Effects of heat loss and viscosity friction at walls on flame acceleration and deflagration to detonation transition

The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.     

Monte Carlo analysis of polymer translocation with deterministic and noisy electric fields

Polymer translocation through the nanochannel is studied by means of a Monte Carlo approach, in the presence of a static or oscillating external electric voltage. The polymer is described as a chain molecule according to the two-dimensional “bond fluctuation model”. It moves through a piecewise linear channel, which mimics a nanopore in a biological membrane. The monomers of the chain interact with the walls of the channel, modelled as a reflecting barrier. We analyze the polymer dynamics, concentrating on the translocation time through the channel, when an external electric field is applied. By introducing a source of coloured noise, we analyze the effect of correlated random fluctuations on the polymer translocation dynamics.     

Dynamics of polyelectrolyte transport through a protein channel as a function of applied voltage

We study the transport of dextran sulfate through a protein channel as a function of applied voltage. Below 60 mV, the chain's entrance to the pore is hindered by an entropic barrier; above 60 mV, the strong local electric field forces the chain entrance. The effective charge of the polyelectrolyte inside the pore is reduced. We observe two types of blockades which have durations that decrease when the applied voltage increases. The shortest is a straddling time between the polyelectrolyte and the pore; the longest is the translocation time. The translocation time obeys an exponential dependence upon applied voltage.     

Optical guiding of elliptical laser beam in nonuniform plasma

Guiding of laser beam in plasma channel formed by short ionizing laser pulse is studied in this paper. When a delayed pulse propagates through this channel diffraction, refraction and self-phase modulation phenomena results which are not balanced resulting in increase/decrease in beam width. These are studied using direct Variational Technique. In this paper cross-focussing phenomena is not observed. Parameters like beam width and longitudinal-phase delay of elliptical laser beam are also studied. It has been observed that the propagation of semi-major axis and semi-minor axis of elliptical beam are independent leading to oscillating guided beam.     

Translocation of Polymer Chains Through a Channel with Complex Geometries

采用nPERMis (new pruned-enriched rosenbluth method with importance sampling) 算法,研究了高分子链在通道中穿行的力学行为. 在管道穿行的过程中,计算了其作用力,发现进入中间通道的过程其对应的作用力f和第一个单体在x轴方向的位置关系曲线有一个平台(f>0). 由于高分子链的受限减少了高分子链的构象数目和熵,从而增加了其自由能, 因此只有在外力的作用下,高分子链才可以进入中间管道. 当高分子运动到某一位置后,第二个平台开始形成(f<0),这时高分子链自发进入右通道. 这是因为在右通道中高分子链的自由能降低的比左通道中高分子链的自由能升高的快. 右通道中的高分子链自发地拉动左通道中的高分子链. 研究了链长、左、右通道宽度对穿孔有很大影响. 通过这些研究可以详细解释各部分穿行时间不同的原因.     

Fluids absorbed in structured pores

A model of fluids absorbed in a pore with walls patterned with parallel channels is used to demonstrate some of the unity that can be proved to hold between the statistical mechanics of fluids absorbed in structured pores and of fluids adsorbed at unstructured walls and at edges/wedges where walls meet. In particular, the work done to reversibly shear a corrugated pore immersed in liquid is related to the difference in the density profile structure of liquid adsorbed near the edges of the channels. When the channel dimensions are mesoscopic or macroscopic but the minimum pore width is microscopic, statistical mechanics generates remarkable links between the surface tension of planar wall-fluid interfaces or, more generally, the solvation free energy of a planar pore, and the density profile at the sides of a channel wall in the vicinity of edges and wedges.     

