Controlling extrudate volume fraction through poroelastic extrusion of entangled looped fibers.

When a suspension of spherical or near-spherical particles passes through a constriction the particle volume fraction either remains the same or decreases. In contrast to these particulate suspensions, here we observe that an entangled fiber suspension increases its volume fraction up to 14-fold after passing through a constriction. We attribute this response to the entanglements among the fibers that allows the network to move faster than the liquid. By changing the fiber geometry, we find that the entanglements originate from interlocking shapes or high fiber flexibility. A quantitative poroelastic model is used to explain the increase in velocity and extrudate volume fraction. These results provide a new strategy to use fiber volume fraction, flexibility, and shape to tune soft material properties, e.g., suspension concentration and porosity, during delivery, as occurs in healthcare, three-dimensional printing, and material repair.

Fiber Buckling in Confined Viscous Flows: An Absolute Instability Described by the Linear Ginzburg-Landau Equation.

We explore the dynamics of a flexible fiber transported by a viscous flow in a Hele-Shaw cell of height comparable to the fiber height. We show that long fibers aligned with the flow experience a buckling instability. Competition between viscous and elastic forces leads to the deformation of the fiber into a wavy shape convolved by a Bell-shaped envelope. We characterize the wavelength and phase velocity of the deformation as well as the growth and spreading of the envelope. Our study of the spatiotemporal evolution of the deformation reveals a linear and absolute instability arising from a local mechanism well described by the Ginzburg-Landau equation.

Spontaneous localized fluid release on swelling fibres.

When immersed into a favourable solvent, many fibres, in particular vegetable, wood or animal fibres, will absorb liquid and swell. When a single drop of solvent is deposited, the fibre first locally swells at the drop position, then the liquid slowly diffuses within the fibre. We study the absorption dynamics of several drops placed on a fibre of fixed length. We show that during absorption, there is a swelling-induced global change in the tension of the fibre. If the drops are close enough to one another, this change induces the release of fluid out of the fibre (i.e. deswelling) in previously fluid-saturated regions. We identify the mechanisms underlying this transient localized fluid release, and identify the conditions for which it occurs in order to build a phase diagram as a function of the drops volume and distance, both experimentally and numerically using a linear poroelastic model.

Correction: Controlling wet adhesion with elasticity.

Correction for 'Controlling wet adhesion with elasticity' by Camille Duprat et al., Soft Matter, 2020, 16, 6463-6467, DOI: 10.1039/D0SM00618A.

Modeling of aerosol transmission of airborne pathogens in ICU rooms of COVID-19 patients with acute respiratory failure.

The COVID-19 pandemic has generated many concerns about cross-contamination risks, particularly in hospital settings and Intensive Care Units (ICU). Virus-laden aerosols produced by infected patients can propagate throughout ventilated rooms and put medical personnel entering them at risk. Experimental results found with a schlieren optical method have shown that the air flows generated by a cough and normal breathing were modified by the oxygenation technique used, especially when using High Flow Nasal Canulae, increasing the shedding of potentially infectious airborne particles. This study also uses a 3D Computational Fluid Dynamics model based on a Lattice Boltzmann Method to simulate the air flows as well as the movement of numerous airborne particles produced by a patient's cough within an ICU room under negative pressure. The effects of different mitigation scenarii on the amount of aerosols potentially containing SARS-CoV-2 that are extracted through the ventilation system are investigated. Numerical results indicate that adequate bed orientation and additional air treatment unit positioning can increase by 40% the number of particles extracted and decrease by 25% the amount of particles deposited on surfaces 45s after shedding. This approach could help lay the grounds for a more comprehensive way to tackle contamination risks in hospitals, as the model can be seen as a proof of concept and be adapted to any room configuration.

Dynamics of drop absorption by a swelling fiber.

Swelling of individual fibres exposed to a favorable solvent may affect the mechanical properties or shape of a fibrous material. We provide experimental results of the absorption dynamics of a single drop deposited on a swellable fibre and show that the total absorption time is highly dependent on the fibre geometry and drop volume. The curvature of the fibre prevents the total spreading of a drop even if the fluid is fully wetting the fibre. For drops larger than a critical volume, a local saturation of the fibre is thus reached below the drop, leading to a strong increase in the total absorption time. These observations are then rationalized with a simple pseudo-diffusive model to understand the drop absorption and fibre swelling dynamics. This minimal model provides quantitative predictions of the total absorption time.

Controlling wet adhesion with elasticity.

We consider the wet adhesion between two deformable fibers. We show that a strong adhesive force can be maintained by coupling the fibers deformation with capillarity. We further identify a regime where, contrary to capillary adhesion, the pull-off force remains constant throughout debonding. In this peeling regime, elasticity, which tends to minimize the deformation of the object, is balanced by capillarity which maximizes the liquid spreading. We show that the adhesive force and the existence region of this self-adjusted peeling regime depend on a single dimensionless parameter, and can thus be controlled by tuning the material properties.

Microfluidic in situ mechanical testing of photopolymerized gels.

Gels are a functional template for micro-particle fabrication and microbiology experiments. The control and knowledge of their mechanical properties is critical in a number of applications, but no simple in situ method exists to determine these properties. We propose a novel microfluidic based method that directly measures the mechanical properties of the gel upon its fabrication. We measure the deformation of a gel beam under a controlled flow forcing, which gives us a direct access to the Young's modulus of the material itself. We then use this method to determine the mechanical properties of poly(ethylene glycol) diacrylate (PEGDA) under various experimental conditions. The mechanical properties of the gel can be highly tuned, yielding two order of magnitude in the Young's modulus. The method can be easily implemented to allow for an in situ direct measurement and control of Young's moduli under various experimental conditions.

Evaporation of drops on two parallel fibers: influence of the liquid morphology and fiber elasticity.

We investigate experimentally the evaporation of liquid accumulated on a pair of parallel fibers, rigid or flexible. The liquid wetting the fibers can adopt two distinct morphologies: a compact drop shape, whose evaporation dynamics is similar to that of an isolated aerosol droplet, or a long liquid column of constant cross-section, whose evaporation dynamics depends upon the aspect ratio of the column. We thus find that the evaporation rate is constant for drops, while it increases strongly for columns as the interfiber distance decreases, and we propose a model to explain this behavior. When the fibers are flexible, the transition from drops to columns can be induced by the deformation of the fibers because of the capillary forces applied by the drop. Thus, we find that the evaporation rate increases with increasing flexibility. Furthermore, complex morphology transitions occur upon drying, which results in spreading of the drop as it evaporates.

Selective spreading and jetting of electrically driven dielectric films.

We study the electrically driven spreading of dielectric liquid films in wedge-shaped gaps across which a potential difference is applied. Our experiments are in a little-studied regime where, throughout the dynamics, the electrical relaxation time is long compared to the time for charge to be convected by the fluid motion. We observe that at a critical normal electric field the hump-shaped leading edge undergoes an instability in the form of a single Taylor cone and periodic jetting ensues, after which traveling waves occur along the trailing thin film. We propose a convection-dominated mechanism for charge transport to describe the observed dynamics and rationalize the viscosity dependence of the self-excited dynamics.