Diffusive-to-ballistic transition in grain boundary motion studied by atomistic simulations

Condensed Matter journal club

Diffusive-to-ballistic transition in grain boundary motion studied by atomistic simulations

  • Event time: 11:30am
  • Event date: 24th February 2012
  • Speaker: Elsen Tjhung (Formerly School of Physics & Astronomy, University of Edinburgh)
  • Location: Room 2511,

Event details

Abstract

An adapted simulation method is used to systematically study grain boundary motion at velocities and driving forces across more than five orders of magnitude. This analysis reveals that grain boundary migration can occur in two modes, depending upon the temperature (T ) and applied driving force (P). At low P and T , grain boundary motion is diffusional, exhibiting the kinetics of a thermally activated system controlled by grain boundary self-diffusion. At high P and T , grain boundary migration exhibits the characteristic kinetic scaling behavior of a ballistic process. A rather broad transition range in both P and T lies between the regimes of diffusive and ballistic grain boundary motion, and is charted here in detail. The recognition and delineation of these two distinct modes of grain boundary migration also leads to the suggestion that many prior atomistic simulations might have probed a different kinetic regime of grain boundary motion (ballistic) as compared to that revealed in most experimental studies (diffusional).
PRB vol. 84, article 214102 (2012)
pdf version of paper } }@meeting{html, date = {24th February 2012}, speaker = {Elsen Tjhung}, title = {Osmotic spreading of Bacillus subtilis biofilms driven by an extracellular matrix}, author = {A Seminara, TE Angelini, JN Wilking, H Vlamakis, S Ebrahim, R Kolter, DA Weitz, MP Brenner}, abstract = {Bacterial biofilms are organized communities of cells living in association with surfaces. The hallmark of biofilm formation is the secretion of a polymeric matrix rich in sugars and proteins in the extracellular space. In Bacillus subtilis, secretion of the exopolysaccharide (EPS) component of the extracellular matrix is genetically coupled to the inhibition of flagella-mediated motility. The onset of this switch results in slow expansion of the biofilm on a substrate. Different strains have radically different capabilities in surface colonization: Flagella-null strains spread at the same rate as wild type, while both are dramatically faster than EPS mutants. Multiple functions have been attributed to the EPS, but none of these provides a physical mechanism for generating spreading. We propose that the secretion of EPS drives surface motility by generating osmotic pressure gradients in the extracellular space. A simple mathematical model based on the physics of polymer solutions shows quantitative agreement with experimental measurements of biofilm growth, thickening, and spreading. We discuss the implications of this osmotically driven type of surface motility for nutrient uptake that may elucidate the reduced fitness of the matrix-deficient mutant strains.
PNAS vol. 109(9), pages 1116-1121 (2012)
pdf version with supplimentary information

Authors

Chuang Deng and Christopher A. Schuh