复合场下优化产生粒子对能量分布宽度的特性研究

采用计算量子场论的方法,对振荡场加稳恒场的组合外场下真空中正反粒子对的产生特性进行了研究.通过一系列的对比得到当振荡场的宽度减小时,一方面可增加正反粒子对的产生量,另一方面也可减小正反粒子对的能量分布宽度从而得到能量单一性更好的粒子对.同时,通过分析产生量、能量分布宽度与振荡场宽度的关系可得出,仅在一定范围内减小振荡场的宽度可使能量分布更加集中,则能量分布宽度趋于某个极限值.因此,要得到产生量多且能量分布集中的正反粒子对应选择合适的参数,这可为今后的实验设计提供数据参考.     

Dynamics and stability of premixed hydrogen-air flames in square microchannels with wall temperature gradients

The dynamics and stability of premixed hydrogen-air flames in square microchannels with heated walls were investigated through three-dimensional direct numerical simulations. The inlet velocity and equivalence ratio were 1.5 m/s and 0.5. The effect of the wall temperature gradient characteristics on the flame dynamics and stability was examined varying the width and location of the wall temperature gradient for a channel height of 1.5 mm. Five distinct flame modes were observed at different wall temperature profiles: flame with repetitive extinction-ignition (FREI), pulsating flame, laterally oscillating flame, spinning flame, and steady flame modes. Furthermore, transitions between these flame modes were observed for specific inflow and boundary conditions. The effect of the channel height on the flame stability was investigated by varying the channel height from 1.0 mm to 1.677 mm for a fixed wall temperature gradient. As the channel height was increased, four of the flame modes, namely, FREI, laterally oscillating flame, spinning flame, and steady flame modes appeared sequentially. To determine whether this sequential appearance was associated with the variation of the wall heat loss, the maximum wall temperature was changed by small amounts. For a lower wall temperature, the laterally oscillating flame mode transitioned to the FREI mode, and for a higher wall temperature, unstable flame modes such as the FREI and laterally oscillating flame modes disappeared, resulting in stable flame.     

Near-wall conductivity effect under a space-charge-saturated sheath in the Hall thruster

In this paper, we adopt the modified Morozov secondary electron emission model to investigate the influence of the characteristic of a space-charge-saturated sheath near the insulated wall of the Hall thruster on the near-wall conductivity, by the method of two-dimensional (2D) particle simulation (2D+3V). The results show that due to the sharp increase of collision frequency between the electrons and the wall under the space-charge-saturated sheath, the near-wall transport current under this sheath is remarkably higher than that under a classical sheath, and equals the near-wall transport current under a spatially oscillating sheath in order of magnitude. However, the transport currents under a space-charge-saturated sheath and a spatially oscillating sheath are different in mechanism, causing different current density distributions under the above two sheaths, and a great influence of channel width on the near-wall transport current under a space-charge-saturated sheath.     

Channel-facilitated molecular transport across membranes: attraction, repulsion, and asymmetry

Transport of molecules across membrane channels is investigated theoretically using exactly solvable discrete stochastic site-binding models. It is shown that the interaction potential between molecules and the channel has a strong effect on translocation dynamics. The presence of attractive binding sites in the pore accelerates the particle current for small concentrations outside the membrane, while for large concentrations, surprisingly, repulsive binding sites yield the most optimal transport. In addition, the asymmetry of the interaction potential also strongly influences the channel transport. The mechanism underlying these phenomena is discussed using the details of particle dynamics at the binding sites.     

分支通道内液-液自发渗吸规律研究

自发渗吸过程中两相界面在分支通道内的破裂和汇合为强非稳态过程, 难以用经典理论及常规数值计算方法准确描述。本文采用格子玻尔兹曼方法研究孔隙结构分支通道基本单元内的液-液自发渗吸行为, 对比出入口通道大小、两相黏度差异对液-液自发渗吸行为的影响。入口通道的大小对两相界面破裂进入分支通道内的竞争自发渗吸行为起控制作用, 而两相流体黏度控制了自发渗吸整体行为。本研究为复杂孔隙结构内液-液自发渗吸行为定量表征提供研究基础。     

Spectral dynamics studies on the supercontinuum channel of femtosecond laser pulses propagation in air

