Séminaires

Osmotic driven phenomena : towards local tree-on-a-chip observations

Sap circulation in vascular plants are governed by passive mechanisms exploited by the trees. Crude sap, composed of water, is transported in the xylem vessels, from roots to leaves by evapotranspiration. This results in a negative pressure driven flow at typically -10bar. At the leaf, crude is transformed into elaborated sap, containing sucrose (20-30% weight) and nutriments to be transported to the plant growing parts, in the phloem vessels. This sap circulation relies on osmotic pressure gradients, through the Munch mechanism. It relies on the fact that xylem and phloem are separated by hemi-permeable membranes. Because of the sucrose concentration difference across the membrane, water is osmotically pumped from xylem to phloem, pushing the elaborated sap.

In this talk, I discuss how we mimic these mechanisms using microfluidic devices, close to the tree vessel sizes. We integrate into microfluidic chips permeable membranes made of mixture of polymers (higher molecular weight), water, an initiator and inhibitor and colloid tracers. We characterize their permeation properties to water and solutes, considering only steric repulsions, to find their molecular weight cut off (MWCO). Using solutions of tabulated osmotic pressures, above the cut-off, we show that we are able to maintain water stable down to -26bar in these systems. We mimic a xylem-like geometry using different concentration to induce a water flow at negative pressure. Next, we focus on the phloem part using sucrose, below the MWCO, inducing transient osmotic flows and membrane deformation at high concentration (20-30% weight). Using Raman spectroscopy we measure sucrose concentrations to quantify the sucrose invasion across the membrane and relate it with transport theory. Finally, due to high osmotic pressures and sucrose gradients in our systems we evidence that colloid tracers can migrate spontaneously from high to low sucrose regions by diffusiophoresis.