Salt solutions and crystals

What is the common factor between rock-based artwork near the sea, concrete structures in mountain areas, water harvesting strategies in dry lands, or atmospheric aerosols? Salts, surfaces, and confinement. The ongoing SINCS project (funded by ANR) aims at characterizing how water interacts with complex, nanoscale structures in the presence of salts. The EHAWEDRY project (funded by the European Union) aims at using this understanding to develop new electrodes for energy harvesting and storage.

I started developing these themes at the end of my stay at Cornell University, with preliminary results on how the capillary/osmotic competition shapes 1) the dynamic response of filling and emptying [17] and 2) the adsorption and desorption of water in nanoporous media [18]

Micro/Nanofluidics

In nature (plants, soil, rocks etc.) and technology (membranes, cement etc.), liquids often flow in microscale and nanoscale spaces, where macroscopic laws of fluid mechanics are not necessarily valid anymore. I use original nanofluidic chips to probe the properties of highly confined liquids, e.g. their viscous and capillary responses [14], or phoresis/osmosis effects arising from the interaction between added solutes or particles and the confining walls [17].

Pattern formation

• Drying of solutions or suspensions on surfaces, and the crystallization/deposit patterns that emerge (applications e.g. in printing).
• A large part of the ongoing SINCS project is also to understand invasion-percolation patterns due to phase change and collective effects in disordered, nanoscale porous structures.
• Spontaneous spatio-temporal patterns appear when drying multiscale porous media, due to the nonlinear coupling between thermally-activated nucleation of vapor bubbles (cavitation) and poroelastic relaxations [12].



• Applying electric fields to suspensions of lipid vesicles (used e.g. as drug carriers or as model systems for living cells) results in selective aggregation patterns [1,2].

Physics of Plants

How water moves, evaporates and interacts with the structure in plants is crucial for the plant's function and survival. Here are a few examples:

• Carnivorous plants use mechanical instabilities to be able to move fast without muscles. In Utricularia, a combination of continuous water pumping, well-designed trap architecture and membrane buckling enables ultra-fast suction (~1 ms time scale, 600 g accelerations) of preys [3, 4] and spontaneous "breathing" of the trap body that follows very regular time patterns, due to spontaneous oscillations appearing in this strongly nonlinear system [5, 6].



• Plants use evaporation to drive flow of water from the roots to the leaves in a process called transpiration, which allows trees to achieve transport up to ~100 meter elevations completely passively. One consequence is that the liquid inside of a tree is often under negative pressure (tension), i.e. a metastable state. With MEMS devices, one can directly measure the tension within a tree, which is important to manage watering and optimal maturation of crops. [10].
• Due to negative pressures, bubbles can form within the sap channels, either by air aspiration or by nucleation (cavitation), which become more likely in dry conditions [19]. This embolism is deadly if too widespread. We studied directly the dynamics of these bubbles using a combination of optics and acoustics in real tree tissues [11]or with artificial systems (see below).

Cavitation and Bubble Dynamics

Cavitation (spontaneous nucleation of vapor bubbles) occurs in many situations including plants, boat propellers, during curing of cement, etc. Some species of shrimps and ferns also use it to their advantage for hunting and dispersion, respectively.

I have been especially interested in water cavitation within solid microstructures, such as plants (see above) or artificial systems made of polymer, silicon and/or glass.

• The combination of fluid motion, wall elasticity, geometric confinement and liquid compressibility results in unusual bubble dynamics, with oscillations much faster than for regular bubbles, and with interesting break-up instabilities and ripening [8, 9, 16, 19].
• Confinement also results in unexpected stability diagrams for bubbles, with e.g. bifurcations and critical-point behavior. In particular, strong confinements should be able to prevent cavitation completely, making a metastable liquid stable [16, 19].
• In extended porous systems with many possible nucleation sites, cavitation events can also "communicate", resulting in events that seem to occur by bursts, and relief in between the bursts because of temporary rehydratation of the medium [12].

Porous Media

Various studies mentioned above relate to fundamental questions about transport and phase change in porous media, which have led me to investigate phenomena such as imbibition, drying, capillary condensation, pervaporation, cavitation, osmosis, cristallization, deliquescence etc. in nanoporous media [12-15, 17-18].

I have mainly used porous silicon as a model system, but am also working with nanoporous glasses (e.g. Vycor), anodic alumina, hydrogels, or even quasi-2D lithographically etched structures.