Changing Time History in Moving Boundary Problems

A class of diffusion-stress equations modeling transport of solvent in glassy polymers is considered. The problem is formulated as a one-phase Stefan problem. It is shown that the moving front changes like √t initially but quickly behaves like t as t increases. The behavior is typical of stress-dominated transport. The quasi-steady state approximation is used to analyze the time history of the moving front. This analysis is motivated by the small time solution

Free boundary problems in controlled release pharmaceuticals. I: diffusion in glassy polymers

This paper formulates and studies two different problems occurring in the formation and use of pharmaceuticals via controlled release methods. These problems involve a glassy polymer and a penetrant, and the central problem is to predict and control the diffusive behavior of the penetrant through the polymer. The mathematical theory yields free boundary problems which are studied in various asymptotic regimes

Controlled Drug Release Asymptotics

The solution of Higushi's model for controlled release of drugs is examined when the solubility of the drug in the polymer matrix is a prescribed function of time. A time-dependent solubility results either from an external control or from a change in pH due to the activation of pH immobilized enzymes. The model is described as a one-phase moving boundary problem which cannot be solved exactly. We consider two limits of our problem. The first limit considers a solubility much smaller than the initial loading of the drug. This limit leads to a pseudo-steady-state approximation of the diffusion equation and has been widely used when the solubility is constant. The second limit considers a solubility close to the initial loading of the drug. It requires a boundary layer analysis and has never been explored before. We obtain simple analytical expressions for the release rate which exhibits the effect of the time-dependent solubility

Tipping points near a delayed saddle node bifurcation with periodic forcing

We consider the effect on tipping from an additive periodic forcing in a canonical model with a saddle node bifurcation and a slowly varying bifurcation parameter. Here tipping refers to the dramatic change in dynamical behavior characterized by a rapid transition away from a previously attracting state. In the absence of the periodic forcing, it is well-known that a slowly varying bifurcation parameter produces a delay in this transition, beyond the bifurcation point for the static case. Using a multiple scales analysis, we consider the effect of amplitude and frequency of the periodic forcing relative to the drifting rate of the slowly varying bifurcation parameter. We show that a high frequency oscillation drives an earlier tipping when the bifurcation parameter varies more slowly, with the advance of the tipping point proportional to the square of the ratio of amplitude to frequency. In the low frequency case the position of the tipping point is affected by the frequency, amplitude and phase of the oscillation. The results are based on an analysis of the local concavity of the trajectory, used for low frequencies both of the same order as the drifting rate of the bifurcation parameter and for low frequencies larger than the drifting rate. The tipping point location is advanced with increased amplitude of the periodic forcing, with critical amplitudes where there are jumps in the location, yielding significant advances in the tipping point. We demonstrate the analysis for two applications with saddle node-type bifurcations

Multirhythmicity in an optoelectronic oscillator with large delay

An optoelectronic oscillator exhibiting a large delay in its feedback loop is studied both experimentally and theoretically. We show that multiple square-wave oscillations may coexist for the same values of the parameters (multirhythmicity). Depending on the sign of the phase shift, these regimes admit either periods close to an integer fraction of the delay or periods close to an odd integer fraction of twice the delay. These periodic solutions emerge from successive Hopf bifurcation points and stabilize at a finite amplitude following a scenario similar to Eckhaus instability in spatially extended systems. We find quantitative agreements between experiments and numerical simulations. The linear stability of the square-waves is substantiated analytically by determining stable fixed points of a map.Comment: 14 pages, 7 figure

Excitable-like chaotic pulses in the bounded-phase regime of an opto-radiofrequency oscillator

We report theoretical and experimental evidence of chaotic pulses with excitable-like properties in an opto-radiofrequency oscillator based on a self-injected dual-frequency laser. The chaotic attractor involved in the dynamics produces pulses that, albeit chaotic, are quite regular: They all have similar amplitudes, and are almost periodic in time. Thanks to these features, the system displays properties that are similar to those of excitable systems. In particular, the pulses exhibit a threshold-like response, of well-defined amplitude, to perturbations, and it appears possible to define a refractory time. At variance with excitability in injected lasers, here the excitable-like pulses are not accompanied by phase slips.Comment: 2nd versio

Synchronization of tunable asymmetric square-wave pulses in delay-coupled optoelectronic oscillators

We consider a model for two delay-coupled optoelectronic oscillators under positive delayed feedback as prototypical to study the conditions for synchronization of asymmetric square-wave oscillations, for which the duty cycle is not half of the period. We show that the scenario arising for positive feedback is much richer than with negative feedback. First, it allows for the coexistence of multiple in- and out-of-phase asymmetric periodic square waves for the same parameter values. Second, it is tunable: The period of all the square-wave periodic pulses can be tuned with the ratio of the delays, and the duty cycle of the asymmetric square waves can be changed with the offset phase while the total period remains constant. Finally, in addition to the multiple in- and out-of-phase periodic square waves, low-frequency periodic asymmetric solutions oscillating in phase may coexist for the same values of the parameters. Our analytical results are in agreement with numerical simulations and bifurcation diagrams obtained by using continuation techniques.This work benefited from the financial support of Ministerio de Economía y Competitividad (Spain) and Fondo Europeo de Desarrollo Regional under Projects No. FIS2012-30634 (INTENSE@COSYP) and No. TEC2012-36335 (TRIPHOP); European Social Fund and Govern de les Illes Balears under programs Grups Competitius and Formació de Personal Investigador; Fonds de la Recherche Scientifique-FNRS (Belgium); and the Belgian Science Policy Office under Grant No. IAP-7/35 “photonics@be.”Peer Reviewe

Tuning the period of square-wave oscillations for delay-coupled optoelectronic systems

We analyze the response of two delay-coupled optoelectronic oscillators. Each oscillator operates under its own delayed feedback. We show that the system can display square-wave periodic solutions that can be synchronized in phase or out of phase depending on the ratio between self- and cross-delay times. Furthermore, we show that multiple periodic synchronized solutions can coexist for the same values of the fixed parameters. As a consequence, it is possible to generate square-wave oscillations with different periods by just changing the initial conditions. © 2014 American Physical Society.We are grateful for financial support from MINECO, Spain and FEDER under Projects No. FIS2007-60327 (FISICOS), No. TEC2009-14101 (DeCoDicA), No. FIS2012-30634 (INTENSE@COSYP), and No. TEC2012-36335 (TRIPHOP); from the EC Project PHOCUS (No. FP7-ICT-2009-C-240763); and from European Social Fund and Comunitat Autònoma de les Illes Balears. T.E. acknowledges support from the FNRS (Belgium). This work benefited from the support of the Belgian Science Policy Office under Grant No. IAP-7/35 “photonics@be”.Peer Reviewe

Analytical theory of external cavity modes of a semiconductor laser with phase conjugate feedback

SCOPUS: cp.pinfo:eu-repo/semantics/publishe