Monte Carlo simulation studies of the translocation of homopolymers of length N driven through a channel have been performed. We find that the translocation time tau depends on temperature in a nontrivial way. For temperatures below some critical temperature theta(c), tau approximately T-1.4, whereas for T>theta(c), tau increases with temperature. The low temperature results are in good agreement with experimental findings as is the dependence of tau on the driving field strength. The velocity of translocation displays the same characteristics as found in experiment but the N dependence of tau shows the linear relationship observed in experiment only for large values of N. A possible reason for this is suggested.
There currently exists a wide range of powerful techniques for probing surfaces, mainly involving the use of electron or ion beams under high- or ultra-high-vacuum conditions. Recently there have been major efforts to develop surface sensitive optical probes that have the inherent advantage that they can be applied in more challenging environments such as in high pressures or under liquids and in real time. The most powerful of these techniques to emerge ( years ago) is reflection anisotropy spectroscopy (RAS), which early on demonstrated its ability to distinguish different reconstructions of GaAs(001) and to detect monolayer-growth-related oscillations similar to those routinely obtained using reflection high-energy electron diffraction. This article describes some aspects of the development of the RAS technique since that time, focusing on our own theoretical and experimental studies concerning the (001) surfaces of cubic semiconductors which have been prepared by molecular beam epitaxy. These studies demonstrate that in surface chemistry, structure and electronic properties RAS has made powerful contributions to the study of such surfaces.
We show that Langevin dynamics can be used to obtain force-extension curves for a single polymer chain under varying solvent conditions. We find that the chains obey Hooke-type and Pincus regime behavior for Theta and good solvents. However, in poor solvents a coil-strand coexistence is observed in the equilibrium state and this leads to a quite different type of deformation behavior.
Summary: Molecular dynamics simulation studies of the translocation of charged homopolymers of length, N, driven by an electric potential gradient through a channel have been performed. We find that the translocation time, τ, displays an inverse power dependence on the temperature of the simulation τ ∼ (T − T0)−7/4, which is in very good agreement with experimental results. In addition, the dependence of τ on the driving field strength and the velocity of translocation on the polymer length N have also been obtained. The results suggest that such minimalist models are useful in modelling biological processes and that the molecular dynamics method is a suitable approach for carrying out these simulations.Snapshot of the polymer during the simulation.imageSnapshot of the polymer during the simulation.
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