Mechanism for Spontaneous Growth of Nanopillar Arrays in Ultrathin Films Subject to a Thermal Gradient

Several groups have reported spontaneous formation of periodic pillar-like arrays in molten polymer nanofilms confined within closely spaced substrates maintained at different temperatures. These formations have been attributed to a radiation pressure instability caused by acoustic phonons. In this work, we demonstrate how variations in the thermocapillary stress along the nanofilm interface can produce significant periodic protrusions in any viscous film no matter how small the initial transverse thermal gradient. The linear stability analysis of the interface evolution equation explores an extreme limit of B\'{e}nard-Marangoni flow peculiar to films of nanoscale dimensions in which hydrostatic forces are altogether absent and deformation amplitudes are small in comparison to the pillar spacing. Finite element simulations of the full nonlinear equation are also used to examine the array pitch and growth rates beyond the linear regime. Inspection of the Lyapunov free energy as a function of time confirms that in contrast to typical cellular instabilities in macroscopically thick films, pillar-like elongations are energetically preferred in nanofilms. Provided there occurs no dewetting during film deformation, it is shown that fluid elongations continue to grow until contact with the cooler substrate is achieved. Identification of the mechanism responsible for this phenomenon may facilitate fabrication of extended arrays for nanoscale optical, photonic and biological applications.Comment: 20 pages, 9 figure

Validation of the granular temperature prediction of the kinetic theory of granular flow by particle image velocimetry and discrete particle model

In order to give a detailed description of the hydrodynamics in large industrial scale fluidized beds, continuum models are required. Continuum models often use the kinetic theory of granular flow (KTGF) to provide closure equations for the internal momentum transport in the particulate phase. In this work the outcome of the continuum model is compared with both an experimental technique and detailed simulations, i.e. particle image velocimetry (PIV) and the discrete particle model (DPM).\ud PIV is used for the measurement of an instantaneous velocity field of the flow in the front plane of a fluid bed. The classical PIV analysis is extended to enable the measurement of the local velocity fluctuations in the interrogation area, i.e. the granular temperature. In the DPM, each particle is tracked individually. In this model detailed collision models can be incorporated, rendering the DPM a valuable research tool to validate the underlying assumptions in the KTGF concerning the particle-particle interactions and the particle velocity distribution functions.\ud The aforementioned experimental and numerical techniques are used to measure the granular temperature distribution around a single bubble rising in a gas-fluidized bed. It was found that the results of PIV and the DPM are very similar. Although the initial bubble shape and size are well predicted by the continuum model, it fails once the bubble has detached from the bottom plate. Further research in the area of KTGF closures is needed to improve the predictions of the TFM

Discrete element modeling and fibre optical measurements for fluidized bed spray granulation

Spout fluidized beds are frequently used for the production of granules or\ud particles through granulation. The products find application in a large variety of\ud applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized\ud beds have a number of advantageous properties, such as a high mobility of the particles,\ud which prevents undesired agglomeration and yields excellent heat transfer properties. The\ud particle growth mechanism in a spout fluidized bed as function of particle-droplet\ud interaction has a profound influence on the particle morphology and thus on the product\ud quality. Nevertheless, little is known about the details of the granulation process. This is\ud mainly due to the fact that the granulation process is not visually accessible. In this work\ud we use fundamental, deterministic models to enable the detailed investigation of\ud granulation behaviour in a spout fluidized bed. A discrete element model is used\ud describing the dynamics of the continuous gas-phase and the discrete droplets and\ud particles. For each element momentum balances are solved. The momentum transfer\ud among each of the three phases is described in detail at the level of individual elements.\ud The results from the discrete element model simulations are compared with local\ud measurements of particle volume fractions as well as particle velocities by using a novel\ud fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were\ud carried out for two different cases using Geldart B type aluminium oxide particles: a\ud freely bubbling fluidized bed and a spout fluidized bed with the presence of droplets. It is\ud demonstrated how the discrete element model can be used to obtain information about the\ud interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Eventually\ud this kind of information can be used to obtain closure information required in more coarse\ud grained model

Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses

Experiments by several groups during the past decade have shown that a molten polymer nanofilm subject to a large transverse thermal gradient undergoes spontaneous formation of periodic nanopillar arrays. The prevailing explanation is that coherent reflections of acoustic phonons within the film cause a periodic modulation of the radiation pressure which enhances pillar growth. By exploring a deformational instability of particular relevance to nanofilms, we demonstrate that thermocapillary forces play a crucial role in the formation process. Analytic and numerical predictions show good agreement with the pillar spacings obtained in experiment. Simulations of the interface equation further determine the rate of pillar growth of importance to technological applications.Comment: 5 pages, 4 figure

Comparison of fibre optical measurements and discrete element simulations for the study of granulation in a spout fluidized bed

Spout fluidized beds are frequently used for the production of granules or particles through granulation. The products find application in a large variety of applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized beds have a number of advantageous properties, such as a high mobility of the particles, which prevents undesired agglomeration and yields excellent heat transfer properties. The particle growth mechanism in a spout fluidized bed as function of particle-droplet interaction has a profound influence on the particle morphology and thus on the product quality. Nevertheless, little is known about the details of the granulation process. This is mainly due to the fact that the granulation process is not visually accessible. In this work we use fundamental, deterministic models to enable the detailed investigation of granulation behaviour in a spout fluidized bed. A discrete element model is used describing the dynamics of the continuous gas-phase and the discrete droplets and particles. For each element momentum balances are solved. The momentum transfer among each of the three phases is described in detail at the level of individual elements. The results from the discrete element model simulations are compared with local measurements of particle volume fractions as well as particle velocities by using a novel fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were carried out for three different cases using Geldart B type aluminium oxide particles: a freely bubbling fluidized bed; a spout fluidized bed without the presence of droplets and a spout fluidized bed with the presence of droplets. It is demonstrated how the discrete element model can be used to obtain information about the interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Eventually this kind of information can be used to obtain closure information required in more coarse grained models

