Soft Membrane Interferometric Stress Sensor – SOFTER

Compliant walls play a significant role in many systems of interest; deformable interfaces abound in systems of biological significance. To understand surface properties of gels (like wetting or adhesion), long-range deformations must be considered. Mammalian synovial joints display remarkable lubrication properties; however, the working mechanisms responsible for that performance, linking charged macromolecules and compliant boundaries, are still not well understood. In that sense, new exploring tools are needed for the precise exploration of the response to external perturbations of compliant boundaries. The main objective of SOFTER is to develop a surface stress sensor (surface membrane sensor, SMS) based on precise determination of the deformation of a compliant membrane exposed to a changing environment, by using multiple beam interferometry. We also aim to build the necessary theoretical/numerical tools to rationalize the novel information gathered.
Using this new tool, a combined theoretical-experimental approach will be implemented to investigate three important topics in soft matter. First, we will explore the electrical double layer at a metal-solution interface and its behavior when it is pushed out of equilibrium, by measuring the changed pressure field on the metal-liquid boundary. A team associated to SOFTER has recently shown that upon the application of an alternating field, a long-range repulsive force emerges between oppositely charged surfaces. We have theorized that this force is a consequence of the field-induced excess of ions between the surfaces, which results in a non-homogeneous pressure in a non-conformal contact. By using the SMS to be developed in SOFTER, we will be able to explore this scenario and its implications. Second, we will explore the behavior of responsive grafted polyelectrolyte brushes, and how by applying an electric field one can control the surface morphology and surface energy of polyelectrolyte brushes, including the possibility of triggering order-disorder transitions. Finally, we will study the elastohydrodynamic EHD coupling emerging when a solid object moves near an elastic membrane. Two teams associated to SOFTER performed the first direct quantitative measurements of the EHD lift force at the nanoscale. We plan to investigate much further this important problem for nanoscience and biology, as the EHD coupling will be greatly amplified by the slender geometry at stake.