Institute of Fundamental Technological Research

The expansion of a plume generated during laser ablation is studied with the Direct Simulation Monte Carlo method. The plume is a mixture of four disparate molecular mass components and expands in vacuum or into ambient gas. The time dependence of deposition rate is studied and the transition from an initial vacuum-like to a diffusion-like regime of expansion in ambient gas is shown. The lack of stoichiometry increases with the ratio of molecular masses of ablated particles and at disparate masses the stoichiometry is seriously affected. Ambient gas worsens the stoichiometry unless it supplies particles compensating the backward and sideward flows of plume constituents.

laser deposition, plume expansion, DSMC

The work presents the results of the simulations of water flows through narrow channels (Poiseuille flows) performed using the molecular dynamics method, for two different channel widths (equal to 5 and 10 diameters of the water molecule) and for two different materials of the channel walls (copper and quartz).
In the simulations, physical properties of the materials and their electrostatic interactions were considered. The obtained results are compared with the analytical solutions for a micropolar fluid flow taking account of the experimentally obtained rheological constants of water.

nanoflows, micropolar fluid, molecular dynamics simulation, nanochannels

The paper presents the results of numerical simulation of processes aimed at production of nanostructures with the use of oil emulsions in water. The appropriate molecular models of water and oil, as well as the model of the substance which would sediment at the water – oil interface, are looked for. Such substance, after suitable solidification, would become the main component of the produced material. For the described simulations, the Molecular Dynamics method has been used throughout this paper.

thin layers, contact surface, nonmixing liquids

Współczesne technologie materiałowe są jedną z najszybciej rozwijających się dziedzin nauki i techniki. Szczególnie prężnie rozwijaj się nano- i mikrotechnologie. Jedna z takich nowoczesnych nanotechnologii, badana obecnie w kilku europejskich ośrodkach, wykorzystuje efekt gromadzenia się substancji na granicy faz emulsji. Użyta emulsja ma bardzo drobną strukturę nano-kropli oleju w wodzie. Trzecia substancja, dzięki odpowiednim właściwościom molekularnym, osiadając na powierzchni styku faz cieczy pokrywa powierzchnie kropel oleju. Po usunięciu emulsji substancja ta, zachowując strukturę, zostaje utwardzona i tworzy nano-materiał. W prezentowanej technologii emulsja spełnia rolę matrycy, na której powstaje struktura wytwarzanego materiału. Technik rozdrabniania emulsji jest wiele; stosuje się m.in. aparaty miksujące (homogenizatory), które w przepływie ścinającym rozrywają krople oleju na mniejsze lub mikrokanały o podobnym działaniu [2]. Nanomateriały o prezentowanej strukturze będą miały wiele interesujących właściwości, takich jak lekkość, elastyczność czy wytrzymałość mechaniczną, co zapewniają silne oddziaływania międzyatomowe w układzie oraz porowatość substancji. Tworzywa o takiej budowie mogą znaleźć zastosowanie w różnych dziedzinach np. medycynie czy aerodynamice. Celem naszych prac nad omawianą technologią wytwarzania nanomateriałów było stworzenie numerycznego modelu zjawiska powstawania nanostruktur w emulsjach. Ponieważ tak drobne układy wymagają modelowania na poziomie atomowym, do opisu procesów zachodzących w cieczach posłużono się metodą Dynamiki Molekularnej. Bazując na symulacjach numerycznych zbudowano modele molekularne cieczy tworzących emulsję oraz zaproponowano kilka typów substancji, które mogłyby wytworzyć pożądaną warstwę na granicy faz. Zaproponowane modele testowano numerycznie, poszukując kombinacji oddziaływań międzyatomowych zapewniającej powstawanie oczekiwanej nanostruktury

nanostruktury, emulsje, nanomateriały

The very serious problem connected with long distance transport of gase-
ous fuels is connected with the fact that detonation may occur inside the duct if some
air leaks into it. Detonation is particularly dangerous for compressors which “push”
the gas through pipelines. It is therefore necessary to use some “detonation dampers”
to protect these compressors.
The commonly used detonation damper has a form of a matrix of narrow chan-
nels, placed across the pipe transporting gas. Detonation wave is supposed to be ex-
tinguished due to cooling by cold walls of these channels. To achieve efficient
damping the channels should be very narrow. Our earlier simulations [1, 2, 3] indi-
cate, that preferable widths should be of the order of 0.005mm. This is not acceptable
for practical reasons – in real applications it is close to 0.5mm. In such channels cool-
ing the gas by heat transfer is inefficient and cannot extinguish the flame (cooling ef-
fect is proportional to perimeter of the cross-section and amount of gas to be cooled
to cross-section surface).
Rarefaction wave generated behind sharp increase of channel cross-section is
an alternative phenomenon, which may be used to cool the burning gas [1]. In our
earlier papers we tested several channel shapes with increase of cross-section [2, 3].
Here we compare channels 0.5mm wide, without and with sharp increases and de-
creases of cross-section. Such channels were found to be the best for practical appli-
cations.

detonation waves, detonation damping, narrow channels

One of the important contemporary technological problems is connected withnecessity of extinguishing detonations, which may occur inpipelines transporting gaseousfuels. To achieve this goal usually a matrix of narrow channels is placed across the flowinside the pipeline. In our recent papers [1], [2] we have shown, that channels with sharpchanges of cross-section should be more efficient in this respect than traditionally usedstraight channels with constant cross-section area. In the present paper we demonstratehow detonation behaves in channels with changes of cross-section under realistic conditions– if the channel cross-section is of dimensions acceptable technologically. At the same timewe take into account the fact, that if friction and heat exchange at the walls are present,gas flowing through the channels accelerates and its densitydecreases considerably. Theresult of our considerations is a selection of, possibly, optimum shape of the channels ofa detonation damper.

