Funded PhD thesis proposal: Ionic Nano-Rheology at Solid/Electrolyte Interfaces

ESPCI, Soft Matter Science and Engineering Laboratory (SIMM)
10 rue Vauquelin, 75005 Paris, France

PhD advisor: Jean COMTET; ;

Understanding the dynamics and transport of ionic charges at the interface between solid surfaces and liquid electrolytes is key to a number of physical, biological, and chemical processes of interest, from biophysical transport, nanofiltration, catalysis and electrochemistry, up to geology and environmental sciences. These interfaces are furthermore at the heart of energy-related technological applications, ranging from energy storage in super-capacitors or batteries, to emerging energy harvesting approaches such as blue energy or hydro-voltaic technologies.
Buried between two condensed phases, solid/electrolyte interfaces are characterized by an “electric double layer” structure, with a spatial separation of charges, between (1) the electronic charges hosted in the solid substrate, (2) the molecular scale of the charges and ions bound to the surface of the solid, known as the surface charges or the "Stern layer" and (3) the diffuse cloud of counterions in the liquid (the Debye layer), which screens the surface potential over characteristic nanometric distances.
The deformable nature of the liquid phase, along with the multi-scale character of the interface leads to strong electrokinetic couplings between solvent flow and ion dynamics at these interfaces. Recently, a number of observations have thus revealed fundamental gaps in our understanding of electrolyte transport, showing deep and fundamentally misunderstood couplings between solvent flow and ionic charge dynamics. One can cite among others long-standing discrepancies between static and dynamic surface charges [1], controversial reports of flow-induced shift of the surface charges [2] or peculiar “electro-viscous” over-dissipation during deformation of the electric double layer [3].

These observations call for the development of novel experimental methodology to quantitatively probe the intricate dynamics between liquid and ions at these interfaces. The aim of this PhD is precisely to investigate the out-of-equilibrium nano-rheological response of the ionic double layer under nanometric confinement, and the peculiar interactions of the electrolyte with the confining substrates. This goal will be achieved thanks to the development of state-of-the-art experimental methodology based on Quartz Tuning-Fork Atomic Force Microscopy techniques developed in the team [4, 5, 6], combined with a fine control of electrolyte physicochemistry along with the bulk and surface properties of the confining substrates.

We will confine the electrolyte between the colloidal probe of the AFM and the surface. Using the forced oscillatory motion of the probe, we will induce a nanoscale flow of liquid in the gap. By measuring the dynamic rheological response associated to this out-of-equilibrium liquid flow we will gain unprecedented insights on the dynamics of the confined liquid and ions, ultimately shedding new lights on the molecular transport processes taking place at the intimate scale of these solid/liquid interfaces. Specifically, we will focus on the following effects:
1) Role of ionic surface mobility. First, we envision that the dynamic spatial charge heterogeneities induced by the convective flow of liquid could be partially relaxed by the mobility of surface charges, allowing us to reveal the dynamics of this last interfacial layer. The signature of these dynamics in the surface force measurements should allow us to distinguish between fixed surface charges on ionizable surfaces (leading to strong spatial charge heterogeneities and electro-viscous over-dissipation) or mobile charges on non-ionizable surface (which should relax charge imbalances).
2) Mixed ionic and liquid friction. In a second step, we will probe the overlooked interfacial couplings between hydrodynamic and ionic friction at these interfaces. The presence of ions adsorbed (and potentially mobile) at the interface could affect the frictional interactions of the liquid. Slippage will be extrapolated from the long- range hydrodynamic dissipation. By accessing the variation of slip length with pH and ion concentration, we will probe how ions adsorbed at the interface affect liquid friction with the solid.
3) Electronic couplings. We will then extend these measurements towards conductive surfaces, for which additional peculiar electronic couplings might arise. Nano-rheological measurements on conductive surfaces will be coupled with electrical measurements of the induced currents between the tip and the surface. We will then probe the effect of an electronic gate (polarization) between the probe and the surface, to study the effect of a normal electric field on confined charge dynamics.
4) Dynamics under molecular confinement. Finally, these dynamic nano-rheological approaches will be extended toward molecular-scale confinement of the electrolyte, allowing us to directly probe the transport properties of ions deep within the Stern layer.

The student will acquire expertise in nano-assembly and micro-fabrication, signal processing for advanced dynamic Atomic Force Microscopy, as well as fundamental knowledge of dynamic transport at solid/liquid interface, of relevance for a broad class nanofluidic problems of interest. We are thus looking for a student strongly motivated by innovative experimental work, with a background in physics.

Applications should be sent to (include CV and grades).


[1] Joly et al. (2018). Current Opinion in Colloid & Interface Science, 37, 101-114.
[2] Bonn et al. (2014). Science, 344(6188), 1138-1142.
[3] Van den Ende et al. (2018). The Journal of Physical Chemistry B, 122.2, 933-946.
[4] Comtet et al. (2017). Nature materials, 16(6), 634-639.
[5] Comtet et al. (2018).Nature communications, 8(1), 15633.
[6] Comtet et al. (2019). Nature, 569(7756), 393-397.