Biomedical subjects

José A Miranda

Publications and source records attributed to José A Miranda.

13 recordsLinked to original sources

PubMed2006

Transcervical intrapartum amnioinfusion for preterm premature rupture of the membranes.

OBJECTIVES: To investigate the effect of transcervical amnioinfusion on the management of labour and neonatal outcomes in preterm premature rupture of the membranes. STUDY DESIGN: This clinical trial included 86 patients with premature rupture of the membranes between weeks 27 and 35 of gestation. Patients were randomly assigned to receive amnioinfusion via a two-way catheter or to the control group. Clinical management was otherwise the same in both groups. RESULTS: Amnioinfusion decreased the frequency of variable decelerations in fetal heart rate (27.9% versus 53.5%, p<0.05) and the rate of obstetric interventions motivated by nonreassuring fetal status (13.6% versus 52.4%, p<0.05). At delivery, pH values were significantly higher in the treatment group than in the conventionally managed control group (median 7.29 versus 7.27). CONCLUSIONS: Intrapartum transcervical amnioinfusion for preterm premature rupture of the membranes reduced the number of interventions needed because of nonreassuring fetal status, and improved neonatal gasometric values without increasing maternal or fetal morbidity.

Acid-Base Equilibrium↗

PubMed2006

Numerical study of pattern formation in miscible rotating Hele-Shaw flows.

The dynamics of the diffusing interface separating two miscible fluids in a rotating Hele-Shaw cell is studied by intensive and highly accurate numerical simulations. We perform numerical experiments in a wide range of parameters, focusing on the influence of viscosity contrast and Korteweg stresses on the shape of the interfacial patterns. A great variety of morphological behaviors is systematically introduced, and a wealth of interesting phenomena related to finger competition dynamics, filament stretching, and interface pinch off are reveal. Our simulations exhibit miscible patterns that bear a strong resemblance to their immiscible counterparts for larger Korteweg stresses. The quantitative equivalence between such stresses and the usual immiscible surface tension is studied. The concept of an effective interfacial tension is considered, allowing the direct and precise calculation of the important fingering properties under miscible circumstances. Our results show excellent agreement with existing experiments and simulations for corresponding immiscible displacements. This agreement refers to a striking similarity between miscible and immiscible pattern morphologies, and also to an accurate prediction for the typical number of miscible fingering structures formed. Our findings suggest that the effective interfacial tension is both qualitatively and quantitatively equivalent to its immiscible counterpart.

Journal Article↗

PubMed2006

Stretching of a confined ferrofluid: influence of viscous stresses and magnetic field.

An analytical investigation is presented for the stretch flow of a viscous Newtonian ferrofluid highly confined between parallel plates. We focus on the development of interfacial instabilities when the upper plate is lifted at a described rate, under the action of an applied magnetic field. We derive the mode-coupling differential equation for the interface perturbation amplitudes and study both linear and nonlinear flow regimes. In contrast to the great majority of works in stretch flow we take into account stresses originated from velocity gradients normal to the ferrofluid interface. The impact of such normal stresses is accounted for through a modified Young-Laplace pressure jump interfacial boundary condition, which also includes the contribution from magnetic normal traction. We study how the stability properties of the interface and the shape of the emerging patterns respond to the combined action of normal stresses and magnetic field, both in the presence and absence of surface tension. We show that the inclusion of normal viscous stresses introduces a pertinent dependence on the initial aspect ratio, indicating that the number of fingers formed would be overestimated if such stresses are not taken into account. At early linear stages it is found that such stresses regularize the system, acting as an effective interfacial tension. At weakly nonlinear stages we verified that normal stresses reduce finger competition, which can be completely suppressed with the assistance of an azimuthal magnetic field. We have also found that the magnetic normal traction introduces a purely nonlinear contribution to the problem, revealing the key role played by the magnetic susceptibility in the control of finger competition.

Journal Article↗

PubMed2005

Viscosity contrast effects on fingering formation in rotating Hele-Shaw flows.

