Accelerated boundary integral method for multiphase flow in non-periodic geometries

An accelerated boundary integral method for Stokes flow of a suspension of deformable particles is presented for an arbitrary domain and implemented for the important case of a planar slit geometry. The computational complexity of the algorithm scales as $O(N)$ or $O(N\log N$), where N is proportional to the product of number of particles and the number of elements employed to discretize the particle. This technique is enabled by the use of an alternative boundary integral formulation in which the velocity field is expressed in terms of a single layer integral alone, even in problems with non-matched viscosities. The density of the single layer integral is obtained from a Fredholm integral equation of the second kind involving the double layer integral. Acceleration in this implementation is provided by the use of General Geometry Ewald-like method (GGEM) for computing the velocity and stress fields driven by a set of point forces in the geometry of interest. For the particular case of the slit geometry, a Fourier-Chebyshev spectral discretization of GGEM is developed. Efficient implementations employing the GGEM methodology are presented for the resulting single and the double layer integrals. The implementation is validated with test problems on the velocity of rigid particles and drops between parallel walls in pressure driven flow, the Taylor deformation parameter of capsules in simple shear flow and the particle trajectories in pair collisions of capsules in simple shear flow. The computational complexity of the algorithm is verified with results from several large scale multiparticle simulations.     

Persistency of single-stranded DNA: The interplay between base sequences and base stacking

The chain persistency of single-stranded (ss) DNA at a high-salt limit mainly arises from the so-called base-stacking interaction between consecutive bases along the strand. Stacking is appreciable for purine–purine (e.g., A–A) and purine–pyrimidine stacks (e.g., A–T), but it is weak for pyrimidine stacks (i.e., T–T, T–C, and C–C). We study how base stacking can stiffen the strand by classifying bases into two subclasses: stacking pairs (i.e., purine–purine and purine–pyrimidine) and non-stacking (i.e., pyrimidine–pyrimidine) pairs. With this simplification, we develop an exactly solvable model for calculating the stacking-induced persistence length ${\ell }_{\mathit{stack}}$ of heterogeneous ssDNA. It is shown that ${\ell }_{\mathit{stack}}$ is mainly determined by the occurrence rate of purines; intrinsic correlations in real DNA sequences barely influence ${\ell }_{\mathit{stack}}$. Our approach leads to a reasonable estimate of ${\ell }_{\mathit{stack}}\approx 2b$–$3b$ (under typical conditions), where b is the inter-base distance.     

Investigations of half-metallic ferromagnetism and thermoelectric properties of cubic XCrO3 (X = Ca,Sr,Ba) compounds via first-principles approaches

In this paper, the physical aspects of the cubic phase XCrO₃ ($\mathrm{X}=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba}$) perovskites are studied by employing full-potential linearized augmented plane wave plus local orbital ($\text{FP-LAPW}+\mathit{lo}$) method. These compounds have been found stable in ferromagnetic (FM) phase since they possess lower energy in FM phase compared to non-FM phase and their stability is also confirmed by calculating the enthalpy of formation (ΔH). The electronic structures of these compounds are analyzed with Trans and Blaha modified Becke–Johnson potential (TB-mBJ) for both spin up and spin down channels, which indicate their half-metallic characters. Analysis of density of states (DOS) shows major contributions of O-2p states in the valence band and Cr 3d-state in conduction band. A comparative analysis of crystal field effect ($\mathrm{\Delta }{\mathrm{E}}_{\mathrm{crystal}}$) and the exchange energies (direct ${\mathrm{\Delta }}_x(d)$ and indirect ${\mathrm{\Delta }}_x(pd)$) tells about the main part of electronic spin in ferromagnetic character. The calculated magnetic moments make these compounds favorable for spintronic applications. In the end, thermoelectric parameters are computed for 200 K–800 K temperature range to explore potential of these compounds for applications in renewable energy devices.