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Matteo Pasquali

Publications and source records attributed to Matteo Pasquali.

10 recordsLinked to original sources
PubMed2006

Dynamics of individual single-walled carbon nanotubes in water by real-time visualization.

Individual single-walled carbon nanotubes (SWNTs) in aqueous suspension are visualized directly by fluorescence video microscopy. The fluorescent tagging is simple, biocompatible, and does not modify the SWNTs. The dynamics of individual SWNTs in water are observed and quantified for the first time. We measure the confined rotational diffusion coefficient and find it in reasonable agreement with predictions based on confined diffusion of dilute Brownian rods. We determine the critical concentration at which SWNTs in suspensions start interacting. By analyzing the fluctuating shape of SWNTs in the 3 to 5 microm range, we determine that their persistence length ranges between 32 and 174 microm, in agreement with theoretical estimates; thus, commonly available SWNTs in liquids can be considered as rigid Brownian rods in the absence of imposed external fields or self-attractive forces.

Journal Article↗
PubMed2006

Dynamics of polymer translocation through nanopores: theory meets experiment.

The dynamics of translocation of polymer molecules through nanopores is investigated via molecular dynamics. We find that an off-lattice minimalist model of the system is sufficient to reproduce quantitatively all the experimentally observed trends and scaling behavior. Specifically, simulations show (i) two translocation regimes depending on the ratio of pore and polymer length, (ii) two different regimes for the probability of translocation depending on applied voltage, (iii) an exponential dependence of translocation velocity upon applied voltage, and (iv) an exponential decrease of the translocation time with temperature. We also propose a simple theoretical explanation of each of the observed trends within a free energy landscape framework.

Computer Simulation↗
PubMed2006

Isotropic-nematic phase transition of single-walled carbon nanotubes in strong acids.

We present the first quantitative assessment of the maximum amount of nanotubes that can exist in the isotropic phase () of single-walled carbon nanotubes (SWNTs) in Brønsted-Lowry acids. We employ a centrifugation technique in conjunction with UV-vis-nIR spectroscopy to quantify , which is also the critical concentration of the isotropic-nematic transition of SWNTs in strong acids. Centrifugation of biphasic dispersions of SWNTs, that is, acid dispersions consisting of an isotropic phase in equilibrium with an ordered nematic liquid crystalline phase, results in a clear phase separation, where the isotropic phase is supernatant. Dilution of the isotropic phase with a known amount of acid followed by UV-vis-nIR absorbance measurements yields , that is, the maximum concentration of SWNTs that can exist in the isotropic phase in a given acid for a given SWNTs' length distribution. At low SWNT concentration (below 200 ppm) in superacids, light absorbance in the range from 400 to 1400 nm scales linearly with concentration. This Beer's law behavior yields calibration curves for measuring SWNTs' concentration in acids. We find that the critical concentration of the isotropic-nematic transition increases with acid strength in accordance with the previously proposed sidewall protonation mechanism for dispersing SWNTs in acids.

Journal Article↗
PubMed2006

Hemolysis estimation in a centrifugal blood pump using a tensor-based measure.

Hemolysis in the GYRO centrifugal blood pump, under development at the Baylor College of Medicine, Houston, TX, is numerically predicted using the newly proposed tensor-based blood-damage model, as well as a traditional model. Three typical operating conditions for the pump are simulated with a special-purpose finite element-based flow solver, and a novel approach for tracing the pathlines in discretely represented time-varying flow in a complex domain is presented, and 271 pathlines are traced through the pump. Hemolysis is computed along the pathlines, and the accumulated hemolysis at the outflow is converted into standard clinical units. The cumulative hemolysis at the outlet of the pump is weighted with the flow rate associated with the pathlines, and a temporal average is obtained by releasing the tracer particles at different time intervals. Numerical predictions are compared to experimental hemolysis studies performed according to the American Society for Testing and Materials standards at the Baylor College of Medicine. The tensor-based blood-damage model is found to match very well with the experimental results, whereas the traditional model overpredicts the hemolysis. The success of the tensor-based blood-damage model is attributed to its construction, which accounts for blood-specific physical properties and phenomena. Hemolysis values at the typical operating conditions of the pump are found to be within the clinically accepted range.

Computer Simulation↗
PubMed2005

Brownian dynamics algorithm for bead-rod semiflexible chain with anisotropic friction.