The different finger morphologies that arise at the interface separating two immiscible fluids in a rotating Hele-Shaw cell are studied numerically. The whole range of viscosity contrast is analyzed and a variety of fingering patterns systematically introduced, including the case in which the inner fluid is less viscous than the outer one. Our numerical results demonstrate that both the magnitude and the sign of the viscosity contrast strongly affect the shape of the emerging fingers, and also their length distribution. We have also found that the occurrence and location of pinch-off singularities are remarkably modified when the inner fluid is less viscous: instead of generating an isolated detaching drop, a full finger is disconnected from the interface. Finally, we have verified that the finger competition phenomena revealed by our simulations are correctly predicted by a weakly nonlinear analysis of the pattern development, showing that such important finger competition dynamics is already set at relatively early stages of interfacial evolution.

Journal Article↗

PubMed2005

Numerical study of miscible fingering in a time-dependent gap Hele-Shaw cell.

We perform a detailed numerical study of the evolution of a miscible fluid droplet in a time-dependent gap Hele-Shaw cell. The development of the emerging fingering instabilities is systematically analyzed by intensive and highly accurate numerical simulations. We focus on the influence of three relevant physical parameters on the interface dynamics: the Pélclet number Pe, the viscosity contrast A, and the Korteweg stress parameter delta. Consistently with conventional miscible Saffman-Taylor studies in constant-gap Hele-Shaw cells, our results demonstrate that more vigorous fingering is observed at higher Pe and larger A. Concerning the specific role of Pe and A, we deduce two general results: higher Péclet number favors branching around a nearly circular region (which leads to longer interfacial lengths); while larger viscosity contrast results in more significant finger penetrations (which is quantitatively expressed by larger diameter of gyration). We have also verified that the Korteweg stress parameter delta does act as an effective interfacial tension: it stabilizes the miscible interface, leading to fingering patterns that present a greater resemblance with the structures obtained in similar immiscible situations. Finally, we have identified the development of a visually striking phenomenon in the limit of high Pe, large A , and relatively small delta: some outward fingers pinch, and subsequent droplet detachment is observed. We show that such a droplet detachment process can be prevented by the action of stronger interfacial stresses. This last finding provides additional evidence for the claim that the Korteweg stresses can be treated as an ersatz interfacial tension in diffusing fluids.

Journal Article↗

PubMed2004

Finger competition dynamics in rotating Hele-Shaw cells.

We report analytical results for the development of interfacial instabilities in rotating Hele-Shaw cells. We execute a mode-coupling approach to the problem and examine the morphological features of the fluid-fluid interface at the onset of nonlinear effects. The impact of normal stresses is accounted for through a modified pressure jump boundary condition. A differential equation describing the early nonlinear evolution of the interface is derived, being conveniently written in terms of three relevant dimensionless parameters: viscosity contrast A , surface tension B , and gap spacing b . We focus our study on the influence of these parameters on finger competition dynamics. It is deduced that the link between finger competition and A , B , and b can be revealed by a mechanism based on the enhanced growth of subharmonic perturbations. Our results show good agreement with existing experimental and numerical investigations of the problem both in low and high A<0 limits. In particular, it is found that the condition of vanishing A suppresses the dynamic competition between fingers, regardless of the value of B and b . Moreover, our study enables one to extract analytical information about the problem by exploring the whole range of allowed values for A , B , and b . Specifically, it is verified that pattern morphology is significantly modified when the viscosity contrast -1< or =A< or =1 varies: increasingly larger values of A>0 (A<0) lead to enhanced competition of outward (inward) fingers. Within this context the role of B and b in determining different finger competition behaviors is also discussed.

Journal Article↗

PubMed2004

Adhesion phenomena in ferrofluids.

One efficient way of determining the bond strength of adhesives is to measure the force or the work required to separate two surfaces bonded by a thin adhesive film. We consider the case in which the thin film is not a conventional adhesive material but a high viscosity ferrofluid confined between two narrowly spaced parallel flat plates subjected to an external magnetic field. Our theoretical results demonstrate that both the peak adhesive force and the separation energy are significantly influenced by the action and symmetry properties of the applied field. Specifically, we show that the adhesive strength of a ferrofluid is reduced if the applied magnetic field is perpendicular to the plates or if the applied field is in plane and exhibits azimuthal symmetry. Conversely, the adhesive strength can be either enhanced or reduced if the applied field is in plane and is directed radially outward. This establishes an interesting connection between adhesion and ferrohydrodynamic phenomena, allowing the control of important adhesive properties by magnetic means.

Journal Article↗

PubMed2004

Time-dependent gap Hele-Shaw cell with a ferrofluid: evidence for an interfacial singularity inhibition by a magnetic field.

