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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2020)

Microswimmers are objects that are able to propel themselves actively in fluid environments. The typical motion of bacteria such as E. coli can be described by the run-and-tumble model, which consists of successive relatively straight runs in conjunction with erratic changes of the swimming direction known as tumbles. The process of diffusion of microswimmers in porous media is crucial in many biological and environmental systems, and their motility is critical for processes such as bioinfection, bioremediation and biodegradation. In this thesis, the diffusion of microswimmers in porous media is studied numerically based on the run-and-tumble model. The results show that an increase in volume fraction of the obstacles comprising the porous medium leads to a decrease in effective long-time diffusivity. In cases where obstables are allowed to overlap, the diffusivity drops even more dramatically with volume fraction than in the case of no overlap. The effect of varying the range of pillar dimensions is also studied. In the case of overlapping pillars, it is found that the diffusivity increases slightly when the domain is filled with

    Cover page: Diffusion of Run-and-Tumble Microswimmers in porous media
    • Article
    • Peer Reviewed
    Recent Work (2014)
    We study behavior of the restricted maximum likelihood (REML) estimator under a misspecified linear mixed model (LMM) that has received much attention in recent gnome-wide association studies. The asymptotic analysis establishes consistency of the REML estimator of the variance of the errors in the LMM, and convergence in probability of the REML estimator of the variance of the random effects in the LMM to a certain limit, which is equal to the true variance of the random effects multiplied by the limiting proportion of the nonzero random effects present in the LMM. The aymptotic results also establish convergence rate (in probability) of the REML estimators as well as a result regarding convergence of the asymptotic conditional variance of the REML estimator. The asymptotic results are fully supported by the results of empirical studies, which include extensive simulation studies that compare the performance of the REML estimator (under the misspecified LMM) with other existing methods.
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2021)
      Metal-ceramic interfaces are scientifically interesting and technologically important. However, the transition of chemical bonding character from a metal to a nonoxide ceramic is not well understood. The effects of solute segregation and interfacial structural transitions are even more elusive. In this study, aberration-corrected electron microscopy is combined with atomic-resolution energy-dispersive x-ray and electron energy loss spectroscopy to investigate Ti-, V-, and Cr-segregated WC-Co interfaces as model systems. The experiments reveal the general anisotropic formation of reconstructed trilayer-like superstructures with segregant-specific compositional profiles that facilitate the transition from covalent to metallic electronic structures. Density functional theory calculations confirm the gradual increasing metallicity from WC to Co in the interfacial trilayers via increasing metallic solute concentration. This study uncovers unprecedented details of the sophisticated interfacial superstructures at metal-ceramic interfaces. It sheds light on how a metal transits to a ceramic at a "general" interface with strong segregation.
        Cover page: Interfacial superstructures and chemical bonding transitions at metal-ceramic interfaces
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