Gelb, Lev D.

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/4958

Lev Geld is an Associate Professor of Material Science and Engineering. Currently he is interested in using molecular simulation to study:

  • First-principles Monte Carlo simulations of phase equilibria at extreme conditions
  • Multiscale simulations of sol-gel materials
  • Cyberinfrastructure for phase-space mapping
  • Capillary phenomena

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      On the Permeability of Colloidal Gels
      (Amer Inst Physics, 2019-01-04) Gelb, Lev D.; Graham, Alan L.; Mertz, Alex M.; Koenig, Peter H.; 0000-0003-0291-5098 (Gelb, LD); Gelb, Lev D.
      We reexamine and refine analytical theories for permeability in colloidal networks, with particular focus on constants and identification of approximations. The new theories are compared against numerical simulations of Stokes flow through the networks and reveal nearly quantitative power-law predictions for both pore size and permeability at low volume fractions, with systematic deviations observed only at high volume fractions. Comparison with two previously published experimental data sets yields mixed results: in one case, very good agreement is found, while in the other, only the scaling is correctly predicted. In fractal gel networks, the permeability is commonly modeled as a power-law function of volume fraction, with the fractal dimension of the network determining the power-law exponent. To quantitatively probe the influence of gel structure on permeability, we investigate this relation in structures generated by diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA) and, for contrast, non-overlapping uniform random dispersions of particles. Geometric analyses are used to determine network pore size distributions, fractal dimensions, and percolation characteristics. High-fidelity simulations of the slow viscous flow of Newtonian fluids are used to obtain first-principles-based velocity and fields and hence network permeabilities. Interestingly, the effective pore size that determines permeability is found to be somewhat larger than that measured by a method based on the insertion of spherical probes. Empirical inclusion of a fractal dimension dependence on volume fraction is found to yield quantitative results for permeabilities over the entire volume fraction range studied, in both DLCA and RLCA materials. Published under license by AIP Publishing.
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      Toward Understanding Weak Matrix Effects in TOF SIMS
      (American Vacuum Society) Gelb, Lev D.; Walker, Amy V.; 0000 0001 3758 9240 (Walker, AV); 0000-0003-0291-5098 (Gelb, LD); Gelb, Lev D.; Walker, Amy V.
      Chemical imaging methods, including imaging mass spectrometry, are increasingly used for the analysis of samples ranging from biological tissues to electronic devices. A barrier to wider adoption of imaging mass spectrometry is the presence of matrix effects which complicate quantitative analysis. Interactions between an analyte molecule and its surroundings (the "matrix") can substantially alter both the yield and type of ions observed. Furthermore, such "intrinsic" effects can be confused with nonlinear response due to detector saturation and other instrument-related complications. As a result, quantitative analyses of time-of-flight secondary ion mass spectrometry (TOF SIMS) data that attempt to account for matrix effects are rare. The authors discuss analysis of such data using maximum a posteriori reconstruction based on physically motivated models, and present progress toward the quantitative extraction of chemical concentration profiles and component spectra in the presence of matrix effects, using mixed self- assembled alkanethiolate monolayers as a test system. The authors demonstrate that the incorporation of matrix effects to lowest order using a series-expansion approach is an effective strategy and that doing so provides improved quantitative performance in measuring surface compositions and can also yield information about interactions between species during the SIMS process. Published by the AVS.
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      Nested Sampling of Isobaric Phase Space for the Direct Evaluation of the Isothermal-Isobaric Partition Function of Atomic Systems
      (American Institute of Physics) Wilson, Blake A.; Gelb, Lev D.; Nielsen, Steven O.; Wilson, Blake A.; Gelb, Lev D.; Nielsen, Steven O.
      Nested Sampling (NS) is a powerful athermal statistical mechanical sampling technique that directly calculates the partition function, and hence gives access to all thermodynamic quantities in absolute terms, including absolute free energies and absolute entropies. NS has been used predominately to compute the canonical (NVT) partition function. Although NS has recently been used to obtain the isothermal-isobaric (NPT) partition function of the hard sphere model, a general approach to the computation of the NPT partition function has yet to be developed. Here, we describe an isobaric NS (IBNS) method which allows for the computation of the NPT partition function of any atomic system. We demonstrate IBNS on two finite Lennard-Jones systems and confirm the results through comparison to parallel tempering Monte Carlo. Temperature-entropy plots are constructed as well as a simple pressure-temperature phase diagram for each system. We further demonstrate IBNS by computing part of the pressure-temperature phase diagram of a Lennard-Jones system under periodic boundary conditions.

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