We consider the flow of a ferrofluid droplet in a Hele-Shaw cell with a time-dependent gap width. When the surface tension and applied magnetic field are zero, interfacial instabilities develop and the droplet breaks. We execute a mode-coupling approach to the problem and focus on understanding how the development of singularities is affected by the action of an external field. Our analytical results indicate that the introduction of an azimuthal magnetic field profoundly modifies pattern formation, allowing the inhibition of interfacial singularities. We suggest the magnetic field can be used as a controllable parameter to discipline singular behavior.

Journal Article↗

PubMed2004

Shear-induced effects in confined non-Newtonian fluids under tension.

We study the influence of shear effects on the adhesive performance of a non-Newtonian fluid under tension, confined between two parallel flat plates. The upper plate is subjected to a pulling force, which is recorded during the separation process. We approach the problem analytically, and use a modified Darcy's law in the weak shear limit to derive the adhesive force and the separation energy. Our theoretical results demonstrate that, for relatively small separations, the adhesion strength is considerably reduced (enhanced) if the fluid is shear thinning (thickening). For larger plate separations, shear effects become negligible, and usual Newtonian behavior is observed. These findings are confirmed by a numerical solution of a more realistic version of the problem, which considers weak shear effects, plus the intrinsic elasticity of the lifting apparatus.

Journal Article↗

PubMed2003

Mode-coupling approach to non-Newtonian Hele-Shaw flow.

The Saffman-Taylor viscous fingering problem is investigated for the displacement of a non-Newtonian fluid by a Newtonian one in a radial Hele-Shaw cell. We execute a mode-coupling approach to the problem and examine the morphology of the fluid-fluid interface in the weak shear limit. A differential equation describing the early nonlinear evolution of the interface modes is derived in detail. Owing to vorticity arising from our modified Darcy's law, we introduce a vector potential for the velocity in contrast to the conventional scalar potential. Our analytical results address how mode-coupling dynamics relates to tip splitting and sidebranching in both shear-thinning and shear-thickening cases. The development of non-Newtonian interfacial patterns in rectangular Hele-Shaw cells is also analyzed.

Journal Article↗

PubMed2003

Controlling fingering instabilities in rotating ferrofluids.

We perform a detailed analytic and numerical study of the evolution of a ferrofluid drop confined to a rotating Hele-Shaw cell in the presence of an azimuthal magnetic field. Our results demonstrate that the centrifugally driven interfacial instabilities can be simply controlled with the use of a current-carrying wire. We compare an analytic linear analysis to our computational results and show that a number of observed features cannot be explained by linear theory alone, including a "diamond ring" instability that results when a droplet is nearly stabilized.

Journal Article↗

PubMed2002

Nonlinear effects due to gravity in a conical Hele-Shaw cell.

In this work we study the viscous fingering instability in a conical Hele-Shaw cell under the presence of gravity. We focus on understanding how the dynamical evolution of the fingering patterns is affected by the combined action of gravity and cell topology. Gravity-induced nonlinear effects are studied by a mode-coupling approach. Our results show that the interplay between gravity and cell topology leads to important effects, and profoundly modifies pattern evolution. We have found that the most dramatic consequences refer to finger tip behavior. Depending on the relative values of fluids' densities and viscosities, finger tip splitting reaches maximum intensity at well defined, preferred values of the cell opening angle. In fact, finger tip splitting can be completely replaced by finger tip sharpening as the cell angle is varied. Finger competition dynamics is also significantly changed: it is considerably enhanced (restrained) if the displaced fluid is more (less) dense.

Journal Article↗

PubMed2002

Analytical approach to viscous fingering in a cylindrical Hele-Shaw cell.

We report analytical results for the development of the viscous fingering instability in a cylindrical Hele-Shaw cell of radius a and thickness b. We derive a generalized version of Darcy's law in such cylindrical background, and find it recovers the usual Darcy's law for flow in flat, rectangular cells, with corrections of higher order in b/a. We focus our interest on the influence of the cell's radius of curvature on the instability characteristics. Linear and slightly nonlinear flow regimes are studied through a mode-coupling analysis. Our analytical results reveal that linear growth rates and finger competition are inhibited for an increasingly larger radius of curvature. The absence of tip-splitting events in cylindrical cells is also discussed.

Journal Article↗