCM48 respectively. Experimental results are critically compared to closed-form analytical expressions and numerical simulations. 相似文献
2.
惯性约束聚变(ICF)冷冻靶中氘氘(D2)、氘氚(DT)等燃料冰层在靶丸中的分布由靶丸所处的温度场决定。在氘氘冷冻靶中,垂直温度梯度引起的气-液界面张力梯度可以抵消重力作用,使氘氘液体在靶丸内均匀分布;然后在氘氘的三相点附近缓慢降温,可以实现燃料冰层的均化。在氘氘冷冻靶均化实验系统上,采用温度梯度结合制冷速率与制冷过程控制的方法,实现了1mm直径、30μm壁厚的辉光放电聚合物(GDP)靶丸中氘氘冰层的均化,对背光阴影图像中亮环位置进行分析表明:氘氘冰层的平均厚度为185.56μm,均匀度为80.2%,模数-功率谱曲线中模数2~100对应的内表面粗糙度为2.26μm。 相似文献
3.
《Superlattices and Microstructures》2001,30(4):169-179
Nanoprobe near-field photoluminescence (PL) of InGaAs(P) dots with quasi-zero-dimensional (quasi-0D) confinement with various degrees of 0D/2D has been investigated by studying probe-induced pressure effects and probe-bias effects. Fine PL peaks of 0D confinement are superimposed on quantum well (QW) peaks for quasi-0D structures, which proves the coexistence of 0D and two-dimensional (2D) confinement in the same layer. Large blue shifts of approximately 100 meV were observed to occur with pressure increase for 0D fine PL peaks, but no shift was observed for the QW peak. The fine 0D peaks were eliminated by larger probe-induced pressures, which should be attributed to carrier diffusion rather than to Γ–X crossover in energy levels. The QW peak increased with the positive probe bias, while 0D fine PL peaks showed a smaller increase with red shifts up to 8–9 meV. The results obtained can be explained by the excitation of immobile excitons in 0D potentials to mobile carriers in the 2D (QW) layer. 相似文献
4.
5.
Pierre Levitz 《Molecular physics》2019,117(7-8):952-959
The confined dynamics of water molecules inside a pore involves an intermittence between adsorption steps near the interface and surface diffusion and excursions in the pore network. Depending on the strength of the interaction in the layer(s) close to the surface and the dynamical confinement of the distal bulk liquid, exchange dynamics can vary significantly. The average time spent in the surface proximal region (also called the adsorption layer) between a first entry and a consecutive exit allows estimating the level of ‘nanowettablity’ of water. As shown in several seminal works, NMRD is an efficient experimental method to follow such intermittent dynamics close to an interface. In this paper, the intermittent dynamics of a confined fluid inside nanoporous materials is discussed. Special attention is devoted to the interplay between bulk diffusion, adsorption and surface diffusion on curved pore interfaces. Considering the nano or meso length scale confinement of the pore network, an analytical model for calculating the inter-dipolar spin–lattice relaxation dispersion curves is proposed. In the low-frequency regime (50?KHz–100?MHz), this model is successfully compared with numerical simulations performed using a 3D-off lattice reconstruction of Vycor glass. Comparison with experimental data available in the literature is finally discussed. 相似文献
6.
Thermophoresis is particle drift induced by a temperature gradient. By measuring the full temperature dependence of this effect for polystyrene latex suspensions, we show that the thermophoretic mobility (or "thermal diffusion coefficient") D(T) is basically independent on particle size, in particular, when the interfacial properties of the colloidal particles are carefully standardized by adsorbing a surfactant layer on the particle surface. Even more, all investigated systems show values of D(T) which are very close to those measured for simple micellar solutions of the adsorbed surfactant. Our findings could be of relevance for downsizing microfluidics to the nanometric range. 相似文献
7.
O. Bénichou D. S. Grebenkov P. E. Levitz C. Loverdo R. Voituriez 《Journal of statistical physics》2011,142(4):657-685
We present an exact calculation of the mean first-passage time to a target on the surface of a 2D or 3D spherical domain,
for a molecule alternating phases of surface diffusion on the domain boundary and phases of bulk diffusion. The presented
approach is based on an integral equation which can be solved analytically. Numerically validated approximation schemes, which
provide more tractable expressions of the mean first-passage time are also proposed. In the framework of this minimal model
of surface-mediated reactions, we show analytically that the mean reaction time can be minimized as a function of the desorption
rate from the surface. 相似文献
8.
Müller J Lupton JM Rogach AL Feldmann J Talapin DV Weller H 《Physical review letters》2004,93(16):167402
We demonstrate a universal correlation between the spectral linewidth and position of the excitonic transition in the spectral jitter observed from single elongated colloidal quantum dots. Breaking the symmetry of electron and hole confinement as well as of the spatial directions for surface charge diffusion enables us to microscopically track meandering surface charges, providing a novel probe of the particle's nanoenvironment. Spectral diffusion exhibits only a weak temperature dependence, which allows us to uncover the single particle homogeneous linewidth of 50 meV at room temperature. 相似文献
9.
