Investigation of the mesoscale couplings in nanofluidics using nonlinear optical techniques
Transport of fluids and ions confined at the nanoscale strongly deviates from the continuum description of hydrodynamics. These exotic nanofluidic properties take their roots in the combination, at the nanoscale, of physical phenomena such as charge effects, fluctuations or fluid slippage. Such effects can be harvested for applications such as desalination, blue-energy production, or ultrafiltration for healthcare. Recently, it has been discovered that beyond the chemical reactivity of interfaces, the electronic properties of the confining materials also strongly modify nanofluidic transport. The aim of this project is to understand the molecular nature of these couplings happening at the mesoscale, where the atomic scale of electronic properties meets the bulk scale of the continuum and classical physics of electrolytes. This requires to develop new experimental tools to go beyond the state-of-the-art techniques mainly based on current measurements. Indeed, despite their precision, they only quantify charge transport regardless of the species involved and cannot distinguish water/surface (slippage) from ion/surface interactions (surface charge). To disentangle these effects, we will use new fast nonlinear optical techniques to reveal the molecular nature of the couplings inside channels (nanotubes and 2D channels) made of hexagonal boron nitride (hBN) and graphite. These twin materials will allow us to probe the electronic nature of the couplings: indeed, they share the same crystallographic structure but differ by their electronic properties (insulator versus conductor). We will focus in particular on two objectives: (i) developing a label-free (pump-probe) method able to measure nanoflows in situ and using it to study the effects of ion density, walls’ electronic properties and channel geometry (1D, 2D) on water slippage, and (ii) using Sum Frequency Generation spectroscopy to identify the nature of the surface charge of graphene and hBN interfaces.