Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Slippery Flows: From Molecular Perspective to Applications

With the system length scales becoming comparable to the molecular dimensions, flow of fluids through nanofluidic channels leads to several non-intuitive and interesting phenomena which are otherwise not observed at larger length scales. Water, despite not being a complex fluid as per classical hydrodynamics, has complexities associated at the molecular levels, giving rise to anomalous properties. Nature widely uses this amalgamation of the continuum properties and the molecular attributes in nanopores and membranes; hydrophobic self-cleaning leaves and scales, antifogging functionality of insect eyes, natural adhesion in gecko limbs, water transport in cells, hydrophilic leaves of pitcher plant etc. Development of manufacturing techniques and discovery of materials like carbon nanotubes and graphene have made it plausible to employ nanofluidics to laboratory and industrial applications. The precise control over fluid steering and retention, over these scales, can form the basis of several applications related to fluid transport, mixing, drug delivery and energy conversion. Through extensive molecular dynamics simulations of water in nanochannels, we develop a comprehensive theoretical understanding of the molecular phenomena and the upscale implications on its dynamics providing a blueprint for explorative experimental studies.
The presentation begins with the discussion of anomalous filling rates of nanocapillaries arising from the interfacial interaction, leading to a dynamic slip length along the capillary axis. The subsequent analysis discusses the effect of size and hydration properties of ionic inclusions on slip length of saline solutions and using it to tune the wettability. Next, contrary to common belief that hydrophobic surfaces trivially cause water to slip, we show that this conceptual paradigm is a mimesis of the interfacial fluid structure which may or may not coincide with the intrinsic wettability. Our molecular scale simulation results emphatically demonstrate that hydrophobic surfaces can manifest features that are otherwise typically associated with hydrophilicity, causing water to stick. Our analysis provides some details on the underlying molecular mechanisms responsible for such non-intuitive results, and pinpoints the interaction between the ionic inclusions and water structuration in the interfacial region to bring out this remarkable effect. Finally, we bring these results in perspective with electrokinetic phenomena at nanoscales. The development of surface charge is shown to alter the slip length and the flow dynamics non-intuitively, which affects the performance of nanofluidic devices in a remarkable manner. We further discuss the blueprint for optimizing various parameters to maximize energy conversion efficiencies in nanofluidic circuits, within the physical realm of miniaturized power plant on a chip.