Mesoscopic media such as porous materials or colloidal dispersions strongly influence the dynamics of the embedded fluid. In the strong-adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long-time pathology, enlarging the time domain of the embedded-fluid dynamics towards the low-frequency regime. An interesting way to probe such a slow interfacial process is to use the field-cycling NMR relaxometry. This technique is used here to probe the fluid dynamics in two types of interfacial systems: i) a colloidal glass made of thin and flat particles; ii) a fully saturated porous media, the Vycor glass. Experimental results are critically compared to either a simple theoretical model of NMR dispersion involving elementary steps of the fluid dynamics near an interface (loops, trains, tails) or Brownian-dynamics simulations performed inside 3D reconstructions of these confined systems.Received: 1 January 2003, Published online: 14 October 2003PACS:
76.60.Es Relaxation effects - 61.43.Gt Powders, porous materials - 82.70.Dd Colloids 相似文献
10.
M. Grinfeld 《Interface Science》1994,1(3):237-241
The relaxation of internal stresses due to interfacial diffusion in a two-phase solid is studied theoretically with the help of the onsageristic approach of irreversible thermodynamics. In this note we derive an expression for the rate at which internal stresses associated with misfit caused by bonding a flat surface of one material to a rough surface of another. The two phases are treated as isotropic clastic substances. It is assumed that the components of only of the solids are capable of leaving their positions of migrating along the interface. The driving force for this process is minimization of total energy-clastic plus interfacial energy. We show that the time constant for relaxing these stresses is proportional to the cube of the wavelength of the roughness. 相似文献
11.
J.-F. Rupprecht O. Bénichou D. S. Grebenkov R. Voituriez 《Journal of statistical physics》2012,147(5):891-918
We present an exact calculation of the mean first-passage time to a target on the surface of a 2D or 3D spherical domain, for a molecule alternating phases of surface diffusion on the domain boundary and phases of bulk diffusion. We generalize the results of Bénichou et al. in (J. Stat. Phys. 142:657, 2011) and consider a biased diffusion in a general annulus with an arbitrary number of regularly spaced targets on a partially reflecting surface. The presented approach is based on an integral equation which can be solved analytically. Numerically validated approximation schemes, which provide more tractable expressions of the mean first-passage time are also proposed. In the framework of this minimal model of surface-mediated reactions, we show analytically that the mean reaction time can be minimized as a function of the desorption rate from the surface. 相似文献
12.
R. Böhme K. Zimmer B. Rauschenbach 《Applied Physics A: Materials Science & Processing》2006,82(2):325-328
The backside ablation of a absorbing carbon layer onto fused silica is studied in air and water confinement in comparison.
The confinement influences the etch rate and the laser fluence dependence of the etch rate significantly while the threshold
fluence is almost the same. The different confinement of the laser induced plasma results in the observed rate saturation
in the case of air and in a linear growing rate in the case of water confinement at medium laser fluences. The less dense
air confinement permits a faster plasma expansion of the laser plume than in the case of water confinement and effects consequently
the interaction time and interaction strength of the laser plume with the fused silica surface. The differences in the laser-plasma-substrate
interaction cause the observed rate saturation at weak interaction (air) and the growing etch rate at strong interaction (water).
Thus, the confinement situation controls the interaction process in the case of backside ablation and should be considered
in indirect material processing methods such as LIBWE and LESAL, too.
PACS 81.65.C; 81.05.K; 79.20.D; 61.80.B; 42.55.L; 68.45.D 相似文献
13.
J.-M. Zanotti K. Lagrené N. Malikova P. Judeinstein K. Panesar J. Ollivier S. Rols M. Mayne-L’Hermite M. Pinault P. Boulanger 《The European physical journal. Special topics》2012,213(1):129-148
In numerous real life situations, molecular systems are not found in bulk but instead trapped in limited volumes of nanometric size: this is nanometric confinement. The complex interplay of the confinement topology, dimensionality (3D to 1D) and surface/volume ratio significantly affects the physical properties of the confined material. After decades of intense fundamental research, we are now entering a time when the unusual properties of fluids under confinement may be tuned to target specific technological objectives. In this paper, we highlight few situations, all related to the fields of energy production or storage, where diverse neutron scattering techniques (imaging, small angle scattering, diffraction, inelastic and quasi-elastic scattering) may help to bridge basic science and applied research. 相似文献
14.
