Carlos I. Mendoza

The 1994 Northridge, California, earthquake: Investigation of rupture velocity, risetime, and high‐frequency radiation

A hybrid global search algorithm is used to solve the nonlinear problem of calculating slip amplitude, rake, risetime, and rupture time on a finite fault. Thirty-five strong motion velocity records are inverted by this method over the frequency band from 0.1 to 1.0 Hz for the Northridge earthquake. Four regions of larger-amplitude slip are identified: one near the hypocenter at a depth of 17 km, a second west of the hypocenter at about the same depth, a third updip from the hypocenter at a depth of 10 km, and a fourth updip from the hypocenter and to the northwest. The results further show an initial fast rupture with a velocity of 2.8 to 3.0 km/s followed by a slow termination of the rupture with velocities of 2.0 to 2.5 km/s. The initial energetic rupture phase lasts for 3 s, extending out 10 km from the hypocenter. Slip near the hypocenter has a short risetime of 0.5 s, which increases to 1.5 s for the major slip areas removed from the hypocentral region. The energetic rupture phase is also shown to be the primary source of high-frequency radiation (1–15 Hz) by an inversion of acceleration envelopes. The same global search algorithm is used in the envelope inversion to calculate high-frequency radiation intensity on the fault and rupture time. The rupture timing from the low- and high-frequency inversions is similar, indicating that the high frequencies are produced primarily at the mainshock rupture front. Two major sources of high-frequency radiation are identified within the energetic rupture phase, one at the hypocenter and another deep source to the west of the hypocenter. The source at the hypocenter is associated with the initiation of rupture and the breaking of a high-stress-drop asperity and the second is associated with stopping of the rupture in a westerly direction.

The rheology of hard sphere suspensions at arbitrary volume fractions : An improved differential viscosity model

We propose a simple and general model accounting for the dependence of the viscosity of a hard sphere suspension at arbitrary volume fractions. The model constitutes a continuum-medium description based on a recursive-differential method where correlations between the spheres are introduced through an effective volume fraction. In contrast to other differential methods, the introduction of the effective volume fraction as the integration variable implicitly considers interactions between the spheres of the same recursive stage. The final expression for the viscosity scales with this effective volume fraction, which allows constructing a master curve that contains all the experimental situations considered. The agreement of our expression for the viscosity with experiments at low- and high-shear rates and in the high-frequency limit is remarkable for all volume fractions.

The rheology of concentrated suspensions of arbitrarily-shaped particles.

We propose an improved effective-medium theory to obtain the concentration dependence of the viscosity of particle suspensions at arbitrary volume fractions. Our methodology can be applied, in principle, to any particle shape as long as the intrinsic viscosity is known in the dilute limit and the particles are not too elongated. The procedure allows to construct a continuum-medium model in which correlations between the particles are introduced through an effective volume fraction. We have tested the procedure using spheres, ellipsoids, cylinders, dumbells, and other complex shapes. In the case of hard spherical particles, our expression improves considerably previous models like the widely used Krieger-Dougherty relation. The final expressions obtained for the viscosity scale with the effective volume fraction and show remarkable agreement with experiments and numerical simulations at a large variety of situations.

Self-assembly of binary nanoparticle dispersions: From square arrays and stripe phases to colloidal corrals

The generation of nanoscale square and stripe patterns is of major technological importance since they are compatible with industry-standard electronic circuitry. Recently, a blend of diblock copolymer interacting via hydrogen bonding was shown to self-assemble in square arrays. Motivated by those experiments we study, using Monte Carlo simulations, the pattern formation in a two-dimensional binary mixture of colloidal particles interacting via isotropic core-corona potentials. We find a rich variety of patterns that can be grouped mainly in aggregates that self-assemble in regular square lattices or in alternate strips. Other morphologies observed include colloidal corrals that are potentially useful as surface templating agents. This work shows the unexpected versatility of this simple model to produce a variety of patterns with high technological potential.

Finite-Fault Analysis of the 2004 Parkfield, California, Earthquake Using Pnl Waveforms

