Dissipative particle dynamics: the equilibrium for finite time steps

Dissipative particle dynamics (DPD) is a relatively new technique which has proved successful in the simulation of complex fluids. We caution that for the equilibrium achieved by the DPD simulation of a simple fluid the temperature depends strongly on the time step. An analytic expression for the dependence is obtained and shown to agree well with simulation results.Comment: 5 pages, LaTeX, 1 Postscript figure, submitted to Europhys.Letts., Algebraic corrections made to final resul

Effect of topology on dynamics of knots in polymers under tension

We use computer simulations to compare the dynamical behaviour of torus and even-twist knots in polymers under tension. The knots diffuse through a mechanism similar to reptation. Their friction coefficients grow linearly with average knot length for both knot types. For similar complexity, however, the torus knots diffuse faster than the even twist knots. The knot-length auto-correlation function exhibits a slow relaxation time that can be linked to a breathing mode. Its timescale depends on knot type, being typically longer for torus than for even-twist knots. These differences in dynamical behaviour are interpreted in terms of topological features of the knots.Comment: 6 pages, 8 figure

Complex dynamics of knotted filaments in shear flow

Coarse-grained simulations are used to demonstrate that knotted filaments in shear flow at zero Reynolds number exhibit remarkably rich dynamic behaviour. For stiff filaments that are weakly deformed by the shear forces, the knotted filaments rotate like rigid objects in the flow. But away from this regime the interplay between between shear forces and the flexibility of the filament leads to intricate regular and chaotic modes of motion that can be divided into distinct families. The set of accessible mode families depends to first order on a dimensionless number that relates the filament length, the elastic modulus, the friction per unit length and the shear rate.Comment: 6 pages, 6 figure

Dynamics of sliding drops on superhydrophobic surfaces

We use a free energy lattice Boltzmann approach to investigate numerically the dynamics of drops moving across superhydrophobic surfaces. The surfaces comprise a regular array of posts small compared to the drop size. For drops suspended on the posts the velocity increases as the number of posts decreases. We show that this is because the velocity is primarily determined by the contact angle which, in turn, depends on the area covered by posts. Collapsed drops, which fill the interstices between the posts, behave in a very different way. The posts now impede the drop behaviour and the velocity falls as their density increases.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let

Rheology of cholesteric blue phases

Blue phases of cholesteric liquid crystals offer a spectacular example of naturally occurring disclination line networks. Here we numerically solve the hydrodynamic equations of motion to investigate the response of three types of blue phases to an imposed Poiseuille flow. We show that shear forces bend and twist and can unzip the disclination lines. Under gentle forcing the network opposes the flow and the apparent viscosity is significantly higher than that of an isotropic liquid. With increased forcing we find strong shear thinning corresponding to the disruption of the defect network. As the viscosity starts to drop, the imposed flow sets the network into motion. Disclinations break-up and re-form with their neighbours in the flow direction. This gives rise to oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure

Tree Streets property survey

As stated in the thesis project, "Increasing demands on decreasing financial resources, both public and private, are causing non-profit agencies to seriously examine their opportunities and threats in the accomplishment of their goals and objectives like never before. Developing, implementing, and monitoring a strategic plan that leads the agency to the successful completion of their mission has taken on new significance. Fundamental to the completion of the various tasks an agency undertakes, is a complete, flexible, and up to date data base. Information has a short shelf life and the need to update information quickly is becoming increasingly important. The purpose of-this project was to create such a data base. Within the delineated geographical boundaries of the Tree Streets neighborhood, residential properties range from the newly restored to the dilapidated. This neighborhood suffers from the highest crime rate in the City of Nashua, as well as associated problems. Key to the restoration of this entire area is a concerted effort by elected representatives, city officials, residents, and many others who share the common goal of a livable, safe, and healthy urban environment that contributes to the quality of life of the City as a whole.Over the last five years the economic profile of ^Nashua has evolved from a deep recession to modest growth level. This current period of growth may offer opportunities to those who are prepared and vigilant of the possibilities that accompany growth. While many factors contribute to the decision making process to pursue opportunities or to let them pass, being adequately prepared to make informed decisions is critical to this process. It's envisioned this project serve as a useful tool for those charged with these responsibilities." (Library-derived description)Yeomans, M. A. (1996). Tree Streets property survey. Retrieved from http://academicarchive.snhu.eduMaster of Science (M.S.)School of Community Economic Developmen

Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces

We report experiments investigating the behaviour of micron-scale fluid droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes. The final droplet shape depends on the droplet size relative to that of the stripes. In particular when the droplet radius is of the same order as the stripe width, the final shape is determined by the dynamic evolution of the drop and shows a sensitive dependence on the initial droplet position and velocity. Numerical solutions of the dynamical equations of motion of the drop provide a close quantitative match to the experimental results. This proves helpful in interpreting the data and allows for accurate prediction of fluid droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui

Turbulence and coarsening in active and passive binary mixtures

Phase separation between two fluids in two-dimensions is investigated by means of Direct Numerical Simulations of coupled Navier-Stokes and Cahn-Hilliard equations. We study the phase ordering process in the presence of an external stirring acting on the velocity field. For both active and passive mixtures we find that, for a sufficiently strong stirring, coarsening is arrested in a stationary dynamical state characterized by a continuous rupture and formation of finite domains. Coarsening arrest is shown to be independent of the chaotic or regular nature of the flow.Comment: 4 pages, 5 figures; discussion on the dependence of the arrest scale on the shear rate has been added; figures have been modified accordingl