Fluids next to solid surfaces or adjacent to solids have physical properties that differ from those found in the bulk phase. These new physical properties are in the focus of this research.
The term “confinement” is often used to describe this nanometer thick interphase of fluid with new physical properties, which occurs in intensified form in a nanometer gap between two solids.
The inner structure of electrical double layers
For example, if the fluid is a salt solution, then the solid surface can acquire a net electrical charge and counter ions from the salt solution form a so-called electrical double layer (EDL). While this phenomenon is long known and used in technological applications, the theoretical understanding of the inner structure is far less clear. For instance, the well-established DLVO theory breaks down in very narrow pores or at salt concentrations as early as 10% of saturation. We study the structure of such EDLs using the extended surface forces apparatus (eSFA) in collaboration with the Laboratory for Surface Science and Technology at the ETH Zürich, the Paul Scherrer Institute and the University of Illinois, USA. We find that the salt (counter) ions carry along a hydration shell, forming some a “soft ball” of a few Ångstom size that can layer next to the surface, like tennis balls on the floor of a box. We can sense these layers of balls using the high force sensitivity of the instrument and even detect sub-atomic transitions, which are related to the change of hydration water structure in the system. Knowing better the inner structure of EDLs is important for biological interaction and also technically relevant fields like rock weathering.
- Espinosa-Marzal, R. M., Drobek T., Balmer T., Heuberger M., (2012). "Hydrated-ion ordering in electrical double layers." Physical Chemistry Chemical Physics 14(17): 6085-6093.
Critical and supercritical Casimir forces
At Empa we have built a unique pressurized surface forces apparatus (pSFA) that can be used to study critical and supercritical fluids like CO2 under confinement. The new physical insights gained there are potentially important for future technological developments in the area of polymer. In particular, we could show that order parameter (=density) fluctuations near the critical point prevail along the extension of the CO2 coexistence line and lead to strong attractive forces in near critical and surprisingly also into supercritical conditions. This force is expected to have a pore stabilizing effect, for example, during pore formation in polymers processed with critical and supercritical fluids.
- Schurtenberger, E. and M. Heuberger (2011). "The extended surface forces apparatus. Part IV: Precision Static Pressure Control." Review of Scientific Instruments 82(103902): 8.
- Schurtenberger, E. and M. Heuberger (2012). "Supercritical Casimir Effect in Carbon Dioxide." Journal of Supercritical Fluids 71: 120-126.
In parallel to measuring surface forces and molecular interactions we are also using a transmission interferometric adsorption sensor (TInAS) to assess the adsorption of molecules onto surfaces. The method is sensitive enough to detect the adsorption isotherm of water on mica crystal surface. More recently we use this method to study the properties of different surface functionalization’s and novel plasma polymer films, for example, in terms of protein adsorption.
- Balmer, T. E., et al. (2008). "The Effect of Surface Ions on Water Adsorption to Mica." Langmuir 24: 1566-1569.
- Heuberger, M. and T. Balmer (2007). "The Transmission Interferometric Adsorption Sensor." Journal of Physics D: Applied Physics 40: 7245-7254.
- Sannomiya, T., et al. (2010). "Optical Sensing and Determination of Complex Refelection Coefficients of Plasmonic Structures using Transmission Interferometric Plasmonic Sensor." Review of Scientific Instruments 81: 053102.