We apply a kinematic finite-fault inversion scheme to Pnl displacement waveforms recorded at 14 regional stations (Δ < 2°) to recover the distribution of coseismic slip for the 2004 Parkfield earthquake using both synthetic Greens func- tions (SGFs) calculated for one-dimensional (1D) crustal-velocity models and empiri- cal Greens functions (EGFs) based on the recordings of a single Mw 5.0 aftershock. Slip is modeled on a rectangular fault subdivided into 2 × 2 km subfaults assuming a constant rupture velocity and a 0.5 sec rise time. A passband filter of 0.1-0.5 Hz is applied to both data and subfault responses prior to waveform inversion. The SGF inversions are performed such that the final seismic moment is consistent with the known magnitude (Mw 6.0) of the earthquake. For these runs, it is difficult to repro- duce the entire Pnl waveform due to inaccuracies in the assumed crustal structure. Also, the misfit between observed and predicted vertical waveforms is similar in char- acter for different rupture velocities, indicating that neither the rupture velocity nor the exact position of slip sources along the fault can be uniquely identified. The pattern of coseismic slip, however, compares well with independent source models derived using other data types, indicating that the SGF inversion procedure provides a general first- order estimate of the 2004 Parkfield rupture using the vertical Pnl records. The best- constrained slip model is obtained using the single-aftershock EGF approach. In this case, the waveforms are very well reproduced for both vertical and horizontal com- ponents, suggesting that the method provides a powerful tool for estimating the dis- tribution of coseismic slip using the regional Pnl waveforms. The inferred slip model shows a localized patch of high slip (55 cm peak) near the hypocenter and a larger slip area (∼50 cm peak) extending between 6 and 20 km to the northwest.

Three-particle correlations in the optical properties of granular composites

The effective dielectric response of a system of spherical inclusions embedded in an otherwise homogeneous matrix has been calculated using, in the Maxwell Garnett formula, a renormalized polarizability instead of the bare one (Phys. Rev. B 38, 5371 (1988)). This renormalized polarizability was given as a functional of the two- and three-particle distribution functions and results were presented for a very crude approximation of the three-particle distribution function. Here we extend these results and analyze the effects of the three-particle correlations by using the best available form of this functional. We compare our results with other theoretical approaches and recent numerical simulations.

Stark effect in a wedge-shaped quantum box

Abstract The effect of an external applied electric field on the electronic ground-state energy of a quantum box with a geometry defined by a wedge is studied by carrying out a variational calculation. This geometry could be used as an approximation for a tip of a cantilever of an atomic force microscope. We study theoretically the Stark effect as function of the parameters of the wedge: its diameter, angular aperture and thickness; as well as function of the intensity of the external electric field applied along the axis of the wedge in both directions; pushing the carrier towards the wider or the narrower parts. A confining electronic effect, which is sharper as the wedge dimensions are smaller, is clearly observed for the first case. Besides, the sign of the Stark shift changes when the angular aperture is changed from small angles to angles θ > π . For the opposite field, the electronic confinement for large diameters is very small and it is also observed that the Stark shift is almost independent with respect to the angular aperture.

Self-assembly of anisotropic soft particles in two dimensions

Abstract.The self-assembly of core-corona discs interacting via anisotropic potentials is investigated using Monte Carlo computer simulations. A minimal interaction potential that incorporates anisotropy in a simple way is introduced. It consists in a core-corona architecture in which the center of the core is shifted with respect to the center of the corona. Anisotropy can thus be tuned by progressively shifting the position of the core. Despite its simplicity, the system self-organizes in a rich variety of structures including stripes, triangular and rectangular lattices, cluster crystals, unusual plastic crystals, and geometrically frustrated phases. Our results indicate that the amount of anisotropy does not alter the lattice spacing and only influences the type of clustering (stripes, micelles, etc.) of the individual particles.Graphical abstract

Electrorheological effect and non-Newtonian behavior of a homogeneous nematic cell under shear flow: Hysteresis, bistability, and directional response

We study the flow of a homogeneous nematic cell under the simultaneous action of an applied electric field and an applied shear flow. Using a hydrodynamic model that describes the response of a flow-aligning nematic liquid crystal (5CB) we obtain the directors configuration and the velocity profile at the steady states. From these results we construct a phase diagram in the electric field vs. shear flow space that displays regions for which the system may have different steady-state configurations of the directors field. The selection of a given steady-state configuration depends on the history of the sample. Due to the competition between shear flow and electric field, the systems viscosity shows a complex non-Newtonian response with regions of shear thickening and thinning. Interestingly, as a consequence of the hysteresis of the system, this response may be asymmetric with respect to the direction of the shear flow. The results also show a moderate electrorheological effect which is also dependent on the history of the sample.

Light propagation and transmission in hybrid-aligned nematic liquid crystal cells: Geometrical optics calculations

The authors present a geometrical approach to calculate the transmission of light in a hybrid-aligned nematic cell under the influence of an applied electric field. Using the framework of geometrical optics they present results for the ray tracing as well as the transmission of light as a function of the applied low frequency voltage. Dispersion effects are included through a wavelength dependent dielectric function. Their results for the transmittance as a function of the applied voltage show oscillations that are in good qualitative agreement with previously obtained experimental measurements.