J.J. McCarthy

Discrete characterization tools for cohesive granular material

Abstract Granular flow is important scientifically as well as industrially. Often, cohesive forces between grains are the norm, rather than the exception; yet, the majority of research in granular materials has been directed at cohesionless materials. Due to the relatively large body of knowledge regarding cohesionless flows, gaining an understanding of the transition from cohesionless to cohesive behavior is of particular interest. In this work, we study systems where the predominant mode of cohesion is due to interstitial liquid (capillary cohesion). Both computations and experiments are used to explore a range of cohesive strengths (from cohesionless to cohesive). We propose two discrete characterization criteria, based on the physical picture of liquid-induced particle-level cohesion, which seem to work well in both static and flowing systems. Finally, we address limitations of this approach and discuss potential extensions to systems dominated by other modes of cohesion.

Computational studies of granular mixing

Particulate systems have proven difficult to probe experimentally in many instances. Simulations of granular flows, and mixing flows in particular, provide a useful means of studying particulate behavior. Mixing flows generate large scale patterns and structures which can be easily visualized. Thus, mixing studies provide a means of indirectly examining granular flows. In this paper we review recent computational studies of tumbler mixing, focusing on two very different, yet complementary, techniques: Particle Dynamics and Lagrangian Simulation. We discuss mixing in different tumbler geometries, as well as segregation and cohesive effects.

Mixing and segregation of granular materials in chute flows

Mixing of granular solids is invariably accompanied by segregation, however, the fundamentals of the process are not well understood. We analyze density and size segregation in a chute flow of cohesionless spherical particles by means of computations and theory based on the transport equations for a mixture of nearly elastic particles. Computations for elastic particles (Monte Carlo simulations), nearly elastic particles, and inelastic, frictional particles (particle dynamics simulations) are carried out. General expressions for the segregation fluxes due to pressure gradients and temperature gradients are derived. Simplified equations are obtained for the limiting cases of low volume fractions (ideal gas limit) and equal sized particles. Theoretical predictions of equilibrium number density profiles are in good agreement with computations for mixtures of equal sized particles with different density for all solids volume fractions, and for mixtures of different sized particles at low volume fractions (nu<0.2), when the particles are elastic or nearly elastic. In the case of inelastic, frictional particles the theory gives reasonable predictions if an appropriate effective granular temperature is assumed. The relative importance of pressure diffusion and temperature diffusion for the cases considered is discussed. (c) 1999 American Institute of Physics.

Conductivity of granular media with stagnant interstitial fluids via thermal particle dynamics simulation

Abstract In this paper, a numerical technique––the thermal particle dynamics method (TPD)––is extended to study heat conduction in granular media in the presence of stagnant interstitial fluids. The method, which generates a multi-particle simulation by explicitly modeling many two-particle interactions, allows bed heterogeneities to be directly included and dynamic temperature distributions to be obtained at the particle-level. Comparison with experimental data shows that TPD yields quantitatively accurate values of the effective thermal conductivity without introducing new adjustable parameters for a wide range of stagnant interstitial media. The model not only sheds light on fundamental issues in heat conduction in particulate materials, but also provides a valuable test bed for existing continuous theories.

Stress effects on the conductivity of particulate beds

Abstract Conduction in particulate materials affects a variety of applications ranging from packed bed and multi-phase reactors to calcining/drying kilns, from the storage of bulk reactive or temperature-sensitive materials to material processing. A multi-scale, multi-physics modeling technique—thermal particle dynamics (TPD)—is used to examine heat conduction through static, two-dimensional beds of granular materials. Results of both experiments and a microstructurally based continuum model compare well with those obtained from TPD simulation. The TPD technique, therefore, may provide a unique test-bed for validating or extending theories of effective properties in granular media by yielding detailed mechanical and thermal information not easily measured in experiments. Moreover, in testing this technique it is found that, at low compressive loads or in high aspect ratio beds, the conductivity of particulate materials becomes highly anisotropic. This suggests that it is possible to dynamically tune the granular microstructure and hence the beds properties.

Fabrication and Characterization of Non-Brownian Particle-Based Crystals

Particle-based crystals have been explored in the literature for applications in molecular electronics, photonics, sensors, and drug delivery. However, much of the research on these crystals has been focused on particles of nano- and submicrometer dimensions (so-called colloidal crystals) with limited attention directed toward building blocks with dimensions ranging from tens to hundreds of micrometers. This can be attributed, in part, to the fact that the underlying thermal effects in these larger systems typically cannot naturally overcome kinetic barriers at the meso- and macroscales so that many of the methods used for nanoscale particle assembly cannot be directly applied to larger components, as they become kinetically arrested in nonequilibrium states. In this work, ultrasonic agitation is being explored as a means of allowing large, non-Brownian microparticles (18-750 μm) to overcome the kinetic barriers to packing in the creation of close-packed, highly ordered, crystalline structures. In addition, we study how the energy input affects bulk particle behavior and describe several new ways to characterize particle-based crystals made from microparticles.

Non-Brownian Particle-Based Materials with Microscale and Nanoscale Hierarchy†

Colloidal crystals are interesting materials owing to their customizable photonic properties, high surface area, and analogy to chemical structures. The flexibility of these materials has been greatly enhanced through mixing particles with varying sizes, compositions, and surface charges. In this way, distinctive patterns or analogies to chemical stoichiometries are produced; however, to date, this body of research is limited to particles with nanoscale dimensions. A simple method is now presented for bottom-up assembly of non-Brownian particle mixtures to create a new class of hierarchically-ordered materials that mimic those found in nature (both in pore distribution as well as stoichiometry). Additionally, these crystals serve as a template to create particle-based inverted crystalline structures with customizable properties.

Discrete Characterization of Cohesion in Gas-Solid Flows

Fluidization and the transport of solid particles either by gravity or by pneumatic means are used in a variety of industrial operations,including fluid catalytic cracking, fluid hydroforming and solid fuel processes such as coal gasification and liquefaction [1]. Despite the fact that a sizeable portion of gas-solid flows are cohesive in nature, the mechanics of cohesive flowing gas-particle systems is still poorly understood, and manipulation/control of the flow variables is still largely done on a trial-and-error basis.Cohesive forces between grains can arise from a variety of sources --such as liquid bridge (capillary) forces, van der Waals forces, orelectrostatic forces -- and may play a significant role in theprocessing of fine and/or moist powders. While recent advances havebeen made in our understanding of liquid-induced cohesion in quasi-static systems at the macroscopic level [2,3], in general, it is still not possible to directly connect this macroscopic understanding of cohesion with a microscopic picture of the particle properties and interaction forces. Moreover, conventional theories on gas-solid flows, make no attempt to distinguish between these modes of cohesion, despite clear qualitative differences (lubrication forces in wet systems or electrostatic repulsion are two good examples).In this work, we extend the previous work on discrete characterization tools of wet granular flows [4], using computations of gas-solid flows, in order to examine the transition from non-cohesive (dry) to cohesive (wet) behavior in gas-solid systems. Gas velocity and bridging liquid surface tension are varied to explore a range of the possible fluidization parameter space and a characterization criterion based on the the physical picture of liquid-induced particle-level cohesion is developed for gas-solid flows. Cohesion between wet particles has been modeled using the concept of liquid bridges. The characterization tool developed, namely the Granular Capillary Number, is tested by measuring both the minimum fluidization velocity as well as the mixing rate in fluidized systems. The systems explored here are traditionally thought to be cohesive but a marked difference is observed as the Granular Capillary Number changes.