We present the spectral dynamics investigations of supercontinuum generation from 2.6 TW femtosecond laser pulses propagation in air. The measured spectra in the supercontinuum channel show a large amount of oscillating structure, which is mostly contributed from four-wave mixing, stimulated Raman scattering and the electronic mechanism self-phase modulation. The temporal coherence length is almost the same as 0.45 ps for each spectral component with the same spectral width of supercontinuum that is confirmed with a Michelson interferometer. The spatial coherence property of supercontinuum is measured by a Young’s double slit diffractometer, which demonstrates that supercontinuum preserves a transverse coherence of about 144 μm because of the divergence of the supercontinuum channel. PACS  42.62.Fi; 42.68.Ay; 43.25.Jh     

Translocation of polymers in a lattice model

Voltage-driven polymer translocation is studied by means of a stochastic lattice model. The model incorporates voltage drop over the membrane as a bias in the hopping rate through the pore and exhibits the two main ingredients of the translocation process: driven motion through the pore and diffusive supply of chain length towards the pore on the cis-side and the drift away from the pore on the trans-side. The translocation time is either bias limited or diffusion limited. In the bias-limited regime the translocation time is inversely proportional to the voltage drop over the membrane. In the diffusion-limited regime the translocation time is independent of the applied voltage, but it is rather sensitive to the motion rules of the model. We find that the whole regime is well described by a single curve determined by the initial slope and the saturation value. The dependence of these parameters on the length of the chain, the motion rules and the repton statistics are established. Repulsion of reptons as well as the increase of chain length decrease the throughput of the polymer through the pore. As for free polymers, the inclusion of a mechanism for hernia creations/annihilations leads to the cross-over from Rouse-like behaviour to reptation. For the experimentally most relevant case (Rouse dynamics) the bimodal power law dependence of the translocation time on the chain length is found.     

The effect of localization on interference. I. Calculated intensities for a feasible optical experiment

A simple geometry utilizing a laser-excited atomic beam as light source, and a nearby oscillating mirror, would permit the observation of a two-channel optical interference effect involving photons which can be localized predominantly in one channel by coincidence observations of the recoiling source atom. A sacrifice of the optimum conditions for photon interference is necessary even when photon localization in one channel is accomplished by an observation of the recoil atom. This necessity arises because the width of the slit defining the atomic beam, and with it the important optical source dimension, must be comparable to the optical wavelength to obtain the small uncertainty in initial total momentum needed for significant localization. Quantum mechanical calculations of the coincidence intensity and singles intensity as a function of mirror position are made for a source width of five quarter wavelengths and are compared to a classical optics calculation of the singles intensity. The coincidence intensity calculation, as expected, predicts a smaller interference effect than classical optics due to the photon localization. The singles intensity calculation also predicts the same reduction in the classical interference effect, as a consequence of the orthoganality of the final atom states.     

Molecular Dynamics simulation of a polymer chain translocating through a nanoscopic pore

The detection of linear polymers translocating through a nanoscopic pore is a promising idea for the development of new DNA analysis techniques. However, the physics of constrained macromolecules and the fluid that surrounds them at the nanoscopic scale is still not well understood. In fact, many theoretical models of polymer translocation neglect both excluded-volume and hydrodynamic effects. We use Molecular Dynamics simulations with explicit solvent to study the impact of hydrodynamic interactions on the translocation time of a polymer. The translocation time τ that we examine is the unbiased (no charge on the chain and no driving force) escape time of a polymer that is initially placed halfway through a pore perforated in a monolayer wall. In particular, we look at the effect of increasing the pore radius when only a small number of fluid particles can be located in the pore as the polymer undergoes translocation, and we compare our results to the theoretical predictions of Chuang et al. (Phys. Rev. E 65, 011802 (2001)). We observe that the scaling of the translocation time varies from τ ∼ N ^11/5 to τ ∼ N ^9/5 as the pore size increases (N is the number of monomers that goes up to 31 monomers). However, the scaling of the polymer relaxation time remains consistent with the 9/5 power law for all pore radii.     

