19 research outputs found
Phase Transitions of Soft Disks in External Periodic Potentials: A Monte Carlo Study
The nature of freezing and melting transitions for a system of model colloids
interacting by a DLVO potential in a spatially periodic external potential is
studied using extensive Monte Carlo simulations. Detailed finite size scaling
analyses of various thermodynamic quantities like the order parameter, its
cumulants etc. are used to map the phase diagram of the system for various
values of the reduced screening length and the amplitude of the
external potential. We find clear indication of a reentrant liquid phase over a
significant region of the parameter space. Our simulations therefore show that
the system of soft disks behaves in a fashion similar to charge stabilized
colloids which are known to undergo an initial freezing, followed by a
re-melting transition as the amplitude of the imposed, modulating field
produced by crossed laser beams is steadily increased. Detailed analysis of our
data shows several features consistent with a recent dislocation unbinding
theory of laser induced melting
Archimedean-like colloidal tilings on substrates with decagonal and tetradecagonal symmetry
Two-dimensional colloidal suspensions subject to laser interference patterns
with decagonal symmetry can form an Archimedean-like tiling phase where rows of
squares and triangles order aperiodically along one direction [J. Mikhael et
al., Nature 454, 501 (2008)]. In experiments as well as in Monte-Carlo and
Brownian dynamics simulations, we identify a similar phase when the laser field
possesses tetradecagonal symmetry. We characterize the structure of both
Archimedean-like tilings in detail and point out how the tilings differ from
each other. Furthermore, we also estimate specific particle densities where the
Archimedean-like tiling phases occur. Finally, using Brownian dynamics
simulations we demonstrate how phasonic distortions of the decagonal laser
field influence the Archimedean-like tiling. In particular, the domain size of
the tiling can be enlarged by phasonic drifts and constant gradients in the
phasonic displacement. We demonstrate that the latter occurs when the
interfering laser beams are not adjusted properly
Phase Transitions of Hard Disks in External Periodic Potentials: A Monte Carlo Study
The nature of freezing and melting transitions for a system of hard disks in
a spatially periodic external potential is studied using extensive Monte Carlo
simulations. Detailed finite size scaling analysis of various thermodynamic
quantities like the order parameter, its cumulants etc. are used to map the
phase diagram of the system for various values of the density and the amplitude
of the external potential. We find clear indication of a re-entrant liquid
phase over a significant region of the parameter space. Our simulations
therefore show that the system of hard disks behaves in a fashion similar to
charge stabilized colloids which are known to undergo an initial freezing,
followed by a re-melting transition as the amplitude of the imposed, modulating
field produced by crossed laser beams is steadily increased. Detailed analysis
of our data shows several features consistent with a recent dislocation
unbinding theory of laser induced melting.Comment: 36 pages, 16 figure
Density-dependent pair interactions in 2D colloidal suspensions
We experimentally determine effective interparticle potentials in
a two-dimensional (2D) colloidal system of charge-stabilized
polystyrene particles at different particle densities ρ.
Density variation is achieved by means of a scanned optical laser
tweezer which serves to create a boundary box for the system. By
changing the size of this boundary box, ρ can be
systematically varied without having to prepare a new system. From
the measured radial distribution functions we can then obtain the
effective pair potentials of the particles. While for low
particle densities perfect agreement with Yukawa-like potentials
is observed, considerable deviations from this form are found at
higher densities. We interpret this result as a many-body effect
produced by macroion screening which is expected to become more
pronounced as the density is increased
Numerical Investigations of Complex Nano-Systems
Quantum effects, structures and phase transitions in Nano-systems have been studied. An overview is given on the results of our computations on atomic wires, clusters, pore condensates, Bose fluids, elastic properties of model colloids and model colloids in external fields
Knocking out the wall: protocols for gene targeting in Physcomitrella patens.
The moss Physcomitrella patens has become established as a model for investigating plant gene function due to the feasibility of gene targeting. The chemical composition of the P. patens cell wall is similar to that of vascular plants and phylogenetic analyses of glycosyltransferase sequences from the P. patens genome have identified genes that putatively encode cell wall biosynthetic enzymes, providing a basis for investigating the evolution of cell wall polysaccharides and the enzymes that synthesize them. The protocols described in this chapter provide methods for targeted gene knockout in P. patens, from constructing vectors and maintaining cultures to transforming protoplasts and analyzing the genotypes and phenotypes of the resulting transformed lines