Nanofluidics

Nanofluidics explores transport phenomena of fluids at nanometer scales. From the point of view of fluidic operations, nanometric scales allow new fluidic functionalities to be developed, using the explicit benefit of the predominance of surfaces. Typical examples involve preconcentration phenomena, the development of nanofluidic transistors or the recently proposed nanofluidic diodes. Nanofluidics also carries the hope that new properties will emerge by benefiting from the specific phenomena occurring at the smallest scales.

Molecular layer

Confined liquid films with a thickness in the range of a few molecular diameters exhibit different mechanical properties than in the bulk. This variation in mechanical properties of thin liquid films compared with the bulk arises from structural changes. Recent advances in synchrotron radiation sources and beam shaping techniques allow scattering experiments to investigate the in-plane structure of confined liquids.

Hydrodynamic model

The analysis of dynamics of the boundary line of the layering transitions in detail layer by layer thinning of a confined thin liquid film with increasing external normal force on the substrates showed that a simple hydrodynamic model was able to explain the phenomenon. While it is generally accepted that the viscosity of confined liquids increases with decreasing thickness, the order of magnitude is highly debated. Using ultra clean, recleaved mica surfaces, it was found that the viscosity of the model lubricant Octamethylcyclotetrasiloxane (OMCTS) increases by a factor of 10 with decreasing the film thickness from 6 to 2 layers. A new hydrodynamic model showed that the sliding friction of liquid layers on top of the solid substrates is approximately 30 times higher than the mutual friction between adjacent liquid layers. Further, the latter was independent of film thickness and in close agreement with the bulk viscosity.

Applications of nanofluidics

There are a number of applications of nanofluidics, like the use of charged polymers for lubrication, the lotus effect for self-cleaning surfaces, membranes for filtering on size or charge (e.g. for desalination), nanoporous materials, for size exclusion chromatography, passive selective transport in aquaporins, active transport in ion channels, molecular motors like kinesin and charge based filtration in the kidney basal membrane. An important area of application for nanofluidics is separation science. A second area of nanofluidic applications has been the study of fundamental properties of liquids and molecules, e.g. in biophysics and fluid mechanics.