A model of semiflexible bead-rod chain with anisotropic friction can mimic closely the hydrodynamics of a slender filament. We present an efficient algorithm for Brownian dynamics simulations of this model with configuration dependent anisotropic bead friction coefficients. The algorithm is an extension of that given previously for the case of configuration independent isotropic friction coefficients by Grassia and Hinch [J. Fluid Mech. 308, 255 (1996)]. We confirm that the algorithm yields predicted values for various equilibrium properties. We also present a stochastic algorithm for evaluation of the stress tensor, and we show that in the limit of stiff chains the algorithm recovers the results of Kirkwood and Plock [J. Chem. Phys. 24, 665 (1956)] for rigid rods with hydrodynamic interactions.

Journal Article↗
PubMed2004

Macroscopic, neat, single-walled carbon nanotube fibers.

Well-aligned macroscopic fibers composed solely of single-walled carbon nanotubes (SWNTs) were produced by conventional spinning. Fuming sulfuric acid charges SWNTs and promotes their ordering into an aligned phase of individual mobile SWNTs surrounded by acid anions. This ordered dispersion was extruded via solution spinning into continuous lengths of macroscopic neat SWNT fibers. Such fibers possess interesting structural composition and physical properties.

Journal Article↗
PubMed2004

Collapse of a semiflexible polymer in poor solvent.

We investigate the dynamics and pathways of the collapse of a single, semiflexible polymer in a poor solvent via three-dimensional Brownian Dynamics simulations. An example of this phenomenon is DNA condensation induced by multivalent cations. Earlier work indicates that the collapse of semiflexible polymers generically proceeds via a cascade through metastable racquet-shaped, long-lived intermediates towards the stable torus state. We investigate the rate of decay of uncollapsed states, analyze the preferential pathways of condensation, and describe the likelihood and lifespan of the different metastable states. The simulations are performed with a bead-stiff spring model with excluded volume interaction, bending stiffness, and exponentially decaying attractive potential. The semiflexible chain collapse is studied as a function of the three relevant length scales of the phenomenon, i.e., the total chain length L, the persistence length L(p), and the condensation length L(0)=square root of [k(B)TL(p)/u(0)], where u(0) is a measure of the attractive potential per unit length. Two dimensionless ratios, L/L(p) and L(0)/L(p), suffice to describe the dimensionless decay rate of uncollapsed states, which appears to scale as (L/L(p))(1/3)(L(0)/L(p)). The condensation sequence is described in terms of the time series of the well separated energy levels associated with each metastable collapsed state. The collapsed states are described quantitatively through the spatial correlation of tangent vectors along the chain. We also compare the results obtained with a locally inextensible bead-rod chain and with a phantom bead-spring chain. Finally, we show preliminary results on how steady shear flow influences the kinetics of collapse.

Biopolymers↗
PubMed2004

Vorticity alignment and negative normal stresses in sheared attractive emulsions.

Attractive emulsions near the colloidal glass transition are investigated by rheometry and optical microscopy under shear. We find that (i) the apparent viscosity eta drops with increasing shear rate, then remains approximately constant in a range of shear rates, then continues to decay; (ii) the first normal stress difference N1 transitions sharply from nearly zero to negative in the region of constant shear viscosity; and (iii) correspondingly, cylindrical flocs form, align along the vorticity, and undergo a log-rolling movement. An analysis of the interplay between steric constraints, attractive forces, and composition explains this behavior, which seems universal to several other complex systems.

Journal Article↗
PubMed2004

A tensor-based measure for estimating blood damage.

Implantable ventricular assist devices give hope of a permanent clinical solution to heart failure. These devices, both pulsatile- and continuous-flow, are presently used as medium-term bridge to heart transplant or recovery. While long-term use of continuous-flow axial and centrifugal pumps is being explored, the excessive level of blood damage in these devices has emerged as a design challenge. Blood damage depends both on shear stress and exposure time, and device designers have relied traditionally on global space- and time-averaged estimates from experimental studies to make design decisions. Measuring distributions of shear stress levels and the blood cell's exposure to these conditions in complex rotary pump flow is difficult. On the other hand, computational fluid dynamics (CFD) is now being used as a tool for designing viable devices, offering more detailed information about the flow field. A tensor-based blood damage model for CFD analysis is proposed here. The model estimates the time- and space-dependent strain experienced by individual blood cells and correlates it to blood damage data from steady shear flow experiments. The blood cells are modeled as deforming droplets and their deformation is tracked along the pathlines of a computed flow. The model predicts that blood cells in a rapidly fluctuating shear flow can sustain high shear stress levels for very short exposure time without deforming considerably. In the context of mechanical modeling of the implantable Gyro blood pump being developed at Baylor College of Medicine, this suggests that blood cells traversing regions of highly fluctuating shear stress rapidly may not hemolyze significantly.

Erythrocytes↗