On the added mass force at finite Reynolds and acceleration numbers

Numerical simulations of flow around a rigid sphere, subjected to a sudden acceleration (or deceleration) in relative velocity, are considered. Particular attention is paid to the interaction between the imposed sudden acceleration and a preexisting finite Re wake. The results clearly establish the independence of added mass coefficient to the acceleration number and to the state of flow prior to acceleration. A simple reasoning based on the different time scales of the flow is given.     

Modeling and simulation of martensitic phase transitions with a triple point

A framework for modeling complex global energy landscapes in a piecewise manner is presented. Specifically, a class of strain-dependent energy functions is derived for the triple point of Zirconia (ZrO₂), where tetragonal, orthorhombic (orthoI) and monoclinic phases are stable. A simple two-dimensional framework is presented to deal with this symmetry breaking. An explicit energy is then fitted to the available elastic moduli of Zirconia in this two-dimensional setting. First, we use the orbit space method to deal with symmetry constraints in an easy way. Second, we introduce a modular (piecewise) approach to reproduce or model elastic moduli, energy barriers and other characteristics independently of each other in a sequence of local steps. This allows for more general results than the classical Landau theory (understood in the sense that the energy is a polynomial of invariant polynomials). The class of functions considered here is strictly larger. Finite-element simulations for the energy constructed here demonstrate the pattern formation in Zirconia at the triple point.     

A recipe for making materials with negative refraction in acoustics

A recipe is given for making materials with negative refraction in acoustics, i.e., materials in which the group velocity is directed opposite to the phase velocity. The recipe consists of injecting many small particles into a bounded domain, filled with a material whose refraction coefficient is known. The number of small particles to be injected per unit volume around any point x is calculated as well as the boundary impedances of the embedded particles.     

A critical analysis of the glass-forming ability of alloys

Several empirical rules have been proposed during the past few years to synthesize bulk metallic glasses. But, the real reasons for the improved glass-forming ability of these alloys are still not clear and the ability to design alloy compositions to enable synthesis of larger diameter rods has not improved. The present work conducts a critical analysis of the existing data in terms of the different glass-forming criteria and concludes that the available parameters cannot satisfactorily predict the GFA and explain all the observed data. Reasons for this failure have been suggested.     

Analytical model for the magnetophoretic capture of magnetic microspheres in microfluidic devices

Magnetic microspheres are used as mobile substrates in micro-total-analysis systems (μTAS), since the particles can be selectively functionalized to attach different bioconjugates and can be precisely manipulated using external magnetic field gradients. A large number of MEMS-based bio-analytical devices employ magnetophoretic separation as an important step during their operation. An analytical technique is proposed in this paper that describes the magnetophoretic transport of magnetic microspheres under an imposed magnetic field when there is a pressure-driven or electroosmotic flow through a microchannel. Successful magnetophoretic capture occurs if the strength of the field-inducing magnetic dipole exceeds a critical value, or if the particles are larger than a critical size. The magnetophoretic separator performance is characterized in terms of capture efficiency. The analysis shows that the capture efficiency is a function of two independent non-dimensional parameters, λ and γ that in turn involve all the physical design and operating parameters of the microfluidic separator, e.g., the dipole strength, particle size and susceptibility, fluid viscosity and velocity, channel height, and the separation of the dipole. Parametric plots of capture efficiency as function of λ and γ helps in choosing the right design and operation parameter of a practical microfluidic separator for a target level of performance.     

Crystallization behaviors of Al-Ni-La amorphous alloys with trace Ti and B

Effects of the single or simultaneous addition of trace Ti and B (?1 at.%) on the glass-forming ability (GFA) and crystallization behaviors of Al₈₈Ni₆La₆ and Al₈₉Ni₆La₅ amorphous alloys were investigated by X-ray diffraction and differential scanning calorimetry. The addition of trace Ti and B deteriorates the GFA, but improves the thermal stability especially if Ti is added singly. The crystallization onset temperature T_x1 of the first exothermic peak increases by 29.5 and 18.0 K when 1% of the Al atoms in Al₈₈Ni₆La₆ and Al₈₉Ni₆La₅ are replaced by Ti, respectively. However, the primary phase during annealing remains unchanged.     

Using Digital Imaging to Characterize Threshold Dynamic Parameters in Porous Media Based on Lattice Boltzmann Method

Digital images (DI) and lattice Boltzmann method (LBM) are used to characterize the threshold dynamic parameters of porous media. Two-dimensional representations of the porous structure are reconstructed from segmentation of digital images obtained from a series of tiny samples. The threshold pressure gradients and threshold Péclet numbers are researched on seven test samples by using LBM. Numerical results are in agreement with that obtained by integrating Darcy’s law. The results also indicate that fluids can flow through porous media only if the fluid force is large enough to overcome threshold pressure gradient in porous media. One synthetic case is used to further illustrate the applicability of the proposed technique. In addition, the dynamical rules in our model are local, therefore it can be run on parallel computers with well computational efficiency.     

Chromophores of the green fluorescent protein studied in the gas phase

Absorption of gas-phase biomolecules has been studied at the heavy-ion storage ring ELISA. Here we discuss the absorption characteristics of the chromophores of the Green Fluorescent Protein (GFP). The gas-phase absorption maximum of the deprotonated chromophore (anion form) is at 479 nm. This is almost identical to one of the two absorption maxima of the protein, being at 477 nm, which is ascribed to a deprotonated chromophore in the protein. The protonated chromophore (cation form) has a maximum at 406 nm in the gas phase. We compare the gas-phase results with absorption profiles of GFP and chromophores in liquids, and argue that the absorption characteristics of GFP are mainly ascribed to intrinsic chemical properties of the chromophore. Evidently, the special β-can structure of GFP provides shielding of the chromophore from the surroundings without significantly changing the electronic structure of the chromophore through interactions with amino acid side chains. Received 28 December 2001 Published online 13 September 2002     

Relaxation model of the orientational phase transition in fullerite C60

The model of thermal behavior of a thermoelastic medium is developed in the context of the Landau theory of phase transitions. In the framework of this model, two different problems are considered with allowance for order parameter relaxation: the problem of relatively slow uniform heating (cooling) of the medium under external hydrostatic pressure and the problem of order parameter relaxation at thermal isolation. A finite value of the relaxation constant τ of the order parameter is demonstrated to bring about the heating (cooling) rate dependence of the physical quantities, such as specific heat. The relaxation time of the order parameter is shown to be twice larger than the temperature relaxation time, as a consequence of the Landau expansion of the free energy.