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Projekt Druckansicht

Nanofluidische Effekte fern vom Gleichgewicht

Fachliche Zuordnung Statistische Physik, Nichtlineare Dynamik, Komplexe Systeme, Weiche und fluide Materie, Biologische Physik
Förderung Förderung von 2017 bis 2022
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 391007699
 
Erstellungsjahr 2022

Zusammenfassung der Projektergebnisse

Nanometer-scale fluid flows have been studied indirectly in various disciplines for the last 50 years. At such small length scales, the effect of the bounding interfaces on electrostatic and hydrodynamic properties becomes pronounced. Especially in the past decades, the effects of the nanometer-scale interfacial layer have been observed in micro-fabricated fluidic channels, where clear effects appear in channels at the scale of several tens of nanometers. Many of the observed effects on conductance and electrokinetics depend critically on the surface and ions type, yet a theory explaining these effects in terms of the molecular structure of the electrolyte interfaces has been lacking so far. The ultimate scale for observing specific effects is set by the molecular size of the fluid; more precisely, a critical confinement of about 1 nm has generally been accepted as the limit of validity for the Navier-Stokes equation. Until recently, the exploration of this ultimate scale was hindered by technical challenges, as molecular-scale channels could not be fabricated artificially. However, the progress in fabrication technology has now allowed researchers to overcome the challenges that have hindered the development of nanofluidics at the ultimate scales, and artificial devices with confinement down to about one water molecule in size (3 ˚) have been achieved. In particular, the fabrication of nanofluidic channels based on carbon nanotubes and graphene channels allows the study of nanometer-scale flows. A precise and comprehensive theoretical understanding of aqueous interfaces and confined systems is required to fully unlock the potential of these new experimental systems to study interfacial and confined systems at an unprecendented level of precision. From the theoretical side of the project, the primary objectives were the following. 1. To develop a comprehensive theoretical framework for the study of interfacial effects on viscosity, dielectric constant and ion adsorption that can be applied to a range of nanometer-scale experimental systems to describe conductivity, electrokinetic flow, effective surface charge, etc. 2. To perform a complete investigation of transport in nanotubes down to the smallest sizes and to develop a single theoretical framework to describe the various surface-driven transport modes (surface slippage, electro-, diffusio- osmotic, etc.) The theory will include effective viscosity and hydrodynamic boundary conditions, dielectric properties and ion adsorption potentials, including dependence on pH and salt concentration. 3. To vary the nanotube inner surface to study the effect of slip and surface polarity. Drastic differences have been reported between the hydrodynamic properties of boron-nitride and carbon nanotubes of small radius. Our objective is to interpret these differences in terms of an atomistic or sub-atomic model. Summarizing our results, we have first studied the structure of the water layer at solidliquid interfaces, its effect on the interfacial dielectric constant and viscosity, and the adsorption properties of ions and charged surfactants. For enhanced versatility and analytical solvability, these effects are described in a box-model based mean-field theoretical framework. Second, we have studied the effects of ion adsorption as function of ion concentration and pH on the conductivity of carbon nanotubes, both in the static case and at finite frequency. The results are compared with experimental measurements from our collaborators. Finally, we have studied the effects of the water-surface interaction as a function of polarity on surface wetting, as well as the effects of solute-surface interactions on the formation and evaporation of droplets. Overall, we have largely fulfilled the aims stated above, including the direct comparison with experimental data. In the sections below, we describe the separate projects in more detail.

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