Sequence dependence of DNA translocation through a nanopore

We investigate the dynamics of DNA translocation through a nanopore using 2D Langevin dynamics simulations, focusing on the dependence of the translocation dynamics on the details of DNA sequences. The DNA molecules studied in this work are built from two types of bases A and C, which have been shown previously to have different interactions with the pore. We study DNA with repeating blocks A(n)C(n) for various values of n and find that the translocation time depends strongly on the block length 2n as well as on the orientation of which base enters the pore first. Thus, we demonstrate that the measurement of translocation dynamics of DNA through a nanopore can yield detailed information about its structure. We have also found that the periodicity of the block sequences is contained in the periodicity of the residence time of the individual nucleotides inside the pore.     

Computer simulation investigation of diffusion selectivity in graphite slit pores

Equilibrium molecular dynamics simulations are reported of oxygen and nitrogen molecules confined in graphite slit pores. Self- and collective diffusion coefficients have been calculated as a function of pore width, temperature and density for each pure component in the pore space. The aim of this study was to elucidate the mechanism by which oxygen and nitrogen are kinetically separated when air is passed over an adsorbent bed consisting of molecular sieving carbon in the commercial production of oxygen. It was found that a critical pore width exists for each species at which there is a sharp drop in the rate of diffusion (both self- and collective diffusion) of each fluid. The critical pore width is one for which the individual molecules are prevented from rotating freely about one of their axes. The greater length of a nitrogen molecule means that the critical pore width is higher for this species than for oxygen. Consequently, oxygen molecules diffuse substantially faster than nitrogen molecules in the vicinity of the nitrogen critical pore width. From an analysis of correlation functions and their corresponding power spectra it is shown that the restricted rotations, which occur at or below the critical pore width, cause a decoupling of translational and rotational modes, with the net result being a lowering of translational diffusion. The nitrogen critical pore width lies within the range of the mean pore size of most commercial molecular sieving carbons, and so this mechanism may help to explain the high oxygen selectivities reported in the literature.     

非均匀外力对粗粒化DNA穿孔行为影响的模拟研究

通过建立DNA高分子的粗粒化模型,采用分子动力学方法模拟其穿孔行为,研究了不同的孔内非均匀外力对DNA高分子穿孔的影响.外力及高分子链内部势能在分子水平下对单体的综合作用很复杂,某些条件穿孔过程会产生后面粒子超过前面粒子而使高分子链堵塞在孔内的情况.研究还发现,穿孔行为是否成功与孔口力的大小有关,在成功穿孔的情况下,非均匀外力相比于恒力情况穿孔时间至少减少了1/2.这些结果对理解DNA复杂的穿孔机理提供了新的视角.     

Quantum Hall effect in restricted geometries

The confinement of electrons in narrow quasi-two-dimensional conducting channels, modelled with a parabolic well, leads to asymmetric Hall plateaus about complete Landau-level fillings and to saw-toothed oscillations of the dc resistivity _xx as a function of the magnetic field B. The peaks in _xx are displaced to lower B and drastically reduced from their wide-channel values. The peak values of _xx increase with increasing channel width. The corrections to σ_yx for finite channel widths and the response to oscillating electric fields are evaluated.     

Translocation of knotted proteins through a pore

We report the results of molecular dynamics simulations of translocation of knotted proteins through pores. The protein is pulled into the pore with a constant force, which in many cases leads to the tightening of the knot. Since the radius of tightened knot is larger than that of the pore opening, the tight knot can block the pore thus preventing further translocation of the chain. Analyzing six different proteins, we show that the stuck probability increases with the applied force and that final positions of the tightened knot along the protein backbone are not random but are usually associated with sharp turns in the polypeptide chain. The combined effect of the confining geometry of the pore and the inhomogeneous character of the protein chain leads thus to the appearance of topological traps, which can immobilize the knot and lead to the jamming of the pore.