Physico-chemical features of hydrogel capsules having a liquid core
This work focuses on the study of physico-chemical properties of millimetric capsules having a liquid core and a thin alginate membrane. First, we caracterized the diffusion of molecules through the hydrogel shell and estimated the cut-off radius of the semi-permeable membrane. Moreover, the leakage of small hydrophilic solutes can be limited by adding a hydrophobic shell between the aqueous core and the alginate membrane. We demonstrated that the characteristic time of transport is determined by the geometry of the system (hydrophobic shell thickness and capsule diameter) and by the solubility and diffusion coefficient of the solute in the oil phase. Furthermore, swelling of a capsule under an osmotic pressure difference allowed us to identify the viscoelastic and viscoplastic regimes of the membrane under stretching. We deduced the elastic modulus of the membrane which decreases above the plastic threshold. However, when capsules are compressed, the hydrogel is concentrated due to the irreversible evacuation of water. This leads to rigidification of its structure. Then, we focused on the flow of oil-core capsules dispersed in water. There is a friction between capsules that we can control by adding surfactant. Thus we studied its role in the flow of capsules in the constriction of an hourglass. We showed that the flow is constant and does not depend on friction. It is established by the size of the constriction. Indeed, the flow is only determined by the hydrodynamic in fluid regime in the hourglass' neck.