The dual role of viscosity in capillary rise.

The spontaneous rise of a wetting liquid in a capillary tube is classically described by Washburn's law: the meniscus height increases as the square root of time, a law singular for short times, where the velocity diverges. We focus here on the early dynamics of the rise of viscous liquids, and report an initial regime of constant velocity contrasting with Washburn's prediction. This is explained by considering the contact line friction at the liquid front, and confirmed by the influence of prewetting films on the tube walls, whose presence is found to speed up the rise and more generally to provide an ideal framework for quantifying the friction at contact lines.

Flow-driven control of pulse width in excitable media.

Models of pulse formation in nerve conduction have provided manifold insight not only into neuronal dynamics but also the nonlinear dynamics of pulse formation in general. Recent observation of neuronal electrochemical pulses also driving mechanical deformation of the tubular neuronal wall, and thereby generating ensuing cytoplasmic flow, now question the impact of flow on the electrochemical dynamics of pulse formation. Here, we theoretically investigate the classical Fitzhugh-Nagumo model, now accounting for advective coupling between the pulse propagator typically describing membrane potential and triggering mechanical deformations, and thus governing flow magnitude, and the pulse controller, a chemical species advected with the ensuing fluid flow. Employing analytical calculations and numerical simulations, we find that advective coupling allows for a linear control of pulse width while leaving pulse velocity unchanged. We therefore uncover an independent control of pulse width by fluid flow coupling.

Giant slip length at a supercooled liquid-solid interface.

The effect of temperature on friction and slip at the liquid-solid interface has attracted attention over the last 20 years, both numerically and experimentally. However, the role of temperature on slip close to the glass transition has been less explored. Here we use molecular dynamics to simulate a bidisperse atomic fluid, which can remain liquid below its melting point (supercooled state), to study the effect of temperature on friction and slip length between the liquid and a smooth apolar wall in a broad range of temperatures. At high temperatures, an Arrhenius law fits well the temperature dependence of viscosity, friction, and slip length. In contrast, when the fluid is supercooled, the viscosity becomes super-Arrhenian, while interfacial friction can remain Arrhenian or even drastically decrease when lowering the temperature, resulting in a massive increase of the slip length. We rationalize the observed superlubricity by the surface crystallization of the fluid, and the incommensurability between the structures of the fluid interfacial layer and of the wall. This study calls for experimental investigation of the slip length of supercooled liquids on low surface energy solids.

Interactions between proteins and cellulose in a liquid crystalline media: Design of a droplet based experimental platform.

Model systems are needed to provide controlled environment for the understanding of complex phenomena. Interaction between polysaccharides and proteins in dense medium are involved in numerous complex systems such as biomass conversion or plant use for food processing or biobased materials. In this work, cellulose nanocrystals (CNCs) were used to study proteins in a dense and organized cellulosic environment. This environment was designed within microdroplets using a microfluidic setup, and applied to two proteins, bovine serum albumin (BSA) and a GH7 endoglucanase, relevant to food and plant science, respectively. The CNC at 56.5 g/L organized in liquid crystalline structure and the distribution of the proteins was probed using synchrotron deep-UV radiation. The proteins were homogeneously distributed throughout the volume, but BSA significantly disturbed the droplet global organization, preferring partition in hydrophilic external micelles. In contrast, GH7 partitioned with the CNCs showing stronger non-polar interaction but without disruption of the system organization. Such results pave the road for the development of more complex polysaccharides - proteins in-vitro models.

Reversible photocontrol of DNA coacervation.

Coacervate micro-droplets produced by liquid-liquid phase separation are increasingly used to emulate the dynamical organization of membraneless organelles found in living cells. Designing synthetic coacervates able to be formed and disassembled with improved spatiotemporal control is still challenging. In this chapter, we describe the design of photoswitchable coacervate droplets produced by phase separation of short double stranded DNA in the presence of an azobenzene cation. The droplets can be reversibly dissolved with light, which provides a new approach for the spatiotemporal regulation of coacervation. Significantly, the dynamics of light-actuated droplet formation and dissolution correlates with the capture and release of guest solutes. The reported system can find applications for the dynamic photocontrol of biomolecule compartmentalization, paving the way to the light-activated regulation of signaling pathways in artificial membraneless organelles.