One of the important contemporary technological problems is connected with necessity of extinguishing detonation, which may occur in pipelines transporting gaseous fuels. To achieve this goal usually a matrix of very narrow channels is placed inside the pipeline, perpendicularly to its axis. In our recent paper (Walenta and Slowicka (2016)) we have shown, that channels with sharp changes of cross-section should be more efficient in this respect than traditionally used straight channels with constant cross-section area. In this paper we demonstrate how detonation behaves in the channels, in which gas flows under realistic conditions – when friction and heat exchange are present. We take into account the fact, that gas flowing through such channels accelerates and its density decreases considerably.

detonation waves, detonation damping, narrow channels

In the present paper we show the dependence of the shock structure in a dense, noble gas on each of the three non-dimensional parameters: non-dimensional initial density, non-dimensional initial temperature and non-dimensional shock velocity. It will also be demonstrated, that the length scale, most suitable for measuring the thickness of the shock wave in a dense gas, is the sum of the mean free path (calculated the same way as for a dilute gas) and the diameter of a single gas molecule.

The necessity of extinguishing detonation, which may occur in pipelines transporting gaseous fuels, is nowadays a very important technological problem. The standard devices used for it consist of matrices of very narrow channels. Cooling the gas by cold walls of such channels may extinguish the flame and stop detonation. Detonation may also be extinguished if the cross-section of the channel transporting gas increases abruptly at some place. The desired effect may be achieved if the generated rarefaction waves decrease sufficiently the temperature of the flame (Teodorczyk et al. 1988, Cai et al. 2002, Dremin 1999, Walenta et al. 2004). It might be expected, that simultaneous use of both methods – using narrow channels with variable cross-section – should give even better results. Additional profit might come from the fact, that the flow in narrow channels is usually laminar; the abrupt increase of the cross-section would introduce some turbulence and this way enhance cooling by the walls. However, if the cross-section of the channel increases and decreases, the unwanted heating of the gas may occur. To estimate the net esult, which is not obvious, it is necessary to perform suitable simulations and experiments. The present paper is devoted to numerical simulation of the phenomenon.

extinguishing detonation, DSMC

The present paper is a continuation of our earlier work on the
structure of shock waves in dense media. Simple, monoatomic gas, argon, was con
sidered first [1]. It was found, that the shock thickness in dense argon, when re
lated to the mean distance between the molecules (as suggested by P.W. Bridgm
an [2]) decreased with increasing density. To illustrate the dependence of the thickness of the
shock wave on density of the medium, the results for liquid sulfur hexafluoride were
compared with experimental, shock tube results in gas phase at low density.

molecular dynamics simulations, complex liquids, shock waves

In our earlier paper [3] we reported the investigation of the shock structure in dense, monatomic gas - argon. Here we extend our work to dense molecular gases and to liquids. We investigate, in particular, the influence of the electric charges (electric dipoles, quadrupoles etc.) of the molecules on the shock wave structure.

Shock structure, Dense media, Molecular Dynamics

laser ablation, plume expansion, DSMC

shock waves, dense fluids, molecular dynamics simulations

The research, reported in the present paper, was aimed at optimization of devices
extinguishing detonation, which may occur in pipelines transporting gaseous fuels. Such
device, “detonation damper”, is a matrix of narrow channels placed across the pipe. Detonation
is extinguished by heat exchange between the flame and cold walls of the channels.
Improvement of efficiency of the damper was achieved by modification of the channel shape,
so that rarefaction waves, cooling additionally the burning gas, appear in the flow.

Flow Control and Optimisation, Micro- and Nano- flows, Detonation

Pulsed laser deposition is a method frequently used for creating thin films of various materials on
solid substrates. High energy laser pulse causes evaporation of the target material, forming a
plume which subsequently
expands and moves with high speed from the target. Thin film of the
evaporated material is deposited on the substrate placed at some distance in front of the target.
The behavior of the plume influences both the stoichiometry and homogeneity of the deposit
ed
layer
–
the final product of the process. Better understanding of the process of expansion of the
plume, variation of its structure as well as deposition of the material itself is therefore very
important and should give us opportunity for better contro
l of formation of the deposited layer.

laser ablation, plume expansion, DSMC

shock waves, dense fluids, molecular dynamics simulations

shock waves, dense media, molecular dynamics symulations

shock waves, dense fluids, molecular dynamics simulations