A Rock Classification Schema

The need for engineering data on earth materials for use in site selection, design, construction, and maintenance of major engineering structures is generally accepted. Probably the most pressing need is for such data to use in preliminary considerations of site selection and design alternatives. Maps and(or) surveys giving the areal distribution of earth materials and their characteristics, together with topographic maps available for many areas, would permit much preliminary work on engineering structures to be done without the engineer ever having to leave his office

Nonlinear Dynamics of Capacitive Charging and Desalination by Porous Electrodes

The rapid and efficient exchange of ions between porous electrodes and aqueous solutions is important in many applications, such as electrical energy storage by super-capacitors, water desalination and purification by capacitive deionization (or desalination), and capacitive extraction of renewable energy from a salinity difference. Here, we present a unified mean-field theory for capacitive charging and desalination by ideally polarizable porous electrodes (without Faradaic reactions or specific adsorption of ions) in the limit of thin double layers (compared to typical pore dimensions). We illustrate the theory in the case of a dilute, symmetric, binary electrolyte using the Gouy-Chapman-Stern (GCS) model of the double layer, for which simple formulae are available for salt adsorption and capacitive charging of the diffuse part of the double layer. We solve the full GCS mean-field theory numerically for realistic parameters in capacitive deionization, and we derive reduced models for two limiting regimes with different time scales: (i) In the "super-capacitor regime" of small voltages and/or early times where the porous electrode acts like a transmission line, governed by a linear diffusion equation for the electrostatic potential, scaled to the RC time of a single pore. (ii) In the "desalination regime" of large voltages and long times, the porous electrode slowly adsorbs neutral salt, governed by coupled, nonlinear diffusion equations for the pore-averaged potential and salt concentration

Rock Evaluation for Engineered Facilities

The need for comprehensive information on the characteristics and behavior of earth materials has been recognized for many years, perhaps for as long as significant construction has taken place in and on the surface of the earth. In recent years, however, the magnitude and complexity of engineered construction has greatly increased, resulting in a corresponding increase in the need for information on the engineering properties of soil and rock materials, Direct testing of soil and rock can be utilized to furnish necessary information. However, both field and laboratory testing can be extremely expensive, particularly where testing must include applications of stress to large masses of earth material, For this reason, significant technical and economic advantages can be realized through the development of indirect or short-cut methods for obtaining indications of the properties and characteristics of geologic materials. Some years ago the value of topographic maps, aerial photographs, pedologic descriptions, and geological surveys in characterizing soil materials was realized. To make this information useful for engineering studies, a serious effort was initiated to obtain data on the engineering properties of various soil groups and associations established on the basis of geological and pedological surveys. The correlation of performance data with information on areal distribution and location furnished by geologic and pedologic works has proven extremely valuable in the planning and construction of facilities in and on soil. In recent years, the size and importance of structures and facilities designed by engineers and architects has greatly increased. This has produced an increased interest in the rock materials underlying surficial soil layers. A clear need has arisen for a program to provide an engineering evaluation of rock materials for the purposes of location, design, construction, and maintenance of engineered facilities. However, a serious gap exists in the association of engineering characteristics with rock units identified on the basis of geological classifications, Therefore, there is a need for the development of a comprehensive evaluation program which permits utilization of existing data and which aids in the procurement of necessary information on engineering characteristics of rock

Engineering Data System for Bedrock Occurrences and Properties

The initial work plan included the development of a classification system based on index tests. An investigation of previous works in classification of rock on the basis of index tests showed that a variety of classification systems utilizing many different index tests had been developed. However, this survey showed that no generally applicable system had been developed and that little communication had been established between field investigators, facility designers, and those in charge of construction and maintenance of facilities. Therefore, the initial plan for work was modified to include the development of a comprehensive methodology for evaluation of rock. The development of such an evaluation schema was to include the establishment of an information bank to provide access to collected data by any interested individual. The first step in the development of this rock evaluation program was a survey of the categories of information that have been collected concerning geologic materials, particularly rock strata. On the basis of this investigation of existing data, a method was devised to collect, categorize, and present more extensive data on rock materials. The general schema for the evaluation program was then developed. At the present time, a research effort is continuing to test and verify the validity of the evaluation program which has been developed. A final step in this effort will be a full implementation of the rock evaluation program for project planning in Kentucky

Noise and charge transport in polymer thin-film structures

The low frequency noise (LFN) properties of. the field-effect transistors (FETs) using polymers as the semiconducting material in thin-film transistor (TFT) structures are investigated and discussed in terms of the charge carrier transport. Results obtained from several research groups are summarized. Injection-drift limited model (IDLM) for charge transport in amorphous PFETs is discussed. IDLM has some advantages in comparison to the commonly used metal-oxide-semiconductor (MOS) transistor models. A general trend of proportionality between noise power density and the DC power applied to the polymer FET's (PFET's), channel is observed in the data from several research groups. This trend implies mobility fluctuation in PFET as the dominant noise source