Nicole Coutris

Role of Marangoni instability in fabrication of axially and internally grooved hollow fiber membranes.

Hollow fiber membranes (HFMs) are extensively used in different industrial applications. Under some controlled fabrication conditions, axially aligned grooves can be formed on the HFM inner surface during typical immersion precipitation-based phase inversion fabrication processes. Such grooved HFMs are found to be promising for nerve repair and regeneration. The axially aligned grooves appearing on the inner surface of the membrane are considered as hydrodynamic instability patterns. During the immersion precipitation process, a transfer of solvent takes place across the interface between a polymer solution and a nonsolvent. This solvent transfer induces gradients of interfacial tension that are considered to be the driving mechanism for Marangoni instability. The onset of the stationary instability is studied by means of a linear instability theory, and the critical and maximum wavenumbers are determined and discussed in terms of the dimensionless groups characterizing the system: viscosity ratio, diffusivity ratio, Schmidt number, crispation number, adsorption number, Marangoni number, and the polymer bulk concentration. A good agreement is found between the predicted wavelength of the most dangerous wave and the experimental groove width. Consequently, solutal Marangoni instability can explain the groove formation mechanism in HFM fabrication.

Numerical study of axonal outgrowth in grooved nerve conduits

Nerve conduits with grooved inner texture, working as a topographical guidance cue, have been experimentally proved to play a significant role in axonal alignment. How grooved conduits guide axonal outgrowth is of particular interest for studying nerve regeneration. A viscoelastic model of axonal outgrowth in a conduit with a defined grooved geometry characterized by its width in the circumferential direction and its height in the radial direction is developed in this work. In this model, the axon is considered as an elastic beam and the axonal deformation and motion, including stretching, bending and torsion, are described using a Cosserat rod theory. The friction between axon and substrate is also considered in this model as well as the tip outgrowth. It is found that the directional outgrowth of the axon can be significantly improved by the grooved texture: when the groove width decreases or the groove height increases, the axonal elongation in the longitudinal direction of the conduit can be increased, which is in good agreement with experimental observations. This work is the first numerical model to study the effect of the substrate geometry on axonal outgrowth.

Methodology for the Evaluation of Double-Layered Microcapsule Formability Zone in Compound Nozzle Jetting Based on Growth Rate Ratio.

Double-layered microcapsules, which usually consist of a core (polymeric) matrix surrounded by a (polymeric) shell, have been used in many industrial and scientific applications, such as microencapsulation of drugs and living cells. Concentric compound nozzle-based jetting has been favored due to its efficiency and precise control of the core-shell compound structure. Thus far, little is known about the underlying formation mechanism of double-layered microcapsules in compound nozzle jetting. This study aims to understand the formability of double-layered microcapsules in compound nozzle jetting by combining a theoretical analysis and numerical simulations. A linear temporal instability analysis is used to define the perturbation growth rates of stretching and squeezing modes and a growth ratio as a function of the wave number, and a computational fluid dynamics (CFD) method is implemented to model the microcapsule formation process in order to determine the good microcapsule forming range based on the growth ratio curve. Using a pseudobisection method, the lower and upper bounds of the good formability range have been determined for a given materials-nozzle system. The proposed formability prediction methodology has been implemented to model a water-poly (lactide-co-glycolide) (PLGA)-air compound jetting system.

THE INFLUENCE OF MATERIAL PROPERTY VARIANCES AND PRINTING TOLERANCES ON THE MECHANICAL BEHAVIOR OF AN ADDITIVELY MANUFACTURED META-MATERIAL TANK TRACK BACKER PAD

This paper explores the influences that potential variances in material properties and nominal dimensions have on the overall mechanical behavior of an additively manufactured meta-material. The investigation looks at deviations between expected and experimental mechanical responses obtained through performance validation testing. Three sources for discrepancies were identified through literature review and model/experimental comparison. Sensitivity analyses were employed to obtain the significance of the design parameters, and preliminary work in boundary condition improvements is discussed.

Drop-on-Demand: A Scale Analysis

An increasing number of applications deal with dispensing of micro-droplets. For several years the fluids used were mostly Newtonian with low viscosity but there is nowadays a tendency to use more complex fluids. Some recent applications are encountered in the pharmaceutical industry where there is a need of producing on-demand identical droplets of a given size that can be used for drug delivery and in bioengineering where droplets have to contain cells used for tissue manufacturing or organ printing. The objective of the paper is to show that scale analysis is an efficient tool to classify the physical phenomena contributing to the breakup of a jet or of a liquid filament of a complex fluid and to provide guidelines for controlling the droplet formation process.Copyright

Evaluation of Transverse Shear Effect on Film Delamination in Blister Test

The transverse shear effect has been frequently ignored in determining the debonding-related energy release rate and the phase angle in the blister test, resulting in underestimated values. This study aims to study the effect of shear force on the energy release rate and phase angle prediction in the blister test. A generalized approach is proposed to predict them under the effect of shear force. The predictions show that when the ratio of the film thickness to the debonded film window radius is large (such as 0.05), the transverse shear effect cannot be ignored in determining the energy release rate and the phase angle. The study also further illustrates the importance of including the shear force contribution in estimation and how this importance depends on the film thickness to debonded radius ratio, as well as the elastic mismatch.

Study of Double-Layered Microcapsule Formation in Compound Nozzle Jetting

Double-layered microcapsules, which usually consist of a core (polymeric) matrix surrounded by a (polymeric) shell, have been used in many industrial and scientific applications such as microencapsulation of drugs and living cells. Compound concentric nozzle-based jetting has been favored due to its efficiency and precise control of the core-shell compound structure. Thus far, little is known about the underlying formation mechanism of double-layered microcapsules in compound nozzle jetting. This study aims to understand the formability of double-layered microcapsules in compound nozzle jetting by coupling a theoretical analysis with numerical simulations. A linear temporal instability analysis is used to define the perturbation growth rate and its ratio as a function of the wavenumber; and a computational fluid dynamics (CFD) method is implemented to model the microcapsule formation process in order to determine the good microcapsule forming range based on the growth ratio curve. Using a pseudo-bisection method, the lower and upper bounds of the good formability range have been determined for a given materials-nozzle system. The proposed formability prediction methodology has been implemented and validated in modeling a water-poly (lactide-co-glycolide) (PLGA) – air compound jetting system with satisfactory prediction results.Copyright

Numerical Study and Scale Analysis of Inner and Outer Diameters Prediction in Fabrication of Hollow Fiber Membranes for Nerve Regeneration

Recently the semi-permeable hollow fiber membrane (HFM) is finding promising applications in promoting axonal outgrowth for nerve repair and regeneration. It is of interest to model the phase inversion-based HFM fabrication process and control the fabricated HFM geometry. The effect of gravity and surface tension which is frequently ignored in general fiber spinning should be carefully addressed in HFM fabrication modeling. Both the volume of fluid (VOF) method and the scale analysis have been applied to appreciate the effect of gravity and surface tension on the HFM geometry profile. The VOF method-based simulation results reveal that both the gravity and/or surface tension significantly reduce the predicted radii/diameters, while the scale analysis reveals that the gravity or surface tension affects the HFM fabrication process dynamics. Both the approaches warrant the need of including the gravity and surface tension in HFM fabrication process modeling.Copyright