Self-assembly of block copolymers confined in cylindrical nanopores is studied systematically using a simulated annealing technique. For diblock copolymers which form two-dimensional hexagonally packed cylinders with period L0 in the bulk, novel structures such as helices and stacked toroids spontaneously form inside the cylindrical pores. These confinement-induced morphologies have no counterpart in the bulk system and they depend on the pore diameter (D) and the surface-polymer interactions, reflecting the importance of structural frustration and interfacial interactions. On tightening the degree of confinement, transitions from helices to toroids to spheres are observed. Mechanisms of the morphological transitions can be understood based on the degree of structural frustration parametrized by the ratio D/L0. 相似文献
15.
Dependent on the surface chemistry of gold nanocrystals of equal metal core size, two morphological types of self-organized block copolymer-particle blends are observed: (1) the segregation of the nanocrystals to the interfacial areas or (2) the preferential uniform distribution within one of the respective polymer domains. The confinement of the nanocrystals to the narrow interfacial regions of the microstructure in type one blends results in high local particle filling fractions and gives rise to electromagnetic coupling upon light irradiation, accompanied by a pronounced increase in absorbance. 相似文献
16.
Weiyi Lu Taewan Kim Cang Zhao Xi Chen Yu Qiao 《Applied Physics A: Materials Science & Processing》2013,112(4):885-889
Infiltration of ions in a nanoporous carbon is responsive to the external electric field. If the liquid phase is an aqueous solution of electrolyte, the effective solid-liquid interfacial tension decreases as the voltage rises, similar to the electrowetting phenomenon at a large graphite surface. If the liquid phase is an ionic liquid, however, the effective interfacial tension increases with the voltage. The accessible nanopore volume is not dependent on the electric field. The unique phenomena should be related to the confinement effects of the nanopore inner surfaces. 相似文献
17.
Confining a supercooled liquid to spaces of several
nanometer in diameter can lead to dramatic changes in the relaxation
dynamics of the material. In many cases, the effect is reported as a
confinement induced shift of the glass transition temperature
Tg. Both positive and negative values for ΔT
g have been observed and the length scale of the confining
geometry is considered the main variable. We review the dynamics of
glass-forming liquids in both hard and soft confinement of <10 nm
spaces, with focus on results from solvation dynamics experiments.
It is shown that the interface is instrumental in determinig the
dynamics, giving rise to reaxation time gradients across the
cooperativity length scale of the liquid. Depending on the
interfacial conditions, dynamics can become faster or slower for the
same liquid, same size of confinement, and identical experimental
technique used. No indications of true finite size effects are
observed, and the pore or droplet size is relevant only indirectly
through the relative number of molecules near the surface. 相似文献
18.
Semi-analytical method of calculating the electrostatic interaction of colloidal solutions
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We present a semi-analytical method of calculating the electrostatic interaction of colloid solutions for confined and unconfined systems. We expand the electrostatic potential of the system in terms of some basis functions such as spherical harmonic function and cylinder function. The expansion coefficients can be obtained by solving the equations of the boundary conditions, combining an analytical translation transform of the coordinates and a numerical multipoint collection method. The precise electrostatic potential and the interaction energy are then obtained automatically. The method is available not only for the uniformly charged colloids but also for nonuniformly charged ones. We have successfully applied it to unconfined diluted colloid system and some confined systems such as the long cylinder wall confinement, the air–water interfacial confinement and porous membrane confinement. The consistence checks of our calculations with some known analytical cases have been made for all our applications. In theory, the method is applicable to any dilute colloid solutions with an arbitrary distribution of the surface charge on the colloidal particle under a regular solid confinement, such as spherical cavity confinement and lamellar confinement. 相似文献
19.
介绍了在HT-6M托卡马克上,利用双低杂波的组合,成功地实现了准稳态的高约束模式运行. 能量约束时间提高两倍以上,粒子的约束在较高密度下依然改善3倍以上.通过应用波扩散及 电流径向扩散方程计算低杂波电流传播的方法,对一组典型的数据进行数值模拟.计算表明 ,在HT-6M低杂波实验中,由于纵场较低,密度较高,低杂波的能量沉积在离磁轴较远的位 置,使等离子体电流密度分布成为反剪切位形,内部输运垒地形成,大大提高了等离子体的 约束状况.实验数据也给出了反剪切的证据.
关键词:
托卡马克
低杂波
约束改善 相似文献
20.
We have measured one-dimensional (1D) plasmons in an atom wire array on the Si(557)-Au surface by inelastic scattering of a highly collimated slow electron beam. The angular dependence of the excitation energy clearly indicates the strong 1D confinement and free propagation of the plasma wave along the wire. The observed plasmon dispersion is explained very well by a quantum-mechanical scheme which takes into account dynamic exchange-correlation effects, interwire interactions, and spin-orbit splitting of the 1D bands. Although the qualitative feature of the plasmon dispersion is reminiscent of that of a high-density free-electron gas, we detected the substantial influence of electron correlation due to strong 1D confinement. 相似文献